Android学习之路(12) setContentView详解

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一、简介

setContentView我们在Activity中经常见到,它的作用就是把我们的布局文件放在Activity中显示,下面我们根据源码分析setContentView是如何做到的

二、源码分析

1.两种setContentView

注意Activity的setContentView和AppCompatActivity的setContentView是有一些区别的,所以我们要分析两钟setContentView,下面先分析Activity的

2.Activity的setContentView

(1).从Activity的setContentView这个方法开始

public void setContentView(@LayoutRes int layoutResID) {
    getWindow().setContentView(layoutResID);
    initWindowDecorActionBar();
}

可以看到第一句getWindow().setContentView(layoutResID),这个getWindow是获取当前Activity的Window,在Android中Window的实现类是phoneWindow,所以我们要看phoneWindow的setContentView

顺便提一下Activity的window的创建时机是在Activity的attach方法:

android setcontentview,安卓,android,学习

(2).继续跟踪到phoneWindow的setContentView

    public void setContentView(int layoutResID) {
        // Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
        // decor, when theme attributes and the like are crystalized. Do not check the feature
        // before this happens.
        if (mContentParent == null) {
            installDecor(); //⭐这句关键流程
        } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            mContentParent.removeAllViews();
        }
 
        if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
                    getContext());
            transitionTo(newScene);
        } else {
            mLayoutInflater.inflate(layoutResID, mContentParent);//⭐这句关键流程
 
        }
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
        mContentParentExplicitlySet = true; //⭐这个flag记一下
    }

上面代码中我标记了三处重点,我们下面继续分析这三个重点都干了什么,先分析第一个installDecor()

(2.1).分析phoneWindow的setContentView的第一个关键点installDecor()

android setcontentview,安卓,android,学习

installDecor主要是我用红框标记出来的是重点,我们先分析generateDecor(-1)这个方法:

(2.1.1).分析installDecor()方法中的generateDecor(-1)方法

    protected DecorView generateDecor(int featureId) {
        // System process doesn't have application context and in that case we need to directly use
        // the context we have. Otherwise we want the application context, so we don't cling to the
        // activity.
        Context context;
        if (mUseDecorContext) {
            Context applicationContext = getContext().getApplicationContext();
            if (applicationContext == null) {
                context = getContext();
            } else {
                context = new DecorContext(applicationContext, this);
                if (mTheme != -1) {
                    context.setTheme(mTheme);
                }
            }
        } else {
            context = getContext();
        }
        return new DecorView(context, featureId, this, getAttributes());//⭐重点
    }

创建了一个DecorView并且返回之后赋值给了mDecor,我们先看一下这个DecorVIew是什么:

android setcontentview,安卓,android,学习

很明显是一个FrameLayout,这下我们知道了创建了一个FrameLayout类型的DecorView然后赋值给了mDecor变量,下面继续分析installDecor的第二个重点:generateLayout(mDecor)

(2.1.2).分析installDecor的第二个重点:generateLayout(mDecor)

    protected ViewGroup generateLayout(DecorView decor) {
    ...
else {
            // Embedded, so no decoration is needed.
            layoutResource = R.layout.screen_simple; 
            // System.out.println("Simple!");
        }
 
        mDecor.startChanging();
        mDecor.onResourcesLoaded(mLayoutInflater, layoutResource); //⭐重点下面的方法
 
        ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
        if (contentParent == null) {
            throw new RuntimeException("Window couldn't find content container view");
        }
    ...
        return contentParent;
    }
 
    //⭐DecorView的onResourcesLoaded方法
    void onResourcesLoaded(LayoutInflater inflater, int layoutResource) {
        if (mBackdropFrameRenderer != null) {
            loadBackgroundDrawablesIfNeeded();
            mBackdropFrameRenderer.onResourcesLoaded(
                    this, mResizingBackgroundDrawable, mCaptionBackgroundDrawable,
                    mUserCaptionBackgroundDrawable, getCurrentColor(mStatusColorViewState),
                    getCurrentColor(mNavigationColorViewState));
        }
 
        mDecorCaptionView = createDecorCaptionView(inflater);
        final View root = inflater.inflate(layoutResource, null);//⭐重点主线流程
        if (mDecorCaptionView != null) {
            if (mDecorCaptionView.getParent() == null) {
                addView(mDecorCaptionView,
                        new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
            }
            mDecorCaptionView.addView(root,
                    new ViewGroup.MarginLayoutParams(MATCH_PARENT, MATCH_PARENT));
        } else {
 
            // Put it below the color views. ⭐重点主线流程
            addView(root, 0, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
        }
        mContentRoot = (ViewGroup) root;
        initializeElevation();
    }

这个方法的作用就是,通过我们设置的style或者requestWindowFuture等来选出一个系统自带的布局文件,默认的是R.layout.screen_simple,选出布局文件后,通过调用mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);方法inflate出来后,add到DecorView上,我们详细看一下R.layout.screen_simple这个布局文件:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    android:orientation="vertical">
    <ViewStub android:id="@+id/action_mode_bar_stub"
              android:inflatedId="@+id/action_mode_bar"
              android:layout="@layout/action_mode_bar"
              android:layout_width="match_parent"
              android:layout_height="wrap_content"
              android:theme="?attr/actionBarTheme" />
    <FrameLayout
         android:id="@android:id/content"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         android:foregroundInsidePadding="false"
         android:foregroundGravity="fill_horizontal|top"
         android:foreground="?android:attr/windowContentOverlay" />
</LinearLayout>

就是这个布局文件,被inflate到DecorView上,然后通过

ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);

这一句获取到这个是content的FrameLayout,然后返回了这个contentParent

到这里installDecor这个方法分析完了,总结一下,installDecor方法主要就是创建DecorView,然后把选出的布局文件add到DecorView上,然后再通过findViewbyId找到类型是FrameLayout的content赋值给contentParent 返回,其实还有一步是DecorView和phoneWindow结合,这里不细说,以后有FrameWorker源码解析再说。

(2.2).继续分析phoneWindow的setContentView的第二个关键流程重点

mLayoutInflater.inflate(layoutResID, mContentParent);

这句很明显,layoutResID是我们的activity_main.layout这种自己写的布局文件,把它inflate到mContentParent中,通过图片让大家有一个更清晰的感官:

android setcontentview,安卓,android,学习

(2.3).继续分析phoneWindow的setContentView的第三个关键流程重点

mContentParentExplicitlySet = true;

这个flag的作用首先我们先看一段代码:

public class MainActivity extends AppCompatActivity {
 
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        requestWindowFeature(Window.FEATURE_NO_TITLE);
    }
}

这段代码运行会报错:

requestFeature() must be called before adding content

为什么会报这个错误呢,从代码上来找:

    public final boolean requestWindowFeature(int featureId) {
        return getWindow().requestFeature(featureId);
    }
 
    //我们已经知道,getWindow其实获取的是PhoneWindow所以调用的是PhoneWindow的requestFeature
    @Override
    public boolean requestFeature(int featureId) {
        if (mContentParentExplicitlySet) { //⭐这就是报错的根源
            throw new AndroidRuntimeException("requestFeature() must be called before adding content"); 
        }
        final int features = getFeatures();
        final int newFeatures = features | (1 << featureId);
        if ((newFeatures & (1 << FEATURE_CUSTOM_TITLE)) != 0 &&
                (newFeatures & ~CUSTOM_TITLE_COMPATIBLE_FEATURES) != 0) {
            // Another feature is enabled and the user is trying to enable the custom title feature
            // or custom title feature is enabled and the user is trying to enable another feature
            throw new AndroidRuntimeException(
                    "You cannot combine custom titles with other title features");
        }
        if ((features & (1 << FEATURE_NO_TITLE)) != 0 && featureId == FEATURE_ACTION_BAR) {
            return false; // Ignore. No title dominates.
        }
        if ((features & (1 << FEATURE_ACTION_BAR)) != 0 && featureId == FEATURE_NO_TITLE) {
            // Remove the action bar feature if we have no title. No title dominates.
            removeFeature(FEATURE_ACTION_BAR);
        }
 
        if (featureId == FEATURE_INDETERMINATE_PROGRESS &&
                getContext().getPackageManager().hasSystemFeature(PackageManager.FEATURE_WATCH)) {
            throw new AndroidRuntimeException("You cannot use indeterminate progress on a watch.");
        }
        return super.requestFeature(featureId);
    }

看到了报错的根源,其实就是mContentParentExplicitlySet这个flag,在setContentView执行完就设置成了true,所以调用requestWindowFeature(Window.FEATURE_NO_TITLE);方法必须在setContentView之前,否则就会抛出异常,最后从设计的角度分析,为什么要设计这么一个flag呢,或者说为什么非要在setContentView之前执行requestFeature,因为在setContentView中需要通过设置的这些requestWindowFeature的flag去选择一个布局文件然后add到DecorView上,如果在setContentView后面设置就起不到作用,所以有了这个设计。

3.AppCompatActivity的setContentView

(1).AppCompatActivity的setContentView源码

    @Override
    public void setContentView(@LayoutRes int layoutResID) {
        getDelegate().setContentView(layoutResID);
    }

(1.1).getDelegate()

    @NonNull
    public AppCompatDelegate getDelegate() {
        if (mDelegate == null) {
            mDelegate = AppCompatDelegate.create(this, this);
        }
        return mDelegate;
    }

实际上实现类是AppCompatDelegate,getDelegate().setContentView(layoutResID);的setContentView实际上是AppCompatDelegate的setContentView方法

(2).AppCompatDelegate的setContentView方法

    @Override
    public void setContentView(int resId) {
        ensureSubDecor(); //⭐重点主线
        ViewGroup contentParent = mSubDecor.findViewById(android.R.id.content); //⭐重点主线
        contentParent.removeAllViews();
        LayoutInflater.from(mContext).inflate(resId, contentParent);//⭐重点主线
        mAppCompatWindowCallback.getWrapped().onContentChanged();
    }

(2.1).分析ensureSubDecor()方法

    private void ensureSubDecor() {
        if (!mSubDecorInstalled) {
            mSubDecor = createSubDecor(); // ⭐重点主线流程
 
            // If a title was set before we installed the decor, propagate it now
            CharSequence title = getTitle();
            if (!TextUtils.isEmpty(title)) {
                if (mDecorContentParent != null) {
                    mDecorContentParent.setWindowTitle(title);
                } else if (peekSupportActionBar() != null) {
                    peekSupportActionBar().setWindowTitle(title);
                } else if (mTitleView != null) {
                    mTitleView.setText(title);
                }
            }
 
            applyFixedSizeWindow();
 
            onSubDecorInstalled(mSubDecor);
 
            mSubDecorInstalled = true;//⭐这个flag参数
 
            // Invalidate if the panel menu hasn't been created before this.
            // Panel menu invalidation is deferred avoiding application onCreateOptionsMenu
            // being called in the middle of onCreate or similar.
            // A pending invalidation will typically be resolved before the posted message
            // would run normally in order to satisfy instance state restoration.
            PanelFeatureState st = getPanelState(FEATURE_OPTIONS_PANEL, false);
            if (!mIsDestroyed && (st == null || st.menu == null)) {
                invalidatePanelMenu(FEATURE_SUPPORT_ACTION_BAR);
            }
        }
    }

主要的第一句:

mSubDecor = createSubDecor();

mSubDecor是一个ViewGroup类型的对象,下面我们分析createSubDecor()

(2.1.1).ensureSubDecor()方法中的createSubDecor()方法

    private ViewGroup createSubDecor() {
        TypedArray a = mContext.obtainStyledAttributes(R.styleable.AppCompatTheme);
        
        //⭐这个错误是不是曾经见到过,如果用的Theme不是AppCompatTheme的就会报错
        if (!a.hasValue(R.styleable.AppCompatTheme_windowActionBar)) {
            a.recycle();
            throw new IllegalStateException(
                    "You need to use a Theme.AppCompat theme (or descendant) with this activity.");
        }
......
 
        // Now let's make sure that the Window has installed its decor by retrieving it
        ensureWindow(); 
        mWindow.getDecorView();  
 
 
        final LayoutInflater inflater = LayoutInflater.from(mContext);
        ViewGroup subDecor = null;
 
 
        if (!mWindowNoTitle) {
......
        } else {
......
           subDecor = (ViewGroup) inflater.inflate(R.layout.abc_screen_simple, null);
......
        }
......
 
        final ContentFrameLayout contentView = (ContentFrameLayout) subDecor.findViewById(
                R.id.action_bar_activity_content);
 
        final ViewGroup windowContentView = (ViewGroup) mWindow.findViewById(android.R.id.content);
        if (windowContentView != null) {
            // There might be Views already added to the Window's content view so we need to
            // migrate them to our content view
            while (windowContentView.getChildCount() > 0) {
                final View child = windowContentView.getChildAt(0);
                windowContentView.removeViewAt(0);
                contentView.addView(child);
            }
 
            // Change our content FrameLayout to use the android.R.id.content id.
            // Useful for fragments.
            windowContentView.setId(View.NO_ID);
            contentView.setId(android.R.id.content);
 
            // The decorContent may have a foreground drawable set (windowContentOverlay).
            // Remove this as we handle it ourselves
            if (windowContentView instanceof FrameLayout) {
                ((FrameLayout) windowContentView).setForeground(null);
            }
        }
 
        // Now set the Window's content view with the decor
        mWindow.setContentView(subDecor);
......
 
        return subDecor;
    }

上面显示出来的基本上都很重要,我们一句一句分析:

(2.1.1.1).ensureWindow();

    private void ensureWindow() {
        // We lazily fetch the Window for Activities, to allow DayNight to apply in
        // attachBaseContext
        if (mWindow == null && mHost instanceof Activity) {
            attachToWindow(((Activity) mHost).getWindow());
        }
        if (mWindow == null) {
            throw new IllegalStateException("We have not been given a Window");
        }
    }

首先我们要明确AppCompatActivity是继承自Activity的,所以window也是在attach方法中创建的,在AppCompatDelegateImpl中也维护了一个Window类型的变量是mWindow,就是通过这个ensureWindow方法经过检查后赋值过来的。说白了ensureWindow方法就是把AppCompatActivity中的Window对象赋值到AppCompatDelegateImpl对象中,当然对window设置的callBack啥的也换成AppCompatDelegateImpl中的。

(2.1.1.2).mWindow.getDecorView()方法
Window的实现类,所以我们要看PhoneWindow的getDecorView()方法:

    @Override
    public final @NonNull View getDecorView() {
        if (mDecor == null || mForceDecorInstall) {
            installDecor();
        }
        return mDecor;
    }

可以看到是调用了installDecor,和Activity有了相同的部分,我们简单回忆一下installDecor干了啥,首先创建了DecorView,然后通过解析我们style的设置选出合适的系统自带布局文件,把它add到DecorView上,并且返回了一个id是com.android.internal.R.id.content的FrameLayout(将来我们要把我们的activity_mai的layout文件add到这个content上面)。

(2.1.1.3).后面要分析的这一堆代码就是Activity和AppCompatActivity的setContentView的主要区别

....
 subDecor = (ViewGroup) inflater.inflate(R.layout.abc_screen_simple, null);
....
final ContentFrameLayout contentView = (ContentFrameLayout) subDecor.findViewById(
                R.id.action_bar_activity_content);
 
        final ViewGroup windowContentView = (ViewGroup) mWindow.findViewById(android.R.id.content);
        if (windowContentView != null) {
            // There might be Views already added to the Window's content view so we need to
            // migrate them to our content view
            while (windowContentView.getChildCount() > 0) {
                final View child = windowContentView.getChildAt(0);
                windowContentView.removeViewAt(0);
                contentView.addView(child);
            }
 
            // Change our content FrameLayout to use the android.R.id.content id.
            // Useful for fragments.
            windowContentView.setId(View.NO_ID);
            contentView.setId(android.R.id.content);
 
......
        }
 
        // Now set the Window's content view with the decor
        mWindow.setContentView(subDecor);
......
 
        return subDecor;
    }

通过我们设置的style选出一个布局文件,这一步好像在installDecor中已经做过了,这样好像重复了,为什么有这样的一个重复?有这样一个重复是为了不影响原来的代码的同时,把一部分对style处理的逻辑转移到AppCompatDelegateImpl中,例如对windowTitle的隐藏与显示,这里可看出来设计师的设计,通过下面的学习慢慢体会,先看一下这个布局文件:

<androidx.appcompat.widget.FitWindowsLinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/action_bar_root"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:fitsSystemWindows="true">
 
    <androidx.appcompat.widget.ViewStubCompat
        android:id="@+id/action_mode_bar_stub"
        android:inflatedId="@+id/action_mode_bar"
        android:layout="@layout/abc_action_mode_bar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />
 
    <include layout="@layout/abc_screen_content_include" />
 
</androidx.appcompat.widget.FitWindowsLinearLayout>

abc_screen_content_include的布局文件:

<merge xmlns:android="http://schemas.android.com/apk/res/android">
 
    <androidx.appcompat.widget.ContentFrameLayout
            android:id="@id/action_bar_activity_content"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:foregroundGravity="fill_horizontal|top"
            android:foreground="?android:attr/windowContentOverlay" />
 
</merge>

把这个布局文件inflate成view赋值给subDecor,subDecor是一个ViewGroup:

下面重点来了:

final ContentFrameLayout contentView = (ContentFrameLayout) subDecor.findViewById(
        R.id.action_bar_activity_content);

这一句是获取subDecor中的id是 action_bar_activity_content的ContentFrameLayout类型的View赋值给contentView

final ViewGroup windowContentView = (ViewGroup) mWindow.findViewById(android.R.id.content);

windowContentView是获取installDecor中选择的那个布局的id是android.R.id.content的那个FrameLayout

while (windowContentView.getChildCount() > 0) {
    final View child = windowContentView.getChildAt(0);
    windowContentView.removeViewAt(0);
    contentView.addView(child);
}

如果windowContentView中有子View,那就全部转移到contentView上

windowContentView.setId(View.NO_ID);
contentView.setId(android.R.id.content);

然后把windowContentView的id设置成NO_ID,把contentView的id设置成android.R.id.content,这就是想把AppCompatDelegateImple中选的这个系统自带布局文件的content替换掉之前的installDecor方法中布局文件的content,以后我们的activity_main的layout布局文件就加载在替换之后的content上

mWindow.setContentView(subDecor);

这句调用的是phoneWindow的setContentView方法

    @Override
    public void setContentView(View view) {
        setContentView(view, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
    }
 
    @Override
    public void setContentView(View view, ViewGroup.LayoutParams params) {
        // Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
        // decor, when theme attributes and the like are crystalized. Do not check the feature
        // before this happens.
        if (mContentParent == null) {
            installDecor();
        } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            mContentParent.removeAllViews();
        }
 
        if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            view.setLayoutParams(params);
            final Scene newScene = new Scene(mContentParent, view);
            transitionTo(newScene);
        } else {
            mContentParent.addView(view, params); //⭐把contentView添加到mContentParent上
        }
        mContentParent.requestApplyInsets();
        final Callback cb = getCallback();
        if (cb != null && !isDestroyed()) {
            cb.onContentChanged();
        }
        mContentParentExplicitlySet = true;
    }

我们看一下mContentParent是什么:

mContentParent = generateLayout(mDecor);

是我们取消ID的那个之前的content,也就是说把AppCompatDelegateImpl的选择的系统自带布局文件(subDecor),添加到之前的content中,最后返回subDecor注意这个subDecor是系统自带布局inflate出来的,接下来我们通过一张图加深理解:

android setcontentview,安卓,android,学习

(2.1.2).ensureSubDecor()方法中的一个boolean值mSubDecorInstalled
这个mSubDecorInstalled和Activity中的mContentParentExplicitlySet一样,作用也是防止在setContentView之后调用

supportRequestWindowFeature(Window.FEATURE_NO_TITLE);

,我们发现和Activity的不一样了,我们思考一下,为什么调用supportRequestWindowFeature而不是调用requestWindowFeature?

Activity中处理这些style是在PhoneWIndow的installDecor中,而AppCompatActivity在处理一些style时是在AppCompatDelegateImpl中,requestWindowFeature是影响的installDecor,supportRequestWindowFeature是影响的AppCompatDelegateImpl,拿window的Title来举例,看上面放过的两张图,Activity的title是要在installDecor方法中决定的显示与隐藏的,而AppCompatActivity的title是放在AppCompatDelegateImpl中决定显示与隐藏的,我们用AppCompatActivity肯定是要在AppCompatDelegateImpl进行一些操作,而不是对installDecor中进行操作。

下面看一下supportRequestWindowFeature调用逻辑:

    public boolean supportRequestWindowFeature(int featureId) {
        return getDelegate().requestWindowFeature(featureId);
    }
 
    @Override
    public boolean requestWindowFeature(int featureId) {
        featureId = sanitizeWindowFeatureId(featureId);
 
        if (mWindowNoTitle && featureId == FEATURE_SUPPORT_ACTION_BAR) {
            return false; // Ignore. No title dominates.
        }
        if (mHasActionBar && featureId == Window.FEATURE_NO_TITLE) {
            // Remove the action bar feature if we have no title. No title dominates.
            mHasActionBar = false;
        }
 
        switch (featureId) {
            case FEATURE_SUPPORT_ACTION_BAR:
                throwFeatureRequestIfSubDecorInstalled();
                mHasActionBar = true;
                return true;
            case FEATURE_SUPPORT_ACTION_BAR_OVERLAY:
                throwFeatureRequestIfSubDecorInstalled();
                mOverlayActionBar = true;
                return true;
            case FEATURE_ACTION_MODE_OVERLAY:
                throwFeatureRequestIfSubDecorInstalled();
                mOverlayActionMode = true;
                return true;
            case Window.FEATURE_PROGRESS:
                throwFeatureRequestIfSubDecorInstalled();
                mFeatureProgress = true;
                return true;
            case Window.FEATURE_INDETERMINATE_PROGRESS:
                throwFeatureRequestIfSubDecorInstalled();
                mFeatureIndeterminateProgress = true;
                return true;
            case Window.FEATURE_NO_TITLE:
                throwFeatureRequestIfSubDecorInstalled();
                mWindowNoTitle = true;
                return true;
        }
 
        return mWindow.requestFeature(featureId);
    }
 
    //⭐mSubDecorInstalled 这参数眼熟吧
    private void throwFeatureRequestIfSubDecorInstalled() {
        if (mSubDecorInstalled) {
            throw new AndroidRuntimeException(
                    "Window feature must be requested before adding content");
        }
    }

(2.2).AppCompatDelegateImpl中的setContentView(View v)剩下的一起说

ViewGroup contentParent = mSubDecor.findViewById(android.R.id.content); //⭐重点主线
contentParent.removeAllViews();
LayoutInflater.from(mContext).inflate(resId, contentParent);//⭐重点主线

获取contentview,注意这个contentView和Activity的contentView的区别,看上面那张图,把所有的view清除掉,然后把我们的activity_main的layout这种我们自己的布局加载上去

3.分析LayoutInflater.from(mContext).inflate(resId, contentParent);方法

(1).很经典的一道面试题,inflate三个参数都有什么作用
或者说这三种写法有什么区别:

android setcontentview,安卓,android,学习

我们先看源码,看完之后总结:

    public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
        final Resources res = getContext().getResources();
        if (DEBUG) {
            Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
                  + Integer.toHexString(resource) + ")");
        }
 
        View view = tryInflatePrecompiled(resource, res, root, attachToRoot);
        if (view != null) {
            return view;
        }
        XmlResourceParser parser = res.getLayout(resource); //⭐把布局文件用xml解析器解析 
        try {
            return inflate(parser, root, attachToRoot); //调用inflate重载的方法
        } finally {
            parser.close();
        }
    }

继续往下看inflate的重载方法,这个方法中就有这道面试题的答案:

    public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
        synchronized (mConstructorArgs) {
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");
 
            final Context inflaterContext = mContext;
            final AttributeSet attrs = Xml.asAttributeSet(parser);
            Context lastContext = (Context) mConstructorArgs[0];
            mConstructorArgs[0] = inflaterContext;
            View result = root; //⭐重点
 
            try {
                advanceToRootNode(parser); //⭐确保下面的代码是首先解析的根布局标签
                final String name = parser.getName();
 
                if (DEBUG) {
                    System.out.println("**************************");
                    System.out.println("Creating root view: "
                            + name);
                    System.out.println("**************************");
                }
 
                if (TAG_MERGE.equals(name)) {
                    if (root == null || !attachToRoot) {
                        throw new InflateException("<merge /> can be used only with a valid "
                                + "ViewGroup root and attachToRoot=true");
                    }
 
                    rInflate(parser, root, inflaterContext, attrs, false);
                } else { //⭐重点 else单独分析
                    // Temp is the root view that was found in the xml
                    final View temp = createViewFromTag(root, name, inflaterContext, attrs); 
 
                    ViewGroup.LayoutParams params = null;
 
                    if (root != null) {
                        if (DEBUG) {
                            System.out.println("Creating params from root: " +
                                    root);
                        }
                        // Create layout params that match root, if supplied
                        params = root.generateLayoutParams(attrs);
                        if (!attachToRoot) {
                            // Set the layout params for temp if we are not
                            // attaching. (If we are, we use addView, below)
                            temp.setLayoutParams(params);
                        }
                    }
 
                    if (DEBUG) {
                        System.out.println("-----> start inflating children");
                    }
 
                    // Inflate all children under temp against its context.
                    rInflateChildren(parser, temp, attrs, true);
 
                    if (DEBUG) {
                        System.out.println("-----> done inflating children");
                    }
 
                    // We are supposed to attach all the views we found (int temp)
                    // to root. Do that now.
                    if (root != null && attachToRoot) {
                        root.addView(temp, params);
                    }
 
                    // Decide whether to return the root that was passed in or the
                    // top view found in xml.
                    if (root == null || !attachToRoot) {
                        result = temp;
                    }
                }
 
            } catch (XmlPullParserException e) {
                final InflateException ie = new InflateException(e.getMessage(), e);
                ie.setStackTrace(EMPTY_STACK_TRACE);
                throw ie;
            } catch (Exception e) {
                final InflateException ie = new InflateException(
                        getParserStateDescription(inflaterContext, attrs)
                        + ": " + e.getMessage(), e);
                ie.setStackTrace(EMPTY_STACK_TRACE);
                throw ie;
            } finally {
                // Don't retain static reference on context.
                mConstructorArgs[0] = lastContext;
                mConstructorArgs[1] = null;
 
                Trace.traceEnd(Trace.TRACE_TAG_VIEW);
            }
 
            return result;
        }
    }

上面的代码太多我把关于这道题的主要代码逻辑拿出来:

(1.1).advanceToRootNode()这个方法就是确保第一个解析的是布局的根标签

    private void advanceToRootNode(XmlPullParser parser)
        throws InflateException, IOException, XmlPullParserException {
        // Look for the root node.
        int type;
        while ((type = parser.next()) != XmlPullParser.START_TAG &&
            type != XmlPullParser.END_DOCUMENT) {
            // Empty
        }
 
        if (type != XmlPullParser.START_TAG) {
            throw new InflateException(parser.getPositionDescription()
                + ": No start tag found!");
        }
    }

(1.2).else的逻辑单独分析

else {
                    // Temp is the root view that was found in the xml
                    final View temp = createViewFromTag(root, name, inflaterContext, attrs);
 
                    ViewGroup.LayoutParams params = null;
 
                    if (root != null) {
                        if (DEBUG) {
                            System.out.println("Creating params from root: " +
                                    root);
                        }
                        // Create layout params that match root, if supplied
                        params = root.generateLayoutParams(attrs);
                        if (!attachToRoot) {
                            // Set the layout params for temp if we are not
                            // attaching. (If we are, we use addView, below)
                            temp.setLayoutParams(params);
                        }
                    }
 
                    if (DEBUG) {
                        System.out.println("-----> start inflating children");
                    }
 
                    // Inflate all children under temp against its context.
                    rInflateChildren(parser, temp, attrs, true);
 
                    if (DEBUG) {
                        System.out.println("-----> done inflating children");
                    }
 
                    // We are supposed to attach all the views we found (int temp)
                    // to root. Do that now.
                    if (root != null && attachToRoot) {
                        root.addView(temp, params);
                    }
 
                    // Decide whether to return the root that was passed in or the
                    // top view found in xml.
                    if (root == null || !attachToRoot) {
                        result = temp;
                    }
                }

下面逐句分析:

(1.2.1)

final View temp = createViewFromTag(root, name, inflaterContext, attrs);

这一句就是通过name,来反射创建view,下面会详细分析如何创建的view,这个temp就是布局文件的根view

(1.2.2)

if (root != null) {
    if (DEBUG) {
        System.out.println("Creating params from root: " +
                root);
    }
    // Create layout params that match root, if supplied
    params = root.generateLayoutParams(attrs);
    if (!attachToRoot) {
        // Set the layout params for temp if we are not
        // attaching. (If we are, we use addView, below)
        temp.setLayoutParams(params);
    }
}

这就是如果inflate方法的第二个参数root不是null,那就执行这段代码:

params = root.generateLayoutParams(attrs);

这段代码的意思是:我们把新视图的根布局参数传递给root。让root进行转换成适合它自身布局的布局参数(因为不同的布局有不同的特性,例如LinearLayout和FrameLayout,我们要把布局文件的根temp这个View放到root中,就要让temp原来的布局参数转换成适合root这个ViewGroup布局的参数),如果inflate第三个参数attachToRoot是false就把布局文件的根view设置成转换的params

(1.2.3)

rInflateChildren(parser, temp, attrs, true);

之后就继续解析布局文件,通过反射创建View,具体如何解析和创建之后详细分析

(1.2.4)

if (root != null && attachToRoot) {
    root.addView(temp, params);
}

如果root不是null并且attachToRoot是true,把布局文件根生成的temp这个view添加到root上面,注意这个params还是上面转换的那个。

(1.2.5)

if (root == null || !attachToRoot) {
    result = temp;
}

如果root是null或者attachToRoot是false,方法最后的返回值就是这个布局文件生成的View

(1.2.6)

到这里我们就知道了:

1.当root不为null时,attachTORoot是true的时候就直接把我们布局生成的View添加到root(这个root是inflate方法参数的第二个参数)上面,并且方法最后的返回结果是root,如果attachTORoot是false,直接返回我们布局文件生成的View注意这个生成View的layoutParams已经set了,所以可以说:

LayoutInflater.from(this).inflate(R.layout.activity_main,root,true);

等价于

View view = LayoutInflater.from(this).inflate(R.layout.activity_main,root,false);
root.addView(view);

2.当root为null时,直接返回我们布局文件生成的view,注意这个生成的View没有layoutParams

,而且一旦root为null,后面的attachToRoot这个参数就失效了。

(2).分析inflate方法如何解析和创建View的
(2.1).在inflate布局文件的根标签时要注意merge标签

if (TAG_MERGE.equals(name)) {
    if (root == null || !attachToRoot) {
        throw new InflateException("<merge /> can be used only with a valid "
                + "ViewGroup root and attachToRoot=true");
    }

    rInflate(parser, root, inflaterContext, attrs, false);
}

当一个布局文件的根标签是merge时,如果root是null或者attachToRoot是false就报错,说明布局文件跟标签是merge时不能直接生成一个View,必须依附于其他View上。

(2.2).解析不是根标签的布局

    void rInflate(XmlPullParser parser, View parent, Context context,
            AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
 
        final int depth = parser.getDepth();
        int type;
        boolean pendingRequestFocus = false;
 
        while (((type = parser.next()) != XmlPullParser.END_TAG ||
                parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {
 
            if (type != XmlPullParser.START_TAG) {
                continue;
            }
 
            final String name = parser.getName();
 
            if (TAG_REQUEST_FOCUS.equals(name)) {
                pendingRequestFocus = true;
                consumeChildElements(parser);
            } else if (TAG_TAG.equals(name)) {
                parseViewTag(parser, parent, attrs);
            } else if (TAG_INCLUDE.equals(name)) {
                if (parser.getDepth() == 0) {
                    throw new InflateException("<include /> cannot be the root element");
                }
                parseInclude(parser, context, parent, attrs);
            } else if (TAG_MERGE.equals(name)) {
                throw new InflateException("<merge /> must be the root element");
            } else {
                final View view = createViewFromTag(parent, name, context, attrs);
                final ViewGroup viewGroup = (ViewGroup) parent;
                final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
                rInflateChildren(parser, view, attrs, true);
                viewGroup.addView(view, params);
            }
        }
 
        if (pendingRequestFocus) {
            parent.restoreDefaultFocus();
        }
 
        if (finishInflate) {
            parent.onFinishInflate();
        }
    }

(2.2.1).解析不是根布局时,注意对merge、include标签的处理

对merge标签时直接报错,因为merge标签只能用于根标签,对include标签的处理,判断如果include是跟标签则报错,因为include标签不能用作根标签。

(2.2.2).其他的view标签会走创建view,然后通过父布局生成对应的布局参数LayoutParams,然后添加在副布局上

else {
    final View view = createViewFromTag(parent, name, context, attrs);
    final ViewGroup viewGroup = (ViewGroup) parent;
    final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
    rInflateChildren(parser, view, attrs, true);
    viewGroup.addView(view, params);
}

这一段中,createViewFromTag这个方法最重要,后面都是生成LayoutParams和添加到父布局上的逻辑,最后我们分析createViewFromTag这个方法

(2.2.3).createViewFromTag创建view,注意这个Activity和AppCompatActivity有差别

a.先分析Activity的createViewFromTag创建view的流程

    private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {
        return createViewFromTag(parent, name, context, attrs, false);
    }
 
    @UnsupportedAppUsage
    View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
            boolean ignoreThemeAttr) {
        if (name.equals("view")) {
            name = attrs.getAttributeValue(null, "class");
        }
 
        // Apply a theme wrapper, if allowed and one is specified.
        if (!ignoreThemeAttr) {
            final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME);
            final int themeResId = ta.getResourceId(0, 0);
            if (themeResId != 0) {
                context = new ContextThemeWrapper(context, themeResId);
            }
            ta.recycle();
        }
 
        try {
            View view = tryCreateView(parent, name, context, attrs);
 
            if (view == null) {
                final Object lastContext = mConstructorArgs[0];
                mConstructorArgs[0] = context;
                try {
                    if (-1 == name.indexOf('.')) { //⭐重点
                        view = onCreateView(context, parent, name, attrs); //⭐重点
                    } else {
                        view = createView(context, name, null, attrs); //⭐重点
                    }
                } finally {
                    mConstructorArgs[0] = lastContext;
                }
            }
 
            return view;
        } catch (InflateException e) {
            throw e;
 
        } catch (ClassNotFoundException e) {
            final InflateException ie = new InflateException(
                    getParserStateDescription(context, attrs)
                    + ": Error inflating class " + name, e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
 
        } catch (Exception e) {
            final InflateException ie = new InflateException(
                    getParserStateDescription(context, attrs)
                    + ": Error inflating class " + name, e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
        }
    }

我们分析重点:

if (view == null) {
    final Object lastContext = mConstructorArgs[0];
    mConstructorArgs[0] = context;
    try {
        if (-1 == name.indexOf('.')) {
            view = onCreateView(context, parent, name, attrs);
        } else {
            view = createView(context, name, null, attrs);
        }
    } finally {
        mConstructorArgs[0] = lastContext;
    }
}

return view;

if (-1 == name.indexOf(‘.’))这个判断是标签是不是自定义View,带"."的是自定义View,如果不是自定义View,通过onCreateView创建View,如果是自定义View,通过createView创建View

所以我们比较一下为啥不是自定义View要通过onCreateView创建而是自定义VIew的用createView创建

先说一个知识点,LayoutInflater的实现类是PhoneLayoutInflater,onCreateView方法也被重写了:

    private static final String[] sClassPrefixList = {
        "android.widget.",
        "android.webkit.",
        "android.app."
    };   
 
 @Override protected View onCreateView(String name, AttributeSet attrs) throws ClassNotFoundException {
        for (String prefix : sClassPrefixList) {
            try {
                View view = createView(name, prefix, attrs);
                if (view != null) {
                    return view;
                }
            } catch (ClassNotFoundException e) {
                // In this case we want to let the base class take a crack
                // at it.
            }
        }
 
        return super.onCreateView(name, attrs);
    }
 
最后还调用了super:
    protected View onCreateView(String name, AttributeSet attrs)
            throws ClassNotFoundException {
        return createView(name, "android.view.", attrs);
    }

可以看到onCreateView最后还是调用了createVIew,所以onCreateView和createView主要的差别就是这个prefix的前缀,不是自定义View需要有前缀,想想LinearLayout这个类的全类名是"android.widget.LinearLayout",这下知道了这些SDK自带的不是自定义View标签都会在这里补全全类名,最后看一下createView

(2.2.4).createView方法

 static final Class<?>[] mConstructorSignature = new Class[] {
            Context.class, AttributeSet.class}; //View的两个参数
   
 public final View createView(@NonNull Context viewContext, @NonNull String name,
            @Nullable String prefix, @Nullable AttributeSet attrs)
            throws ClassNotFoundException, InflateException {
        Objects.requireNonNull(viewContext);
        Objects.requireNonNull(name);
        Constructor<? extends View> constructor = sConstructorMap.get(name);
        if (constructor != null && !verifyClassLoader(constructor)) {
            constructor = null;
            sConstructorMap.remove(name);
        }
        Class<? extends View> clazz = null;
 
        try {
            Trace.traceBegin(Trace.TRACE_TAG_VIEW, name);
 
            if (constructor == null) {//⭐主要就是通过反射去创建view
                // Class not found in the cache, see if it's real, and try to add it
                clazz = Class.forName(prefix != null ? (prefix + name) : name, false,
                        mContext.getClassLoader()).asSubclass(View.class);
 
                if (mFilter != null && clazz != null) {
                    boolean allowed = mFilter.onLoadClass(clazz);
                    if (!allowed) {
                        failNotAllowed(name, prefix, viewContext, attrs);
                    }
                }
                constructor = clazz.getConstructor(mConstructorSignature);//通过反射获取两个参数的构造方法,两个参数分别是context与AttributeSet(XML的参数集合)
                constructor.setAccessible(true);
                sConstructorMap.put(name, constructor); //用map缓存起来
            } else { //constructor不为null说明map里面有直接用map里面的
                // If we have a filter, apply it to cached constructor
                if (mFilter != null) {
                    // Have we seen this name before?
                    Boolean allowedState = mFilterMap.get(name);
                    if (allowedState == null) {
                        // New class -- remember whether it is allowed
                        clazz = Class.forName(prefix != null ? (prefix + name) : name, false,
                                mContext.getClassLoader()).asSubclass(View.class);
 
                        boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
                        mFilterMap.put(name, allowed);
                        if (!allowed) {
                            failNotAllowed(name, prefix, viewContext, attrs);
                        }
                    } else if (allowedState.equals(Boolean.FALSE)) {
                        failNotAllowed(name, prefix, viewContext, attrs);
                    }
                }
            }
 
            Object lastContext = mConstructorArgs[0];
            mConstructorArgs[0] = viewContext;
            Object[] args = mConstructorArgs;
            args[1] = attrs;
 
            try {
                final View view = constructor.newInstance(args);
                if (view instanceof ViewStub) {
                    // Use the same context when inflating ViewStub later.
                    final ViewStub viewStub = (ViewStub) view;
                    viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
                }
                return view;
            } finally {
                mConstructorArgs[0] = lastContext;
            }
        } catch (NoSuchMethodException e) {
            final InflateException ie = new InflateException(
                    getParserStateDescription(viewContext, attrs)
                    + ": Error inflating class " + (prefix != null ? (prefix + name) : name), e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
 
        } catch (ClassCastException e) {
            // If loaded class is not a View subclass
            final InflateException ie = new InflateException(
                    getParserStateDescription(viewContext, attrs)
                    + ": Class is not a View " + (prefix != null ? (prefix + name) : name), e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
        } catch (ClassNotFoundException e) {
            // If loadClass fails, we should propagate the exception.
            throw e;
        } catch (Exception e) {
            final InflateException ie = new InflateException(
                    getParserStateDescription(viewContext, attrs) + ": Error inflating class "
                            + (clazz == null ? "<unknown>" : clazz.getName()), e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_VIEW);
        }
    }

这么一大堆代码其实就是通过反射去创建View,注意里面的注释

b.再分析AppCompatActivity的createViewFromTag创建view的流程

    private View createViewFromTag(View parent, String name, Context context, AttributeSet attrs) {
        return createViewFromTag(parent, name, context, attrs, false);
    }
 
    @UnsupportedAppUsage
    View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
            boolean ignoreThemeAttr) {
        if (name.equals("view")) {
            name = attrs.getAttributeValue(null, "class");
        }
 
        // Apply a theme wrapper, if allowed and one is specified.
        if (!ignoreThemeAttr) {
            final TypedArray ta = context.obtainStyledAttributes(attrs, ATTRS_THEME);
            final int themeResId = ta.getResourceId(0, 0);
            if (themeResId != 0) {
                context = new ContextThemeWrapper(context, themeResId);
            }
            ta.recycle();
        }
 
        try {
            View view = tryCreateView(parent, name, context, attrs);//⭐重点
 
            if (view == null) {
                final Object lastContext = mConstructorArgs[0];
                mConstructorArgs[0] = context;
                try {
                    if (-1 == name.indexOf('.')) { 
                        view = onCreateView(context, parent, name, attrs);
                    } else {
                        view = createView(context, name, null, attrs); 
                    }
                } finally {
                    mConstructorArgs[0] = lastContext;
                }
            }
 
            return view;
        } catch (InflateException e) {
            throw e;
 
        } catch (ClassNotFoundException e) {
            final InflateException ie = new InflateException(
                    getParserStateDescription(context, attrs)
                    + ": Error inflating class " + name, e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
 
        } catch (Exception e) {
            final InflateException ie = new InflateException(
                    getParserStateDescription(context, attrs)
                    + ": Error inflating class " + name, e);
            ie.setStackTrace(EMPTY_STACK_TRACE);
            throw ie;
        }
    }

这个tryCreatView方法如果尝试创建View失败之后才轮到Activity的创建方式,我们看一下tryCreateView方法:

    public final View tryCreateView(@Nullable View parent, @NonNull String name,
        @NonNull Context context,
        @NonNull AttributeSet attrs) {
        if (name.equals(TAG_1995)) {
            // Let's party like it's 1995!
            return new BlinkLayout(context, attrs);
        }
 
        View view;
        if (mFactory2 != null) {
            view = mFactory2.onCreateView(parent, name, context, attrs);
        } else if (mFactory != null) {
            view = mFactory.onCreateView(name, context, attrs);
        } else {
            view = null;
        }
 
        if (view == null && mPrivateFactory != null) {
            view = mPrivateFactory.onCreateView(parent, name, context, attrs);
        }
 
        return view;
    }

发现有几个变量不认识,mFactory2和mFactory还有mPrivateFactory,我们的AppCompatActivity就是使用的mFactory2,看AppCompatActivity的onCreate方法:

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        final AppCompatDelegate delegate = getDelegate();
        delegate.installViewFactory(); // ⭐
        delegate.onCreate(savedInstanceState);
        super.onCreate(savedInstanceState);
    }

我们继续跟踪这个方法:

    public void installViewFactory() {
        LayoutInflater layoutInflater = LayoutInflater.from(mContext);
        if (layoutInflater.getFactory() == null) {
            LayoutInflaterCompat.setFactory2(layoutInflater, this);
        } else {
            if (!(layoutInflater.getFactory2() instanceof AppCompatDelegateImpl)) {
                Log.i(TAG, "The Activity's LayoutInflater already has a Factory installed"
                        + " so we can not install AppCompat's");
            }
        }
    }
 
//Factory2是一个接口
    public interface Factory2 extends Factory {
        @Nullable
        View onCreateView(@Nullable View parent, @NonNull String name,
                @NonNull Context context, @NonNull AttributeSet attrs);
    }

//LayoutInflater实现了这个接口
class AppCompatDelegateImpl extends AppCompatDelegate
        implements MenuBuilder.Callback, LayoutInflater.Factory2 

这下看到了,AppCompatActivity的oncreate方法中调用installViewFactory方法,获取到layoutInflater对象,AppCompatDelegateImpl实现了Factory2的接口,

LayoutInflaterCompat.setFactory2(layoutInflater, this);

 public static void setFactory2(
            @NonNull LayoutInflater inflater, @NonNull LayoutInflater.Factory2 factory) {
        inflater.setFactory2(factory); // ⭐
 
        if (Build.VERSION.SDK_INT < 21) {
            final LayoutInflater.Factory f = inflater.getFactory();
            if (f instanceof LayoutInflater.Factory2) {
                // The merged factory is now set to getFactory(), but not getFactory2() (pre-v21).
                // We will now try and force set the merged factory to mFactory2
                forceSetFactory2(inflater, (LayoutInflater.Factory2) f);
            } else {
                // Else, we will force set the original wrapped Factory2
                forceSetFactory2(inflater, factory);
            }
        }
    }
 
 
//LayoutInflater类中的方法
    public void setFactory2(Factory2 factory) {
        if (mFactorySet) {
            throw new IllegalStateException("A factory has already been set on this LayoutInflater");
        }
        if (factory == null) {
            throw new NullPointerException("Given factory can not be null");
        }
        mFactorySet = true;
        if (mFactory == null) {
            mFactory = mFactory2 = factory;
        } else {
            mFactory = mFactory2 = new FactoryMerger(factory, factory, mFactory, mFactory2);
        }
    }

可以看到吧Factory2传进了infalter中,所以Inflater的对象中Factory2不是null了。

Factory2不是null了,在执行创建createViewFromTag方法的tryCreateView时:

    public final View tryCreateView(@Nullable View parent, @NonNull String name,
        @NonNull Context context,
        @NonNull AttributeSet attrs) {
        if (name.equals(TAG_1995)) {
            // Let's party like it's 1995!
            return new BlinkLayout(context, attrs);
        }
 
        View view;
        if (mFactory2 != null) {
            view = mFactory2.onCreateView(parent, name, context, attrs);
        } else if (mFactory != null) {
            view = mFactory.onCreateView(name, context, attrs);
        } else {
            view = null;
        }
 
        if (view == null && mPrivateFactory != null) {
            view = mPrivateFactory.onCreateView(parent, name, context, attrs);
        }
 
        return view;
    }

mFactory2不是null了,就会执行 view = mFactory2.onCreateView(parent, name, context, attrs);

我们知道,这个mFactory2其实就是AppCompatDelegateImpl的实例对象,这个设计挺巧妙和上面ensureDecor有点像,添加了一些逻辑,使对view的创建逻辑转移到了AppCompatDelegateImpl中,所以我们下面看AppCompatDelegateImpl的onCreateView方法:

    @Override
    public final View onCreateView(View parent, String name, Context context, AttributeSet attrs) {
        return createView(parent, name, context, attrs);
    }
 
    @Override
    public View createView(View parent, final String name, @NonNull Context context,
            @NonNull AttributeSet attrs) {
        if (mAppCompatViewInflater == null) {
            TypedArray a = mContext.obtainStyledAttributes(R.styleable.AppCompatTheme);
            String viewInflaterClassName =
                    a.getString(R.styleable.AppCompatTheme_viewInflaterClass);
            if ((viewInflaterClassName == null)
                    || AppCompatViewInflater.class.getName().equals(viewInflaterClassName)) {
                // Either default class name or set explicitly to null. In both cases
                // create the base inflater (no reflection)
                mAppCompatViewInflater = new AppCompatViewInflater();
            } else {
                try {
                    Class<?> viewInflaterClass = Class.forName(viewInflaterClassName);
                    mAppCompatViewInflater =
                            (AppCompatViewInflater) viewInflaterClass.getDeclaredConstructor()
                                    .newInstance();
                } catch (Throwable t) {
                    Log.i(TAG, "Failed to instantiate custom view inflater "
                            + viewInflaterClassName + ". Falling back to default.", t);
                    mAppCompatViewInflater = new AppCompatViewInflater();
                }
            }
        }
 
        boolean inheritContext = false;
        if (IS_PRE_LOLLIPOP) {
            inheritContext = (attrs instanceof XmlPullParser)
                    // If we have a XmlPullParser, we can detect where we are in the layout
                    ? ((XmlPullParser) attrs).getDepth() > 1
                    // Otherwise we have to use the old heuristic
                    : shouldInheritContext((ViewParent) parent);
        }
        //⭐
        return mAppCompatViewInflater.createView(parent, name, context, attrs, inheritContext,
                IS_PRE_LOLLIPOP, /* Only read android:theme pre-L (L+ handles this anyway) */
                true, /* Read read app:theme as a fallback at all times for legacy reasons */
                VectorEnabledTintResources.shouldBeUsed() /* Only tint wrap the context if enabled */
        );
    }

可以看到创建了一个mAppCompatViewInflater ,最后调用了mAppCompatViewInflater的createView方法:

    final View createView(View parent, final String name, @NonNull Context context,
            @NonNull AttributeSet attrs, boolean inheritContext,
            boolean readAndroidTheme, boolean readAppTheme, boolean wrapContext) {
        final Context originalContext = context;
 
        // We can emulate Lollipop's android:theme attribute propagating down the view hierarchy
        // by using the parent's context
        if (inheritContext && parent != null) {
            context = parent.getContext();
        }
        if (readAndroidTheme || readAppTheme) {
            // We then apply the theme on the context, if specified
            context = themifyContext(context, attrs, readAndroidTheme, readAppTheme);
        }
        if (wrapContext) {
            context = TintContextWrapper.wrap(context);
        }
 
        View view = null;
 
        // We need to 'inject' our tint aware Views in place of the standard framework versions
        switch (name) {
            case "TextView":
                view = createTextView(context, attrs);
                verifyNotNull(view, name);
                break;
            case "ImageView":
                view = createImageView(context, attrs);
                verifyNotNull(view, name);
                break;
            case "Button":
                view = createButton(context, attrs);
                verifyNotNull(view, name);
                break;
            case "EditText":
                view = createEditText(context, attrs);
                verifyNotNull(view, name);
                break;
            case "Spinner":
                view = createSpinner(context, attrs);
                verifyNotNull(view, name);
                break;
            case "ImageButton":
                view = createImageButton(context, attrs);
                verifyNotNull(view, name);
                break;
            case "CheckBox":
                view = createCheckBox(context, attrs);
                verifyNotNull(view, name);
                break;
            case "RadioButton":
                view = createRadioButton(context, attrs);
                verifyNotNull(view, name);
                break;
            case "CheckedTextView":
                view = createCheckedTextView(context, attrs);
                verifyNotNull(view, name);
                break;
            case "AutoCompleteTextView":
                view = createAutoCompleteTextView(context, attrs);
                verifyNotNull(view, name);
                break;
            case "MultiAutoCompleteTextView":
                view = createMultiAutoCompleteTextView(context, attrs);
                verifyNotNull(view, name);
                break;
            case "RatingBar":
                view = createRatingBar(context, attrs);
                verifyNotNull(view, name);
                break;
            case "SeekBar":
                view = createSeekBar(context, attrs);
                verifyNotNull(view, name);
                break;
            case "ToggleButton":
                view = createToggleButton(context, attrs);
                verifyNotNull(view, name);
                break;
            default:
                // The fallback that allows extending class to take over view inflation
                // for other tags. Note that we don't check that the result is not-null.
                // That allows the custom inflater path to fall back on the default one
                // later in this method.
                view = createView(context, name, attrs);
        }
 
        if (view == null && originalContext != context) {
            // If the original context does not equal our themed context, then we need to manually
            // inflate it using the name so that android:theme takes effect.
            view = createViewFromTag(context, name, attrs);
        }
 
        if (view != null) {
            // If we have created a view, check its android:onClick
            checkOnClickListener(view, attrs);
        }
 
        return view;
    }

可以看到有一个switch,如果View是case中的一种,那就把布局参数传进去重新创建一个AppCompat的View,如果不是case中的一种,进default,这个createView方法直接返回null,

protected View createView(Context context, String name, AttributeSet attrs) {
return null;
}
然后走下面这句:

if (view == null && originalContext != context) {
    // If the original context does not equal our themed context, then we need to manually
    // inflate it using the name so that android:theme takes effect.
    view = createViewFromTag(context, name, attrs);
}
    private View createViewFromTag(Context context, String name, AttributeSet attrs) {
        if (name.equals("view")) {
            name = attrs.getAttributeValue(null, "class");
        }
 
        try {
            mConstructorArgs[0] = context;
            mConstructorArgs[1] = attrs;
 
            if (-1 == name.indexOf('.')) {
                for (int i = 0; i < sClassPrefixList.length; i++) {
                    final View view = createViewByPrefix(context, name, sClassPrefixList[i]);
                    if (view != null) {
                        return view;
                    }
                }
                return null;
            } else {
                return createViewByPrefix(context, name, null);
            }
        } catch (Exception e) {
            // We do not want to catch these, lets return null and let the actual LayoutInflater
            // try
            return null;
        } finally {
            // Don't retain references on context.
            mConstructorArgs[0] = null;
            mConstructorArgs[1] = null;
        }
    }
 
    private View createViewByPrefix(Context context, String name, String prefix)
            throws ClassNotFoundException, InflateException {
        Constructor<? extends View> constructor = sConstructorMap.get(name);
 
        try {
            if (constructor == null) {
                // Class not found in the cache, see if it's real, and try to add it
                Class<? extends View> clazz = Class.forName(
                        prefix != null ? (prefix + name) : name,
                        false,
                        context.getClassLoader()).asSubclass(View.class);
 
                constructor = clazz.getConstructor(sConstructorSignature);
                sConstructorMap.put(name, constructor);
            }
            constructor.setAccessible(true);
            return constructor.newInstance(mConstructorArgs);
        } catch (Exception e) {
            // We do not want to catch these, lets return null and let the actual LayoutInflater
            // try
            return null;
        }
    }

这个方法和Activity的逻辑一致了

总结一下:AppCompatActivity通过LayoutInfater解析创建View时,会通过setFactory2拦截原有Activity的逻辑,去执行AppCompatDelegateImpl的逻辑,在View解析和创建的时候,会先检查,如果是AppCompat新设计的View就是case里的那一堆,就把这个View转换成AppCompat新设计的View,如果不是还是按照之前的逻辑来。

三、总结

1.setContentView的总结

setContentView总的来说就是创建DecorView,DecorView是一个FrameLayout,然后根据style选择系统自带的布局文件,(例如有没有title,这里说一下这个布局文件根布局是linearLayout,如果有title则是有一个viewStub和两个FrameLayout:title的和content的,如果没有title则是一个viewstub和一个content的FrameLayout),添加到DecorView上,最后再把我们自己的的activity_mian这种layout添加到content这个FrameLyout上。

注意Activity和AppCompatActivity有一些区别,但总体上的逻辑是不变的。

2.inflate的总结

总结了inflate三个参数的作用,总结了解析到include、merge等标签时的注意点,总结了如何解析xml文件和如何创建View的重要流程。文章来源地址https://www.toymoban.com/news/detail-798947.html

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