Android JetPack 深入分析ViewModel源码

前言

本文主要分析ViewModel相关源码，相关使用不再赘述，可参考Android ViewModel使用；

ViewModel 概览

Google官方给的ViewModel定义如下：

ViewModel类旨在`以注重生命周期的方式存储和管理界面相关数据`。

ViewModel类`让数据可在发生屏幕旋转等配置更改后继续留存`。

定义主要提到两个关键点：

• 生命周期
  
  上图是Google官网提供的ViewModel生命周期图示，可以看到ViewModel的生命周期是从onCreate创建到完成并销毁Finished，开发中经常结合LiveData一起使用；

• 配置更改后数据留存
  我们知道当屏幕旋转的时候，Activity会销毁重建,如果我们想恢复之前的数据，之前一般是通过onSaveInstanceState()来实现，内部是通过Bundle来实现，此方法仅适合可以序列化再反序列化的少量数据，而不适合数量可能较大的数据，而ViewModel可以很简单的实现这个功能，并存储较大的数据；

源码分析

ViewModel是如何创建的？

 	val viewModel = ViewModelProvider(this).get(MyViewModel::class.java)

以上是ViewModel创建代码

ViewModelProvider(this)做了什么？

### ViewModelProvider.java
    public ViewModelProvider(@NonNull ViewModelStoreOwner owner) {
        this(owner.getViewModelStore(), owner instanceof HasDefaultViewModelProviderFactory
                ? ((HasDefaultViewModelProviderFactory) owner).getDefaultViewModelProviderFactory()
                : NewInstanceFactory.getInstance());
    }

ComponentActivity则实现了ViewModelStoreOwner和HasDefaultViewModelProviderFactory接口；
因此我们只需要关注ComponentActivity中对应的方法；

1. getViewModelStore()；
2. getDefaultViewModelProviderFactory()；

我们先分析getViewModelStore:

	### ComponentActivity.getViewModelStore
   public ViewModelStore getViewModelStore() {
		...
        ensureViewModelStore();
        return mViewModelStore;
    }

可以看到getViewModelStore()调用了ensureViewModelStore()，其对应代码如下：

### ComponentActivity.ensureViewModelStore
    void ensureViewModelStore() {
        if (mViewModelStore == null) {
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
                // Restore the ViewModelStore from NonConfigurationInstances
                mViewModelStore = nc.viewModelStore;
            }
            if (mViewModelStore == null) {
                mViewModelStore = new ViewModelStore();
            }
        }
    }
   static final class NonConfigurationInstances {
        Object custom;
        ViewModelStore viewModelStore;
    }

在ensureViewModelStore方法中，会判断mViewModelStore是否为null，如果为null的话，会先尝试调用getLastNonConfigurationInstance()方法获取，如果获取不到，则直接new创建一个ViewModelStore；

我们看下ViewModelStore类:

public class ViewModelStore {

    private final HashMap<String, ViewModel> mMap = new HashMap<>();

    final void put(String key, ViewModel viewModel) {
        ViewModel oldViewModel = mMap.put(key, viewModel);
        if (oldViewModel != null) {
            oldViewModel.onCleared();
        }
    }

    final ViewModel get(String key) {
        return mMap.get(key);
    }

    Set<String> keys() {
        return new HashSet<>(mMap.keySet());
    }

    /**
     *  Clears internal storage and notifies ViewModels that they are no longer used.
     */
    public final void clear() {
        for (ViewModel vm : mMap.values()) {
            vm.clear();
        }
        mMap.clear();
    }
}

其内部创建了一个HashMap，以ViewModel的CanonicalName为key，ViewModel对象为value，存储Activity中创建的ViewModel对象；

我们再看下getDefaultViewModelProviderFactory()方法：

### ComponentActivity.getDefaultViewModelProviderFactory()
    public ViewModelProvider.Factory getDefaultViewModelProviderFactory() {
		...
        if (mDefaultFactory == null) {
            mDefaultFactory = new SavedStateViewModelFactory(
                    getApplication(),
                    this,
                    getIntent() != null ? getIntent().getExtras() : null);
        }
        return mDefaultFactory;
    }

主要是创建一个SavedStateViewModelFactory对象返回，我们看下SavedStateViewModelFactory的构造方法

    public SavedStateViewModelFactory(@Nullable Application application,
            @NonNull SavedStateRegistryOwner owner,
            @Nullable Bundle defaultArgs) {
        mSavedStateRegistry = owner.getSavedStateRegistry();
        mLifecycle = owner.getLifecycle();
        mDefaultArgs = defaultArgs;
        mApplication = application;
        mFactory = application != null
                ? ViewModelProvider.AndroidViewModelFactory.getInstance(application)
                : ViewModelProvider.NewInstanceFactory.getInstance();
    }

我们看下mFactory的创建，application 肯定不为null，因此会调用ViewModelProvider.AndroidViewModelFactory.getInstance(application)返回一个AndroidViewModelFactory对象

    public static class AndroidViewModelFactory extends ViewModelProvider.NewInstanceFactory {

        private static AndroidViewModelFactory sInstance;


        public static AndroidViewModelFactory getInstance(@NonNull Application application) {
            if (sInstance == null) {
                sInstance = new AndroidViewModelFactory(application);
            }
            return sInstance;
        }

        private Application mApplication;

      
        public AndroidViewModelFactory(@NonNull Application application) {
            mApplication = application;
        }

        @NonNull
        @Override
        public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
            if (AndroidViewModel.class.isAssignableFrom(modelClass)) {
                try {
                    return modelClass.getConstructor(Application.class).newInstance(mApplication);
                } catch (NoSuchMethodException e) {
                    throw new RuntimeException("Cannot create an instance of " + modelClass, e);
                } catch (IllegalAccessException e) {
                    throw new RuntimeException("Cannot create an instance of " + modelClass, e);
                } catch (InstantiationException e) {
                    throw new RuntimeException("Cannot create an instance of " + modelClass, e);
                } catch (InvocationTargetException e) {
                    throw new RuntimeException("Cannot create an instance of " + modelClass, e);
                }
            }
            return super.create(modelClass);
        }
    }

可以看到AndroidViewModelFactory主要是用来生产ViewModel的，其create()方法：根据传入的ViewModel类型【继承自AndroidViewModel或ViewModel】，通过反射创建ViewModel对象；

小结

调用ViewModelProvider(this)方法主要做了以下两件事：

1. 获取ViewModelStore对象，如果ViewModelStore为null，会先尝试调用getLastNonConfigurationInstance()方法获取，获取不到则直接通过new创建，内部通过HashMap实现ViewModel对象的存储；

2. 创建AndroidViewModelFactory对象，其内部提供了create()，使用反射创建对应的ViewModel对象；

get(MyViewModel::class.java)做了什么？

	### ViewModelProvider.java
   public <T extends ViewModel> T get(@NonNull Class<T> modelClass) {
        String canonicalName = modelClass.getCanonicalName();
        if (canonicalName == null) {
            throw new IllegalArgumentException("Local and anonymous classes can not be ViewModels");
        }
        return get(DEFAULT_KEY + ":" + canonicalName, modelClass);
    }

    @NonNull
    @MainThread
    public <T extends ViewModel> T get(@NonNull String key, @NonNull Class<T> modelClass) {
        ViewModel viewModel = mViewModelStore.get(key);

        if (modelClass.isInstance(viewModel)) {
            if (mFactory instanceof OnRequeryFactory) {
                ((OnRequeryFactory) mFactory).onRequery(viewModel);
            }
            return (T) viewModel;
        } else {
            if (viewModel != null) {
            }
        }
        if (mFactory instanceof KeyedFactory) {
            viewModel = ((KeyedFactory) mFactory).create(key, modelClass);
        } else {
            viewModel = mFactory.create(modelClass);
        }
        mViewModelStore.put(key, viewModel);
        return (T) viewModel;
    }

小结

get(MyViewModel::class.java)方法会根据传入的ViewModel的CanonicalName，先从mViewModelStore缓存中查找，如果找到，则直接返回，如果没找到，则调用前面创建的mFactory.create()方法通过反射创建对应的ViewModel，并添加到 mViewModelStore缓存中；

以上便是ViewModel相关的创建逻辑源码分析！

ViewModel是如何实现配置更改后数据恢复的？

我们知道当屏幕旋转配置发生变化时，会调用onDestroy(),在ComponentActivity构造方法中：

   getLifecycle().addObserver(new LifecycleEventObserver() {
            @Override
            public void onStateChanged(@NonNull LifecycleOwner source,
                    @NonNull Lifecycle.Event event) {
                if (event == Lifecycle.Event.ON_DESTROY) {
                    // Clear out the available context
                    mContextAwareHelper.clearAvailableContext();
                    // And clear the ViewModelStore
                    if (!isChangingConfigurations()) {
                        getViewModelStore().clear();
                    }
                }
            }
        });

当监听到Activity调用ON_DESTROY时，会调用getViewModelStore().clear()，清除ViewModelStore内部缓存的ViewModel对象，那为何重新创建后，ViewModel中持有的数据又没有丢失呢？

当Activity销毁时会调用ActivityThread.handleDestroyActivity()方法：

  public void handleDestroyActivity(IBinder token, boolean finishing, int configChanges,
            boolean getNonConfigInstance, String reason) {
        ActivityClientRecord r = performDestroyActivity(token, finishing,
                configChanges, getNonConfigInstance, reason);
        if (r != null) {
         ...
        }
        mSomeActivitiesChanged = true;
    }

handleDestroyActivity方法中又会调用performDestroyActivity()方法：

  ActivityClientRecord performDestroyActivity(IBinder token, boolean finishing,
            int configChanges, boolean getNonConfigInstance, String reason) {
      		...
      		 ActivityClientRecord r = mActivities.get(token);
            if (getNonConfigInstance) {
                try {
                    r.lastNonConfigurationInstances
                            = r.activity.retainNonConfigurationInstances();
                } catch (Exception e) {
                    if (!mInstrumentation.onException(r.activity, e)) {
                        throw new RuntimeException(
                                "Unable to retain activity "
                                + r.intent.getComponent().toShortString()
                                + ": " + e.toString(), e);
                    }
                }
            }
     	...
        return r;
    }

performDestroyActivity方法中会调用activity.retainNonConfigurationInstances()

    NonConfigurationInstances retainNonConfigurationInstances() {
        Object activity = onRetainNonConfigurationInstance();
     
		...
        return nci;
    }

retainNonConfigurationInstances()方法中又调用onRetainNonConfigurationInstance(),其在ComponentActivity中具体实现代码如下：

    public final Object onRetainNonConfigurationInstance() {
        Object custom = onRetainCustomNonConfigurationInstance();
        ViewModelStore viewModelStore = mViewModelStore;
        if (viewModelStore == null) {
            NonConfigurationInstances nc =
                    (NonConfigurationInstances) getLastNonConfigurationInstance();
            if (nc != null) {
                viewModelStore = nc.viewModelStore;
            }
        }

        if (viewModelStore == null && custom == null) {
            return null;
        }

        NonConfigurationInstances nci = new NonConfigurationInstances();
        nci.custom = custom;
        nci.viewModelStore = viewModelStore;
        return nci;
    }

可以看到会将viewModelStore保存到NonConfigurationInstances对象里返回，结合performDestroyActivity源码，最终会保存到ActivityClientRecord.lastNonConfigurationInstances属性中；

当Activity重新创建的时候，会执行ActivityThread.performLaunchActivity()方法：

    private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
       			...
                activity.attach(appContext, this, getInstrumentation(), r.token,
                        r.ident, app, r.intent, r.activityInfo, title, r.parent,
                        r.embeddedID, r.lastNonConfigurationInstances, config,
                        r.referrer, r.voiceInteractor, window, r.configCallback,
                        r.assistToken);

 		...
        return activity;
    }

会调用 activity.attach()方法将lastNonConfigurationInstances对象传入,并赋值给Activity中的mLastNonConfigurationInstances属性；同时在ComponentActivity的构造方法中找到如下代码：

        getLifecycle().addObserver(new LifecycleEventObserver() {
            @Override
            public void onStateChanged(@NonNull LifecycleOwner source,
                    @NonNull Lifecycle.Event event) {
                ensureViewModelStore();
                getLifecycle().removeObserver(this);
            }
        });

而ensureViewModelStore()方法上面我们已经看到，内部会先调用getLastNonConfigurationInstance()方法获取mViewModelStore对象，getLastNonConfigurationInstance()方法返回的正是mLastNonConfigurationInstances属性；从而实现ViewModel数据的恢复！

整体时序图

结语

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