ListView 是一种可以显示一系列项目并能进行滚动显示的 View,每一行的Item可能包含复杂的结构,可能会从网络上获取icon等的一些图标信息,就现在的网络速度要想保持ListView运行的很好滚动流畅是做不到的
所以这里就需要把这些信息利用多线程实现异步加载
实现这样功能的类
- public class AsyncImageLoader {
- private HashMap<String, SoftReference<Drawable>> imageCache;
- public AsyncImageLoader() {
- imageCache = new HashMap<String, SoftReference<Drawable>>();
- }
- public Drawable loadDrawable(final String imageUrl, final ImageCallback imageCallback) {
- if (imageCache.containsKey(imageUrl)) {
- SoftReference<Drawable> softReference = imageCache.get(imageUrl);
- Drawable drawable = softReference.get();
- if (drawable != null) {
- return drawable;
- }
- }
- final Handler handler = new Handler() {
- @Override
- public void handleMessage(Message message) {
- imageCallback.imageLoaded((Drawable) message.obj, imageUrl);
- }
- };
- new Thread() {
- @Override
- public void run() {
- Drawable drawable = loadImageFromUrl(imageUrl);
- imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));
- Message message = handler.obtainMessage(0, drawable);
- handler.sendMessage(message);
- }
- }.start();
- return null;
- }
- public static Drawable loadImageFromUrl(String url) {
- // ...
- }
- public interface ImageCallback {
- public void imageLoaded(Drawable imageDrawable, String imageUrl);
- }
- }
注意这里使用了 SoftReference来缓存图片,允许 GC在需要的时候可以对缓存中的图片进行清理。它这样工作:
· 调用 loadDrawable(ImageUrl, imageCallback),传入一个匿名实现的 ImageCallback接口
· 如果图片在缓存中不存在的话,图片将从单一的线程中下载并在下载结束时通过 ImageCallback回调
· 如果图片确实存在于缓存中,就会马上返回,不会回调 ImageCallback
然后我们还可以根据09google I/0开发者大会提到的方式来继续优化Adapter 使用ViewHolder来减少一些比较费时的操作,譬如inflate XML 和 findViewById()等操作
- public class ImageAndTextListAdapter extends ArrayAdapter<ImageAndText> {
- private ListView listView;
- private AsyncImageLoader asyncImageLoader;
- public ImageAndTextListAdapter(Activity activity, List<ImageAndText> imageAndTexts, ListView listView) {
- super(activity, 0, imageAndTexts);
- this.listView = listView;
- asyncImageLoader = new AsyncImageLoader();
- }
- @Override
- public View getView(int position, View convertView, ViewGroup parent) {
- Activity activity = (Activity) getContext();
- // Inflate the views from XML
- View rowView = convertView;
- ViewCache viewCache;
- if (rowView == null) {
- LayoutInflater inflater = activity.getLayoutInflater();
- rowView = inflater.inflate(R.layout.image_and_text_row, null);
- viewCache = new ViewCache(rowView);
- rowView.setTag(viewCache);
- } else {
- viewCache = (ViewCache) rowView.getTag();
- }
- ImageAndText imageAndText = getItem(position);
- // Load the image and set it on the ImageView
- String imageUrl = imageAndText.getImageUrl();
- ImageView imageView = viewCache.getImageView();
- imageView.setTag(imageUrl);
- Drawable cachedImage = asyncImageLoader.loadDrawable(imageUrl, new ImageCallback() {
- public void imageLoaded(Drawable imageDrawable, String imageUrl) {
- ImageView imageViewByTag = (ImageView) listView.findViewWithTag(imageUrl);
- if (imageViewByTag != null) {
- imageViewByTag.setImageDrawable(imageDrawable);
- }
- }
- });
- imageView.setImageDrawable(cachedImage);
- // Set the text on the TextView
- TextView textView = viewCache.getTextView();
- textView.setText(imageAndText.getText());
- return rowView;
- }
- }
这里我们没有加载完iamge之后直接设定到相应的ImageView上 ,而是通过Tag查找,这里我们重用的View 这里有个listView的引用来通过Tag查找 可见 CallBack的实现
- ImageView imageViewByTag = (ImageView) listView.findViewWithTag(imageUrl);
- if (imageViewByTag != null) {
- imageViewByTag.setImageDrawable(imageDrawable);
- }
这里通过ViewCatch来减少了 findViewById的使用文章来源:https://www.toymoban.com/news/detail-601213.html
- public class ViewCache {
- private View baseView;
- private TextView textView;
- private ImageView imageView;
- public ViewCache(View baseView) {
- this.baseView = baseView;
- }
- public TextView getTextView() {
- if (textView == null) {
- textView = (TextView) baseView.findViewById(R.id.text);
- }
- return titleView;
- }
- public ImageView getImageView() {
- if (imageView == null) {
- imageView = (ImageView) baseView.findViewById(R.id.image);
- }
- return imageView;
- }
- }
总结 :这里主要做了三点优化文章来源地址https://www.toymoban.com/news/detail-601213.html
- 在单一线程里加载图片
- 重用列表中行
- 缓存行中的 View
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