Android进阶：ListView性能优化异步加载图片 使滑动效果流畅
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); }}
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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
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??????? 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
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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; }}??
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????? 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
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ImageView imageViewByTag = (ImageView) listView.findViewWithTag(imageUrl); if (imageViewByTag != null) { imageViewByTag.setImageDrawable(imageDrawable); }?
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????? 这里通过ViewCatch来减少了 findViewById的使用
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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; }}??
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???? 总结 ：这里主要做了三点优化
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在单一线程里加载图片 ? 重用列表中行 缓存行中的 View