ListView性能[译]
时间：2011-04-11 来源：xirihanlin
原文路径：http://blog.jteam.nl/2009/09/17/exploring-the-world-of-android-part-2/
· ArrayAdapter<T> （显示数组对象，使用toString()来显示）
· SimpleAdapter （显示Maps列表）
· SimpleCursorAdapter（显示通过Cursor从DB中获取的信息）
public class ImageAndText {
private String imageUrl;
private String text;
public ImageAndText(String imageUrl, String text) {
this.imageUrl = imageUrl;
this.text = text;
}
public String getImageUrl() {
return imageUrl;
}
public String getText() {
return text;
}
}
public class ImageAndTextListAdapter extends ArrayAdapter<ImageAndText> {
public ImageAndTextListAdapter(Activity activity, List<ImageAndText> imageAndTexts) {
super(activity, 0, imageAndTexts);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
Activity activity = (Activity) getContext();
LayoutInflater inflater = activity.getLayoutInflater();
// Inflate the views from XML
View rowView = inflater.inflate(R.layout.image_and_text_row, null);
ImageAndText imageAndText = getItem(position);
// Load the image and set it on the ImageView
ImageView imageView = (ImageView) rowView.findViewById(R.id.image);
imageView.setImageDrawable(loadImageFromUrl(imageAndText.getImageUrl()));
// Set the text on the TextView
TextView textView = (TextView) rowView.findViewById(R.id.text);
textView.setText(imageAndText.getText());
return rowView;
}
public static Drawable loadImageFromUrl(String url) {
InputStream inputStream;
try {
inputStream = new URL(url).openStream();
} catch (IOException e) {
throw new RuntimeException(e);
}
return Drawable.createFromStream(inputStream, "src");
}
}
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="horizontal"
android:layout_width="fill_parent"
android:layout_height="wrap_content">
<ImageView android:id="@+id/image"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="@drawable/default_image"/>
<TextView android:id="@+id/text"
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>
</LinearLayout>
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);
}
}
· 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
ImageView imageView = (ImageView) rowView.findViewById(R.id.image);
imageView.setImageDrawable(loadImageFromUrl(imageAndText.getImageUrl()));
final ImageView imageView = (ImageView) rowView.findViewById(R.id.image);
Drawable cachedImage = asyncImageLoader.loadDrawable(imageAndText.getImageUrl(), new ImageCallback() {
public void imageLoaded(Drawable imageDrawable, String imageUrl) {
imageView.setImageDrawable(imageDrawable);
}
});
imageView.setImageDrawable(cachedImage);
· findViewById()的昂贵调用
· 每次都inflate XML
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;
}
}
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;
}
}
我已经向大家演示了3种改进ListView性能的方法：
· 在单一线程里加载图片
· 重用列表中行
· 缓存行中的View
xirihanlin 2011.04.06
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