我有以下代码可以使用，但我只是不明白它是如何工作的，开始参考以下链接
http://developer.android.com/reference/android/os/Message.html#sendToTarget%28%29
它明确指出 sendToTarget() 通过 getTarget() 向指定的 Handler 发送消息，否则会抛出 NPE。在下面的代码中，我没有看到 getTarget() 的实现，但它工作正常，请解释它是如何工作的，在此处查找 queueThumbnail() 函数..在我不理解它是如何工作的确切位置的评论中指定
public class ThumbnailDownloader<Token> extends HandlerThread {
private static final String TAG = "ThumbNailDownloader";
private static final int MESSAGE_DOWNLOAD = 0;
Handler mHandler;
Map <Token, String> requestMap = Collections.synchronizedMap(new HashMap<Token,String>());
public ThumbnailDownloader(Handler responseHandler)
{
super(TAG);
}
@SuppressLint("HandlerLeak")
@Override
protected void onLooperPrepared()
{
mHandler = new Handler(){
@Override
public void handleMessage(Message msg)
{
if(msg.what==MESSAGE_DOWNLOAD)
{
@SuppressWarnings("unchecked")
Token token = (Token)msg.obj;
Log.i(TAG, "Got a request for Url:"+requestMap.get(token));
handleRequest(token);
}
}
};
}
public void handleRequest(final Token token)
{
try
{
final String url = requestMap.get(token);
if(url == null) return;
byte[] bitmapBytes = new FlickrFetchr().getUrlBytes(url);
final Bitmap bitmap = BitmapFactory.decodeByteArray(bitmapBytes, 0, bitmapBytes.length);
Log.i(TAG, "Bitmap Created");
mResponseHandler.post(new Runnable(){
public void run()
{
if(requestMap.get(token)!=url) return;
requestMap.remove(token);
mListener.onThumbnailDownloaded(token, bitmap);
}
});
}
catch(IOException ioe)
{
Log.e(TAG, "Error Downloading Image"+ioe);
}
}
public void queueThumbnail(Token token, String url)
{
Log.i(TAG, "Got an URL: "+url);
requestMap.put(token, url);
mHandler.obtainMessage(MESSAGE_DOWNLOAD, token).sendToTarget();
//some other class calls this function
//here the target is not set, but I see the request is handled well
//how is this possible? getTarget() is not set anywhere is here
}