为什么我的所有听众都没有注册？

Question

public interface DownloadListener {
    public void onDownloaded();
}

public class DownloadManager {

    private static DownloadManager instance;

    private DownloadListener mDownloadListener;

    public static synchronized DownloadManager getInstance(){
        if(instance == null)
            instance = new DownloadManager();
        return instance;
    }


    private DownloadManager() {
        myHandler.sendEmptyMessageDelayed(29, 3 * 1000);
    }

    public void registerDownloadListener(DownloadListener downloadListener) {
        mDownloadListener = downloadListener;
    }

    Handler myHandler = new Handler(new Handler.Callback() {
        @Override
        public boolean handleMessage(Message msg) {
            if (msg.what == 29) {
                mDownloadListener.onDownloaded();
                return true;
            }
            return false;
        }
    });
}

public class I implements DownloadListener {

    public I() {
        DownloadManager.getInstance().registerDownloadListener(this);
    }

    @Override
    public void onDownloaded() {
        Log.e("TAG", "I onDownloaded");
    }
}

public class You implements DownloadListener {

    public You() {
        DownloadManager.getInstance().registerDownloadListener(this);
    }

    @Override
    public void onDownloaded() {
        Log.e("TAG", "You onDownloaded");
    }

}

public class PATTERNSActivity extends Activity implements DownloadListener {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        new I();
        new You();

        DownloadManager.getInstance().registerDownloadListener(this);
    }

    @Override
    public void onDownloaded() {
        Log.e("TAG","PATTERNSActivity onDownloaded");
    }

}

I am expecting to get:

I onDownloaded

You onDownloaded

PATTERNSActivity onDownloaded

But I am getting only:

PATTERNSActivity onDownloaded

What could it be the problem?

Answer 1

You keep registered downloaders in a single instance property:

// Last call's downloadListener wins.
public void registerDownloadListener(DownloadListener downloadListener) {
    mDownloadListener = downloadListener;
}

The last one registered is the activity's:

new I();   // First set singleton's property to an instance of I...
new You(); // ...then to an instance of You...
// ...then to the current instance.
DownloadManager.getInstance().registerDownloadListener(this);

---

Edit based on your comment.

public void registerDownloadListener(DownloadListener downloadListener) {
    mDownloadListeners.add(downloadListener);
}

...

public boolean handleMessage(Message msg) {
    if (msg.what != 29) {
        return false;
    }

    for (DownloadListener listener : mDownloadListeners) {
        listener.onDownloaded();
    }

    return true;
}

Answer 2

In your code, this gets executed by calling mDownloadListener.onDownloaded(); in the DownloadManager class.

@Override
public void onDownloaded() {
    Log.e("TAG","PATTERNSActivity onDownloaded");
}

In don't see why the onDownloaded methods of the I and YOU class should be executed, they're never called. Only the OnDownloaded method of your Listener is called.

Answer 3

For starters, I think you are not using a list. You just override the value so you will always get the last one:

public void registerDownloadListener(DownloadListener downloadListener) {
        mDownloadListener = downloadListener;
    }