AsyncQueryHandler onQueryComplete Listener为null

Question

我在AsyncQueryHandler中使用getview(); getview()中的每个项都进行异步查询。

但有时，onqueryComplete()会返回null。

public class CustomAsyncHandler extends AsyncQueryHandler {

    private WeakReference<AsyncQueryListener> mListener;

    public interface AsyncQueryListener {
        void onQueryComplete(int token, Object cookie, Cursor cursor);
    }

    public CustomAsyncHandler(ContentResolver cr, AsyncQueryListener listener) {
        super(cr);
        mListener = new WeakReference<AsyncQueryListener>(listener);
    }

    public CustomAsyncHandler(ContentResolver cr) {
        super(cr);
    }

    /**
     * Assign the given {@link AsyncQueryListener} to receive query events from
     * asynchronous calls. Will replace any existing listener.
     */
    public void setQueryListener(AsyncQueryListener listener) {
        mListener = new WeakReference<AsyncQueryListener>(listener);
    }

    /** {@inheritDoc} */
    @Override
    protected void onQueryComplete(int token, Object cookie, Cursor cursor) {
        final AsyncQueryListener listener = mListener.get();
        if (listener != null) {
            listener.onQueryComplete(token, cookie, cursor);
        } else if (cursor != null) {
            cursor.close();
        }
    }

}

背后的原因是什么？