GLES20RecordingCanvas drawBitmap NullPointerException

Question

我正在尝试定期执行onDraw，例如每30ms执行一次。

private Canvas canvas;
private final long updateInterval = 30;
private long curDeadline;

@Override
protected void onDraw(Canvas canvas)
{
    this.canvas = canvas;
    long now = System.currentTimeMillis();
    if (now < curDeadline) //too early, need to wait
    {
        long delay = curDeadline - now; //e.g. start drawing 5ms later
        handler.postDelayed(runDoDraw, delay);
    }
    else
        doDraw(); //execute immediately
}   

private void doDraw()
{
    long now = System.currentTimeMillis();      
    curDeadline = now+updateInterval;

    canvas.drawBitmap(bmp, 0, 0, null);
    invalidate();
}

private Runnable runDoDraw = new Runnable()
{       
    public void run()
    {
        doDraw();
    }
};

但它会抛出

 java.lang.NullPointerException
    at android.view.GLES20RecordingCanvas.drawBitmap(GLES20RecordingCanvas.java:105)
    at MyGame.doDraw(MyGame.java:963)
    at MyGame.access$1(MyGame.java:945)
    at MyGame$2.run(MyGame.java:1007)
    at android.os.Handler.handleCallback(Handler.java:725)
    at android.os.Handler.dispatchMessage(Handler.java:92)
    at android.os.Looper.loop(Looper.java:175)
    at android.app.ActivityThread.main(ActivityThread.java:5279)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:511)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
    at dalvik.system.NativeStart.main(Native Method)

在source code中，它在第105行说明

super.drawBitmap(bitmap, src, dst, paint);

看起来像是一个错误的方法正在调用drawBitmap。 这是Android的错误吗？