Android SurfaceView，OnDraw被调用但显示没有更新而没有invalidate（）

Question

I'm having a little trouble following a typical Android graphics flow. There are many examples but mine seems not to work. In my main activity I do this:

super.onCreate(savedInstanceState);                           
setContentView(R.layout.activity_main); 

In my resources file I have 3 custom classes that override SurfaceView. In my constructor for these Surface Views I have something like this:

super(context, attrs);
getHolder().addCallback(this);
this.thread = new MyThread(getHolder(),this);

In my thread's run method I do this (lifted from many examples I've seen):

Canvas c;
try {
    c = surfaceHolder.lockCanvas(null);
    synchronized (surfaceHolder) {
        panel.onDraw(c);
    }
} finally {
    if (c != null) {
    surfaceHolder.unlockCanvasAndPost(c);
}

This thread is started in my SurfaceView's 'surfaceCreated' event. In onDraw I do this:

public void onDraw(Canvas canvas) { 
    super.onDraw(canvas);
    <some drawing stuff here using canvas>
}

When I run, the thread does indeed run, and I can see that onDraw is getting called (I put log print command in there), however my graphics don't update as expected. If I call this.invalidate(), however, the graphics do update a handful of times as I expect until I crash with this:

E/AndroidRuntime(748): android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.

I realize according the examples I've seen I shouldn't need to call invalidate in OnDraw, but I did it just to verify that the code in my onDraw is valid. So, it seems to me that without the invalidate() the onDraw event is getting called, and my code should be updating the graphics properly, but the screen never updates. I'm missing something in how the main Activity actually decides that it needs to repaint it's children.

Any ideas would be greatly appreciated. I've spent a while trying to copy stuff from some working demos but I just seem to be stuck. Thanks

Answer 1

the stuff you do in run() does whatever you do - Once. You need to repeat the drawing process.

One way could be to put everything inside a while loop

while(true){
    try {
        c = surfaceHolder.lockCanvas(null);
        synchronized (surfaceHolder) {
            panel.onDraw(c);
        }
    } finally {
        if (c != null) {
        surfaceHolder.unlockCanvasAndPost(c);
    }
}

Answer 2

I already had the same problem. As I could not find any solution, I did a kind of workaround: Create a Bitmap as a buffer and draw to that bitmap. After drawing, draw the complete bitmap to the view itself. Declare these variables in your View class:

Canvas bmpCanvas;
Bitmap bmp;

And use it in your onDraw()-method like shown:

@Override
protected void onDraw(Canvas canvas) {

   if(bmp == null || bmpCanvas == null) {

       bmp = Bitmap.createBitmap(getWidth(), getHeight(), Bitmap.Config.ARGB_8888);
       bmpCanvas = new Canvas(bmp);
   }

   //do drawing-stuff here

   canvas.drawBitmap(bmp, 0, 0, p); //where p is a Paint-Object.
}

Now, to update the view/call this method use either invalidate or postInvalidate() invalidate of your view, depending on whether you call the method in the same thread or in an other thread.