Android: How to get a custom view to redraw partially?

Question

I have a custom view that fills my entire screen. (A piano keyboard) When a user touches the key, it causes invalidate() to be called and the whole keyboard get

Answer 1

Current nice workaround is to manually cache the full canvas to a bitmap:

 private void onDraw(Canvas canvas)
 {
     if (!initialDrawingIsPerformed)
     {
          this.cachedBitmap = Bitmap.createBitmap(getWidth(), getHeight(),
            Config.ARGB_8888); //Change to lower bitmap config if possible.
          Canvas cacheCanvas = new Canvas(this.cachedBitmap);
          doInitialDrawing(cacheCanvas);
          canvas.drawBitmap(this.cachedBitmap, 0, 0, new Paint());
          initialDrawingIsPerformed = true;
     }
     else
     {
          canvas.drawBitmap(this.cachedBitmap, 0, 0, new Paint());
          doPartialRedraws(canvas);
     }
 }

Ofcourse, you need to store the info about what to redraw yourself and preferably not use a new Paint everytime, but that are details.

Also note: Bitmaps are quite heavy on the memory usage of your app. I had crashes when I cached a View that was used with a scroller and that was like 5 times the height of the device, since it used > 10MB memory!

Answer 2

To complement Peterdk's answer, you could save your operations in a Picture instead of a Bitmap.

• A Bitmap will save all pixels, like he said it could take a lot of memory.
• A Picture will save the calls, like drawRect, drawLine, etc.

It depends of what is really heavy in your application : a lot of draw operations, a few draw operations but controlled by heavy calculations, a lot of blank/unused space (prefer Picture) etc...