Question

I have a complex custom view – photo collage.

What is observed is whenever any UI interaction happens, the view is redrawn.

How can I avoid complete redrawing (for example, use a cached UI) of the view specially when I click the “back” button to go back to previous activity because that also causes redrawing of the view.

While exploring the API and web, I found a method – getDrawingCache() – but don’t know how to use it effectively.

How do I use it effectively?

I’ve had other issues with Custom Views that I outline here.

Answer 1

I found a better way than using getDrawingCache.

In the method onDraw, apart from drawing in the natural canvas, I also draw on an memory-only canvas.

Bitmap cacheBmp = Bitmap.Create(....);
Canvas cacheCanvas = new Canvas(cacheBmp);


void onDraw(Canvas c)
{
   if(updateDueToInteraction)
   {
     c.drawXXX(...);
     cacheCanvas.drawXXX(...);
   } else
   {
     c.drawBitmap(cacheBmp, 0, 0);
   }
}