Multiple canvases in a view

Question

I have overridden the onDraw() method as follows:

public void onDraw(Canvas canvas1){
Canvas canvas2 = new Canvas();

Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.graphic1);
Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.graphic2);

canvas1.drawBitmap(
          top,
          new Rect(0, 0, graphic1.getWidth(), graphic1.getHeight()),
          new Rect(0, 0, width, width),
          null);

canvas2.drawBitmap(
          top,
          new Rect(0, 0, graphic2.getWidth(), graphic2.getHeight()),
          new Rect(0, 0, width, width),
          null);

}

Only graphic1 on canvas1 gets displayed, canvas2 and graphic2 do not. How can i get multiple canvases to be displayed on a single view?

Answer 1

As the comment said you're not attaching Canvas2 to anything. You're creating it each frame (which is bad), drawing to it and then letting it go our of scope to be garbage collected. What you should do it create Canvas2 with a backing Bitmap in the constructor of your view and keep it as a member. Then you can draw to it and then blit its Bitmap to Canvas1. For example:

public MyCustomView(Context context)
{
    super(context);
    _canvas2 = new Canvas(_backingBitmap);
}

public void onDraw(Canvas canvas1)
{
Bitmap graphic1 = BitmapFactory.decodeResource(getResources(), R.drawable.graphic1);
Bitmap graphic2 = BitmapFactory.decodeResource(getResources(), R.drawable.graphic2);

canvas1.drawBitmap(
          top,
          new Rect(0, 0, graphic1.getWidth(), graphic1.getHeight()),
          new Rect(0, 0, width, width),
          null);

_canvas2.drawBitmap(
          top,
          new Rect(0, 0, graphic2.getWidth(), graphic2.getHeight()),
          new Rect(0, 0, width, width),
          null);

canvas1.drawBitmap(
          top,
          new Rect(0, 0, _backingBitmap.getWidth(), _backingBitmap.getHeight()),
          new Rect(0, 0, width, width),
          null);
}