How disposable are Canvas objects?

Question

Imagine I have a Bitmap and I need to draw on it. For that I need to wrap it in a Canvas.

Can I wrap it as follows:

Bitmap mBitmap;

public void drawDrawable(Drawable d){
    d.draw(new Canvas(mBitmap));
}

public void drawListOfPathsInRed(List<Path> list){
    Paint paint = new Paint();
    for(Path path : list)
        new Canvas(mBitmap).drawPath(path, paint);
}

How bad would that be? Is there any noticeable overhead to creating many Canvases or any other issue with it?

Answer 1

Canvas itself is not a huge deal, it's a pretty "light" object, but when we're talking about drawing, we really need every bit of performance we can get (more info here: Optimizing the view). To that end, you could move the Canvas creation outside your drawing method, and reuse it with the setBitmap method:

Bitmap mBitmap;
Canvas mCanvas = Canvas();
Paint mPaint = new Paint(); // You should also reuse your Paint object

public void drawDrawable(Drawable d) {
    mCanvas.setBitmap(mBitmap);
    d.draw(mCanvas);
}

public void drawListOfPathsInRed(List<Path> list){
    canvas.setBitmap(mBitmap)
    for (Path path : list) {
        canvas.drawPath(path, paint);
    }
}

To go a step further, you could also extract the call to mCanvas.setBitmap from the drawing step, by doing right after setting your mBitmap variable.