Why does the Android maps compass demo use a delegate for the SmoothCanvas class?

Question

In the Android MapsDemo available in Eclipse for the Google API, they create an inner class SmoothCanvas in MapViewCompassDemo.java. Within this class the re-implement seemingly every method and reroute it to a delegate instance of Canvas.

static final class SmoothCanvas extends Canvas {
    Canvas delegate;

    private final Paint mSmooth = new Paint(Paint.FILTER_BITMAP_FLAG);

    public void setBitmap(Bitmap bitmap) {
        delegate.setBitmap(bitmap);
    }

    public void setViewport(int width, int height) {
        delegate.setViewport(width, height);
    }
    ...

What is the point of the delegate in this instance?

Answer 1

The key point of delegating in that instance is:

private final Paint mSmooth = new Paint(Paint.FILTER_BITMAP_FLAG);

FILTER_BITMAP_FLAG bit. Filtering affects the sampling of bitmaps when they are transformed. Filtering does not affect how the colors in the bitmap are converted into device pixels. That is dependent on dithering and xfermodes.

By activating that flag, drawing bitmaps will basically increase its performance. You will in the example that mSmoth is used in every drawBitmap call.

Answer 2

The value of delegate is passed in to dispatchDraw. The SmoothCanvas class is a wrapper around delegate. By delegating, the Canvas implementation passed to dispatchDraw does all the heavy lifting. The wrapper just allows the injection of the smoothed paint without implementing all the logic of Canvas.