UnsupportedException from Canvas.setBitmap(Bitmap)

Question

I'm creating an app that does drawings and such under the users control and let's them save it. The way I'm trying to achieve this is by using a custom Bitmap on which the canvas draws then saving the resulting Bitmap.

Everything is working as expected, until Canvas.setBitmap(Bitmap) is called.

I get the following error.

03-24 13:47:50.741: E/AndroidRuntime(27888): FATAL EXCEPTION: main
03-24 13:47:50.741: E/AndroidRuntime(27888): Process: example.imageeditor, PID: 27888
03-24 13:47:50.741: E/AndroidRuntime(27888): java.lang.UnsupportedOperationException
03-24 13:47:50.741: E/AndroidRuntime(27888):    at android.view.HardwareCanvas.setBitmap(HardwareCanvas.java:39)

Code which is throwing the exception:

protected void onDraw(Canvas canvas) {
    mResultImage=Bitmap.createBitmap(width,height,mOriginalImage.getConfig());
    canvas.setBitmap(mResultImage);
    canvas.save();

    if(mOriginalImage!=null)
        canvas.drawBitmap(mOriginalImage, width, height, paint);
    else
        canvas.drawText("Image loading...", width/2f-20, height/2, paint);

    canvas.drawText(text, x, y-20, paint);

    canvas.restore();
    super.onDraw(canvas);
}

The android.view.HardwareCanvas isn't even on the reference of android. But I was able to find some information about it. It seems that it's setBitmap(Bitmap) isn't written yet, and that's ok.

My question is why is the onDraw(Canvas) returning a HardwareCanvas class? It isn't even a super class for Canvas.

Bonus question: Any way around this?

Answer 1

If you want to draw on a Bitmap you should create a new Canvas passing the bitmap to it. You should not be allowed to change the target of the canvas your view should be drawn on. So simply create a new canvas with the bitmap and then draw the resulting bitmap on your canvas in the onDraw method.