Question

I’ve made a simple whiteboard on my Android app, where the user can draw something. I want to add an Undo feature so that he will be able to go back to his previous action.

On my Touch Start event, I’ve added the following code which basically adds the current canvas to an ArrayList and create a new one to avoid same reference.

previousDrawing.add(this.canvas);
this.canvas = new Canvas(this.bitmap);

Then, in my undo method, I’ve added the following code:

if (previousDrawing.size() > 0)
{
     // Remove last
    this.canvas = previousDrawing.remove(previousDrawing.size() - 1);
    this.canvas.setBitmap(this.bitmap);
}

It doesn’t work at all. I mean, I’m able to draw on my canvas using this.canvas.drawPath(this.path, this.paint); but not to save and restore a previous canvas.

Could you help me to do that ?

Thanks in advance.

EDIT :
I’ve also tryied to use the saveLayer method. But when I save, I’m no longer able to draw on the canvas. Is this a normal behaviour ?

Answer 1

Try drawBitmap instead of setBitmap

this.canvas.drawBitmap(this.bitmap, 0, 0, null);

Edit

if (previousDrawing.size() > 0)
{
     // Remove last
    this.bitmap = previousDrawing.get(previousDrawing.size() - 1);
    this.canvas.drawBitmap(this.bitmap, 0, 0, null);
    previousDrawing.remove(previousDrawing.size() - 1);
}

Reference link