Drawing on Canvas outside the onDraw() method

Question

Here's my OnDraw() method

void onDraw(Canvas canvas) {
    mCanvas = canvas;
    //invalidate();
    int x = 0;
    Iterator<Letter> it = mNextUpQueue.iterator();
    while(it.hasNext()){
        mCanvas.drawBitmap(it.next().getNext(), mNextUpCoordinates.get(x).x, mNextUpCoordinates.get(x).y, mPaint);
        mCanvas.drawBitmap(mAvailableLetters.get(x).getNotPressed(), mAvailableLettersCoordinates.get(x).x, mAvailableLettersCoordinates.get(x).y, mPaint);
        x++;
    }
}

I have set canvas to a global variable mCanvas. But if I try to paint on mCanvas from outside the onDraw() method I get an error. Is it because I'm doing something wrong or the canvas must always be used from within the onDraw method?

Answer 1

You mustn't take the reference to the Canvas passed, as it is only valid during the onDraw(Canvas) method call.

I recommend reading http://developer.android.com/guide/topics/graphics/2d-graphics.html#draw-with-canvas thoroughly, the possible ways are explained there quite well:

1. To the Canvas provided to the onDraw(Canvas) method, during this method call. This is done in the UI thread, so nothing complicated should be attempted here
2. To a Canvas you created yourself. This could be useful for preparing a bitmap in another thread and then drawing the bitmap to the Canvas given to onDraw(Canvas) method
3. Using a SurfaceView, and obtaining a Canvas object from it with lockCanvas() method.