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.

Answer 2

You can use invalidate(); to call onDraw() and draw canvas depending on your drawing logic.

Example

public class ThumbnailImage extends android.support.v7.widget.AppCompatImageView {

    public static final int FALSE = 0, TRUE = 1, NOT_SET = 2;
    private int drawingStatus;     

    public ThumbnailImage(Context context) {
        super(context);
        init();
    }

    public ThumbnailImage(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public ThumbnailImage(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }

    private void init() {
        ...
        drawingStatus = NOT_SET;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        if (drawingStatus != NOT_SET) {
            if (drawingStatus == TRUE) {
               ...
            } else {
               ...
            }
        }
    }

    public void setDrawingStatus(int drawingStatus) {
        this.drawingStatus = drawingStatus;
        invalidate();
    }

}