iPhone clear UIView alpha at touch location

Question

I want to present an image in a UIView , because I have to customize (void)drawRect:(CGRect)rect . I have to clear the view's alpha where the user "rubs" the screen, so the subview behind this view will become visible.
If I use a solid color it works fine, but when I try to draw an image and clear the view just where the user touched the screen, the image is not drawn. Can you help me, how should I draw the UIImage , to be drawn in the background and to become transparent in a particular rect , please?

This is my code which works with solid color (as it is now):

- (void)drawRect:(CGRect)rect {
    CGContextRef context = UIGraphicsGetCurrentContext();

    CGContextSaveGState(context);

    CGContextSetFillColorWithColor( context, [UIColor greenColor].CGColor );
    CGContextFillRect( context, rect );

    CGContextSetFillColorWithColor( context, [UIColor clearColor].CGColor );
    CGContextSetBlendMode(context, kCGBlendModeClear);
    CGContextFillRect(context, CGRectMake(lastTouch.x, lastTouch.y, 30, 30));

    CGContextRestoreGState(context);
}

Answer 1

There's a good control on github for that purpose already. check out:

https://github.com/akopanev/iOS-Scratch-n-See

I used it as a starting point for a "scratch game" app already. The delegate receives also the information how much of the screen/image is already cleared (ie "scratched away").

Good luck!

Answer 2

I'm not sure to understand your problem... but if the problem is that the drawRect is not called when the user touch the screen... try this:

put a

[self setNeedsDisplay];

in your touch event... a UIGestureRecognizer or overriding the UITouches methods