裁剪时如何避免覆盖drawRect

Question

At the moment i'm overriding drawRect in a view in order to be able to clip a colored rectangle with a mask (using CGContextClipToMask inside drawRect ). Sometimes I change the color of this clipped rectangle. In this case drawRect gets called again, redrawing and clipping the rectangle with the new color. Now I don't want to change the color at once, but animate this. The problem is, that animations are not performed when overriding drawRect (drawRect only gets called one time and immediately).

Is there a way to perform this animation, maybe by subclassing the view, so that I still override drawRect in the superclass, but the animation is somehow performed through the subclass so that drawRect from the superclass gets performed multiple times during the animation?

Or is also possible not to override drawRect at all, and still be able to clip this rectangle with the mask somehow? drawRect looks something like this:

CGContextRef ctx = UIGraphicsGetCurrentContext();
CGImageRef maskImage = [[UIImage imageNamed:maskName] CGImage];
CGContextClipToMask(ctx, rect, maskImage);

CGContextSetFillColorWithColor(ctx, self.currentColor);
CGContextFillRect( ctx, rect );

Answer 1

One solution would be to have a timer that fires to redraw repeatedly over time whenever the color is changed. When the user picks a new color, star an NSTimer that fires, say 10x a second. When the timer fires, update the current color to be partway from the start to the end color, and invalidate the view. After you reach the final color, kill the timer.

Answer 2

I think it is better to use a CALayer to mask your view (don't forget to import QuartzCore):

maskLayer.contents = (id)[UIImage imageNamed:maskName].CGImage; // maskLayer is a CALayer
view.layer.mask = maskLayer;

Now assuming you have a method -(void)animate in your view controller, you have execute it multiple times using a timer, or using recursive calls to performSelector:withObject:afterDelay .