iOS: Drawing just portions of a UImage

Question

I am trying to draw just a custom portion of a UIImage (i.e.: I would like to reveal the portion of the UIImage user touches) and I am having reasonable results by using mask property of the layer.

Something like this on my UIView:

UIBezierPath *maskPath = [UIBezierPath bezierPath];
[maskPath setLineWidth:10.0];
[maskPath moveToPoint:CGPointMake(10.0, 10.0)];
[maskPath addLineToPoint:CGPointMake(100.0, 100.0)];
CAShapeLayer *shapeMaskLayer = [CAShapeLayer layer];
shapeMaskLayer.path = maskPath.CGPath;
[self.layer setMask:shapeMaskLayer];

Then, on drawRect:

- (void)drawRect:(CGRect)rect
{
    [img drawInRect:rect];
}

It works. I am just seeing the portion of the image defined by maskPath.

However, doesn't look that that's the best way of approaching this issue. So my question is: What would be the best way of drawing just a custom portion (it could be any shape) of an image in iOS SDK?.

Answer 1

One thing you could try is simply clipping instead of creating an extra layer. e.g.

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

    UIBezierPath *maskPath = [UIBezierPath bezierPath];
    [maskPath setLineWidth:10.0];
    [maskPath moveToPoint:CGPointMake(10.0, 10.0)];
    [maskPath addLineToPoint:CGPointMake(100.0, 100.0)];

    CGContextAddPath(ctx, maskPath.CGPath);
    CGContextClip(ctx)
    [img drawInRect:rect];
}