CALayer面具无法正常工作

Question

I am trying to mask a UIView to an image using it's layer's mask property. I have seen countless examples that say, "it's just that easy". However, after quite a bit of tweaking, I cannot seem to reproduce the results described. Setting the layer mask only makes the view disappear. Here is the code that I am using:

- (void)setMaskImage:(UIImage *)maskImage
{
    _maskImage = maskImage;

    self.layer.mask.contents = (__bridge id)(_maskImage.CGImage);
}

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self != nil) {
        self.layer.mask = [CALayer layer];
    }

    return self;
}

- (void)setFrame:(CGRect)frame
{
    [super setFrame:frame];

    self.layer.mask.frame = self.layer.bounds;
}

And here is the image that I am trying to use to mask the view: http://cl.ly/0a300G2r133V

Answer 1

One possibility is that the system's not actually using setFrame: to set your view's geometry. It can use setCenter: and setBounds: . Another possibility is that the system is not setting your view's geometry at all, and it's only being set once, in the [super initWithFrame:frame] call, before you add the mask layer.

Anyway, instead of overriding setFrame: , you should override layoutSubviews :

- (void)layoutSubviews {
    [super layoutSubviews];
    self.layer.mask.frame = self.bounds;
}

Answer 2

The following works as expected:

- (void)setMaskImage:(UIImage *)maskImage
{
    if (_maskView == nil) {
        _maskView = [[UIImageView alloc] initWithImage:maskImage];
        _maskView.frame = self.bounds;
        self.layer.mask = _maskView.layer;
    } else {
        _maskView.image = maskImage;
    }
}

- (UIImage *)maskImage
{
    return _maskView.image;
}

- (void)setBounds:(CGRect)bounds
{
    [super setBounds:bounds];

    _maskView.frame = self.bounds;
}

I'm not sure why just using a plain CALayer wasn't working, but this adds the bonus of working well with stretchable images.

Answer 3

You can override the UIView drawRect() method for its custom appearance on the screen. Try to use following approach.

//code for drawRect()

CGContextRef context = UIGraphicsGetCurrentContext();
CGContextAddRect(context,ImageFrame);
CGContextClip(context);
CGContextClearRect(context,ImageFrame);
[super drawRect:rect];

It will make your view transparent in your imageFrame area.