iOS UIView subclass, draw see-through text to background

Disclaimer: I’m writing this without testing, so forgive me if I’m wrong here.

You should achieve what you need by these two steps:

1. Create a CATextLayer with the size of your view, set the backgroundColor to be fully transparent and foregroundColor to be opaque (by [UIColor colorWithWhite:0 alpha:1] and [UIColor colorWithWhite:0 alpha:0]. Then set the string property to the string you want to render, font and fontSize etc.

2. Set your view’s layer’s mask to this layer: myView.layer.mask = textLayer. You’ll have to import QuartzCore to access the CALayer of your view.

Note that it’s possible that I switched between the opaque and transparent color in the first step.

Edit: Indeed, Noah was right. To overcome this, I used CoreGraphics with the kCGBlendModeDestinationOut blend mode.

First, a sample view that shows that it indeed works:

@implementation TestView

- (id)initWithFrame:(CGRect)frame {
  if (self = [super initWithFrame:frame]) {
    self.backgroundColor = [UIColor clearColor];
  }
  return self;
}

- (void)drawRect:(CGRect)rect {
  [[UIColor redColor] setFill];
  UIBezierPath *path = [UIBezierPath bezierPathWithRoundedRect:self.bounds cornerRadius:10];
  [path fill];

  CGContextRef context = UIGraphicsGetCurrentContext();
  CGContextSaveGState(context); {
    CGContextSetBlendMode(context, kCGBlendModeDestinationOut);
    [@"Hello!" drawAtPoint:CGPointZero withFont:[UIFont systemFontOfSize:24]];
  } CGContextRestoreGState(context);
}

@end

After adding this to your view controller, you’ll see the view behind TestView where Hello! is drawn.

Why does this work:

The blend mode is defined as R = D*(1 - Sa), meaning we need opposite alpha values than in the mask layer I suggested earlier. Therefore, all you need to do is to draw with an opaque color and this will be subtracted from the stuff you’ve drawn on the view beforehand.