This project demonstrates how to use a CALayer to mask a UIView.
It defines a custom subclass of UIImageView, MaskableView.
The MaskableView class has a property maskLayer that contains a CALayer.
MaskableView implements an updateBounds() method property so that when the view's bounds change, it's view controller can notify it so it can update it's mask layer and rebuild it's sample mask image.
The MaskableView has a method installSampleMask which builds an image the same size as the image view, mostly filled with opaque black, but with a small rectangle in the center filled with black at an alpha of 0.5. The translucent center rectangle causes the image view to become partly transparent and show the view underneath.
The demo app installs a couple of subviews into the MaskableView, a sample image of Scampers, one of my dogs, and a UILabel. It also installs an image of a checkerboard under the MaskableView so that you can see the translucent parts more easily.
The MaskableView has properties circleRadius, maskDrawingAlpha, and drawingAction that it uses to let the user erase/un-erase the image by tapping on the view to update the mask.
It attaches a custom subclass of UIPanGestureRecognizer, a TouchDownPanGestureRecognizer, to itself, with an action of gestureRecognizerUpdate. The gestureRecognizerUpdate method takes the tap/drag location from the gesture recognizer and uses it to draw a circle onto the image mask that either decreases the image mask's alpha (to partly erase pixels) or increase the image mask's alpha (to make those pixels more opaque.) The custom TouchDownPanGestureRecognizer is a very minor change to a normal UIPanGestureRecognizer that begins sending events on the first touch event rather than waiting for the user to drag.
It also has a property circleCursorColor:UIColor. It defaults to clear. If you set circleCursorColor to a non-clear color the MaskableView draws a "circle cursor" at the current tap point to show the shape the user is drawing into. The demo app's view controller sets the circleCursorColor to a slightly translucent yellow.
Its mask drawing is crude, and only meant for demonstration purposes. It draws a series of discrete circles intstead of rendering a path into the mask based on the user's drag gesture. A better solution would be to connect the points from the gesture recognizer and use them to render a smoothed curve into the mask.
The app's screen looks like this:
