How can I observe changes to opacity of a UIView (possibly via its CALayer property?)

Question

Title is the basic question. Trying to get notified when the opacity on a view(s layer) changes.

This is allowed...

addedView.addObserver(self, forKeyPath: #keyPath(isHidden), options: [.old, .new], context: nil)

But this doesn't compile...

addedView.layer.addObserver(self, forKeyPath: #keyPath(opacity), options: [.old, .new], context: nil)

Any ideas?

Answer 1

From the documentation:

The property name must be a reference to a property that is available in the Objective-C runtime. [...] For example:

// ...
let c = SomeClass(someProperty: 12)
let keyPath = #keyPath(SomeClass.someProperty)

if let value = c.value(forKey: keyPath) {
    print(value)
}

When you use a key-path string expression within a class, you can refer to a property of that class by writing just the property name, without the class name.

extension SomeClass {
    func getSomeKeyPath() -> String {
        return #keyPath(someProperty)
    }
}
print(keyPath == c.getSomeKeyPath())
// Prints "true"

So really, the property name in #keyPath(...) should be qualified in general. Unless the property is in the same class as the #keyPath expression, you should qualify the property name with its enclosing class name. This makes sense, because how else is the compiler going to know which isHidden property you mean?

#keyPath(isHidden) just so happens to work because the class in which you are writing this in also happens to have a isHidden property (probably a UIView subclass?).

You should do:

#keyPath(CALayer.opacity)

for opacity