(edited)
koher
extension Optional { // Produce the `default` value if `self` is nil. subscript(default value: Wrapped) -> Wrapped { get { return self ?? value } set { self = newValue } } // Act like optional chaining on read, while allowing "writes" that drop // the value on the floor if `self` is nil. subscript<T>(droppingWritesOnNil path: WritableKeyPath<Wrapped, T>) -> T? { get { return self?[keyPath: path] } set { if let newValue = newValue { self?[keyPath: path] = newValue } } } } struct Foo { var a: Int = 42 } struct Bar { var foo: Foo? = Foo() } var bar = Bar() bar[keyPath: \Bar.foo[droppingWritesOnNil: \.a]] = -1 print(bar.foo!.a) bar.foo = nil bar[keyPath: \Bar.foo[droppingWritesOnNil: \.a]] = 999 print(String(describing: bar.foo))
Version:
swift-4.1.1-RELEASE
Output:
-1 nil
Error: