Swift Selector

首先我们来看一下下面这个方法： ``` func addTarget(_ target: Any?, action: Selector, for controlEvents: UIControlEvents) ``` 其中的Selector 源自 Objective-C，例如 SEL 类型，以及 @selector() 方法选择器。由于Swift语法体系不含有消息机制，所以这里只能借助Cocoa Framework本身的消息机制来了。而Swift中所引入的Select()也是为了迎合action这里的selector，Swift 中也兼容了这个概念。 ###Selector结构体 先来看一下Selector的源码： ``` public struct Selector : ExpressibleByStringLiteral { /// Create a selector from a string. public init(_ str: String) /// Create an instance initialized to `value`. public init(unicodeScalarLiteral value: String) /// Construct a selector from `value`. public init(extendedGraphemeClusterLiteral value: String) /// Create an instance initialized to `value`. public init(stringLiteral value: String) } extension Selector : Equatable, Hashable { /// The hash value. /// /// **Axiom:** `x == y` implies `x.hashValue == y.hashValue` /// /// - Note: the hash value is not guaranteed to be stable across /// different invocations of the same program. Do not persist the /// hash value across program runs. public var hashValue: Int { get } } extension Selector : CustomStringConvertible { /// A textual representation of `self`. public var description: String { get } } extension Selector : CustomReflectable { /// Returns a mirror that reflects `self`. public var customMirror: Mirror { get } } ``` 从中可以看出selector其实是一个结构体。它实现了字面量的协议，可以使用字符串直接定义selector。 ###Selector的几种初始化方法 ``` #第一种 aButton.addTarget(self, action: Selector("aButtonClick"), for: .touchUpInside) #第二种 bButton.addTarget(self, action: #selector(bButtonClick), for: .touchUpInside) ``` 若是 Swift 中的私有方法，则必须赋予其 Objective-C 的 runtime（运行时）。即在方法名前加上 `@objc`： ``` @IBOutlet weak var wgButton: UIButton! @IBOutlet weak var anotherWgButton: UIButton! override func viewDidLoad() { super.viewDidLoad() wgButton.addTarget(self, action: #selector(wgButtonClick), for: .touchUpInside) } @objc func wgButtonClick(_ button: UIButton) { let btnLabel = button.titleLabel?.text ?? "nil" print(btnLabel) print(#function) } ```