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)
}