Swift 关键字 -- Extension

官方对于 extension 使用的解释Swift Extensions doc

1. 与 OC 分类相似，但是没有名字区别

Extensions are similar to categories in Objective-C. (Unlike Objective-C categories, Swift extensions don’t have names.)

2. 使用的几个场景

Extensions in Swift can:

Add computed instance properties and computed type properties
Define instance methods and type methods
Provide new initializers
Define subscripts
Define and use new nested types
Make an existing type conform to a protocol

2.1 添加计算属性

tips

Extensions can add new computed properties, but they can’t add stored properties, or add property observers to existing properties.
允许添加计算属性，不允许添加存储属性，或者添加已声明属性的观察服务。

/// 添加计算属性
extension Double {
    var m: Double {
        return self
    }
    var km: Double {
        return self / 1000
    }
    var cm: Double {
        return self * 1000
    }
}

let result = 50 * 1.0.cm + 10.m // 10.5.m

2.2 定义对象方法/常量方法

/// 添加方法

class ExtensionDemo {
    
}

/// 对象添加方法
extension ExtensionDemo {
    /// 添加静态方法
    static func staticMethod() -> Void {
        
    }
    /// 添加实例方法
    func instanceMethod() -> Void {
        
    }
}

/// 常量添加方法
extension Int {
    func repetitions(task: () -> Void) {
        for _ in 0.. Int {
        self = x * y
        return self
    }
}

func methodDemo() -> Void {
    ExtensionDemo.staticMethod()
    ExtensionDemo().instanceMethod()
    10.repetitions {
        print("1")
    }
}

2.3支持添加新的构造方法

/// 添加构造方法

extension Int {
    init(x: Double, y: Double) {
        self.init(x * y)
    }
}

2.4 支持 Subscripts

/// 支持 Subscripts

extension Int {
    subscript(index: Int) -> Int {
        return 0
    }
}

let index = 100[0]

2.5 Define and use new nested types

Extensions can add new nested types to existing classes, structures, and enumerations:

classes,
structures,
enumerations:

/// 支持嵌套类型

enum ExtensionEnumDemo {
    case first
    case second
}

/// 枚举支持内嵌对象

extension ExtensionEnumDemo {
    class ExtensionSub {
        
    }
}

/// 对象支持内嵌对象

extension ExtensionDemo {
    
    class ExtensionSub {
        
    }
}

/// 结构体支持内嵌对象

extension Int {
    
    class ExtensionSub {
        
    }
    
    enum Types {
        case first
        case second
    }
}

2.6 Make an existing type conform to a protocol 已有类型支持协议

/// 支持协议


extension UIViewController : UITableViewDelegate {
    
    func cellForRow(at indexPath: IndexPath) -> UITableViewCell? {
        return nil
    }
    
}