【iOS印象】Swift 4.1 新特性

缺乏、安全感 2022-03-16 13:07 238阅读 0赞

整理译自： [What’s New in Swift 4.1][What_s New in Swift 4.1]

**语言特性提升**

*  条件一致性（[Conditional Conformance][]）
    
     *  Equatable Containers：当可选型、数组、字典等集合与容器类型中的元素遵循 Equatable 或 Hashable 协议，则其同样遵循这些协议。包括 Codable 协议。
     *  自定义容器或集合同样可应用泛型配置支持该特性

// Swfit 4.0
    
    // [String] to [Int?]
    let a = ["1","2","x"].map(Int.init)
    a == [1,2,nil]    // 期待值应该是 ‘true’，但是~!
    编译器报错:
    //Binary operator ‘==’ cannot be applied to two ‘[Int?]’ operands.
    --------------------------------------
    // Swift 4.1 新增特性默认实现
    extension Array: Equatable where Element: Equatable { 
        // implementation of == for Array
    }
    
    extension Optional: Equatable where Wrapped: Equatable {
        // implementation of == for Optional
    }
    a == [1,2,nil]                  // true
    a.contains(nil)                 // true
    [[1,2],[3,4],[]].index(of: [])  // 2复制代码

*  JSON Coder 支持驼峰命名与下划线命名转换
    
     *  .keyEncodingStrategy
     *  .keyDecodingStrategy
 *  Equatable 和 Hashable 的一致性：
    
     *  对所有属性都遵循 Equatable 或 Hashable 的 struct 和 enum 默认遵循该协议。
     *  **注意！class 类型不支持这个特性**。
     *  Comparable 协议？继续按业务逻辑自行实现吧。
 *  协议中的关联类型支持递归约束

protocol Phone {
        associatedtype Version
        associatedtype SmartPhone: Phone where SmartPhone.Version == Version, SmartPhone.SmartPhone == SmartPhone
    }复制代码

*  协议中使用 weak 与 unowned 修饰属性将收到编译警告（因为在协议中修饰其没有意义）

class Key {}
    class Pitch {}
    protocol Tune {
        unowned var key: Key { get set }
        weak var pitch: Pitch? { get set }
    }
    class Instrument: Tune {
        var key: Key
        var pitch: Pitch?
        init(key: Key, pitch: Pitch?) {
            self.key = key        
            self.pitch = pitch
        }
    }复制代码

*  typeOfCollection(\_:) 方法中，IndexDistance 改为 Int
 *  flatMap(\_:) 重命名为 compactMap(\_:)

**平台配置与编译参数更新**

*  编译时导入（Build Imports）

// Swift 4.0#
    if os(iOS) || os(tvOS)
        import UIKit
        print("UIKit is available on this platform.")
    #else    
        print("UIKit is not available on this platform.")
    #endif
    
    // Swift 4.1
    #if canImport(UIKit)
        print("UIKit is available if this is printed!")
    #endif复制代码

*  编译时判断目标环境（Target Environments）

// Swift 4.0
    #if (arch(i386) || arch(x86_64)) && (os(iOS) || os(tvOS) || os(watchOS))
        print("Testing in the simulator.")
    #else    
        print("Testing on the device.")
    #endif
    
    // Swift 4.1
    #if targetEnvironment(simulator)
        print("Testing in the simulator.")
    #endif复制代码

[What_s New in Swift 4.1]: https://link.juejin.im?target=https%3A%2F%2Fwww.raywenderlich.com%2F187826%2Fwhats-new-in-swift-4-1
[Conditional Conformance]: https://link.juejin.im?target=https%3A%2F%2Fswift.org%2Fblog%2Fconditional-conformance%2F