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Swift语法(1)- Mirror

Swift语法
字数统计: 924阅读时长: 5 min
2021/04/22 Share

Mirror 反射的用法

Mirror 是 swift 中的反射机制, 可以动态的获取类型以及成员信息, 同时也可以在运行时动态的调用方法和属性等.

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// **** Mirror ****
// 

class Person {
	var name: String = "yangqing"
	var age: Int = 27
	var height: Double = 1.70
}

var p = Person()
var mirror = Mirror(reflecting: p.self)

print("对象类型: \(mirror.subjectType)")
print("对象属性个数: \(mirror.children.count)")
print("对象属性及属性值")

for child in mirror.children {
	print("\(child.label!) ---- \(child.value)")
}

// **** 可以看到属性名和值都已经正常打印 ****


// 其他用法

import UIKit

// **** 开发中使用到 Mirror ****

// **** 1. 对象转换为字典 ****
class Animal {
	var name: String?
	var color: String?
	private var birthday: Date = Date(timeIntervalSince1970: 0)
}


class Cat: Animal {
	var master = "小黑"
	var like: [String] = ["mouse", "fish"]
	override init() {
		super.init()
		color = "黄色"
	}
	
	func mapDic(mirror: Mirror) -> [String : Any] {
		var dic: [String : Any] = [:]
		for child in mirror.children {
			// 如果没有 label 就被抛弃
			if let label = child.label {
				let propertyMirror = Mirror(reflecting: child.value)
				print(propertyMirror)
				dic[label] = child.value
			}
		}
		// 添加父类属性
		if let superMirror = mirror.superclassMirror {
			let superDic = mapDic(mirror: superMirror)
			for p in superDic {
				dic[p.key] = p.value
			}
		}
		return dic
	}
	
}

// Mirror 使用
let cat = Cat()
cat.name = "大橘为重"
let mirror = Mirror(reflecting: cat)
let mirrorDic = cat.mapDic(mirror: mirror)
print(mirrorDic)




// **** 转 JSON ****
struct Address {
	var street: String
}

struct Person: CustomJSONProtocol {
	var name: String = "yangqing"
	var age: Int = 27
	var isMale: Bool = true
	var address: Address? = Address(street: "xizhimen North")
	var height = 1.70
	var like: Array = ["eat", "sleep", "play"]
	var weight: Float = 75
	var some: Int?
}

// ***** 创建一个person对象 *****
let p = Person()

protocol CustomJSONProtocol {
	func toJSON() throws -> Any?
}

enum JSONMapError: Error {
	case emptyKey
	case notConformProtocol
}

extension JSONMapError: LocalizedError {
	var errorDescription: String? {
		switch self {
			case .emptyKey:
				return "key 为空"
			case .notConformProtocol:
				return  "没遵守协议"
		}
	}
}

extension JSONMapError: CustomNSError {
	var errorCode: Int{
		switch self {
			case .emptyKey:
				return 100
			case .notConformProtocol:
				return 101
		}
	}
}

extension CustomJSONProtocol {

	func toJSON() throws -> Any? {

		let mirror = Mirror(reflecting: self)

		guard !mirror.children.isEmpty else { return self }

		var result: [String:Any] = [:]

		for children in mirror.children {
			if let value = children.value as? CustomJSONProtocol{
				if let key = children.label {
					print(key)
					result[key] = try value.toJSON()
				}else {
					throw JSONMapError.emptyKey
				}
			}else {
				throw JSONMapError.notConformProtocol
			}
		}
		return self
	}
}


let p2 = Person()

do {
	print(try p.toJSON()!)
}catch {
	print(error.localizedDescription)
	print((error as? JSONMapError)?.errorCode)
}

struct Point {
	let x: Int, y: Int
}
let p3 = Point(x: 20, y: 30)
String(reflecting: p)

autoclosure

把一句表达式自动封装成闭包 closure, 这样有时候在语法上看起爱就会非常漂亮 . 

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// **** autoclosure ****
func logIfTrue(_ predicate: () -> Bool) {
	if predicate() {
		print("True")
	}
}

logIfTrue({ return 2 > 1 })
logIfTrue({ 2 > 1 })

func logIfTrue2(_ predicate: @autoclosure () -> Bool) {
	if predicate() {
		print("True")
	}
}

logIfTrue2(2 > 1)

associatedtype

关联类型为协议中某个类型提供了一个占位符名称, 其代表的实际类型在协议被遵循时才会被指定. 关联类型通过 associatedtype 关键字来指定.

定义一个协议时, 声明一个或者多个关联类型作为协议定义的一部分将会非常有用. 

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import UIKit

protocol Food { }
struct Meat: Food { }
struct Grass: Food { }

protocol Animal {
	associatedtype F: Food
	func eat(_ food: F)
}

struct Tiger: Animal {
	func eat(_ food: Meat) {
		print("eat \(food)")
	}
}

struct Sheep: Animal {
	func eat(_ food: Grass) {
		print("eat \(food)")
	}
}

func isDangerous<T: Animal>(animal: T) -> Bool {
	if animal is Tiger {
		return true
	} else {
		return false
	}
}

isDangerous(animal: Tiger())
isDangerous(animal: Sheep())


输出: 
true
false

associate object

扩展类添加实例变量不允许的

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class MyClass {
	
}

private var key: Void?

extension MyClass {
	var title: String? {
		get {
			return objc_getAssociatedObject(self, &key) as? String
		}
		set {
			objc_setAssociatedObject(self, &key, newValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
		}
	}
}


// **** test ****

func printTitle(_ input: MyClass) {
	if let title = input.title {
		print("Title: \(title)")
	}else {
		print("没有设置")
	}
}

let a = MyClass()
printTitle(a)

a.title = "Swifter.tips"
printTitle(a)

原文作者：杨庆

原文链接：http://www.yqblog.cn/2021/04/22/Swift%E8%AF%AD%E6%B3%95-1/

发表日期：April 22nd 2021, 6:24:22 pm

更新日期：April 19th 2022, 11:47:39 pm

版权声明：本文采用知识共享署名-非商业性使用 4.0 国际许可协议进行许可

CATALOG
  1. 1. Mirror 反射的用法
  2. 2. autoclosure
  3. 3. associatedtype
  4. 4. associate object
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