SwiftUI mutating 是什么怎么用 （2020年教程）

众所周知，类是引用类型，而结构和枚举是值类型。这意味着类对象共享该对象的单个实例，并在传递给任何函数或新对象时传递相同的引用，而值类型是建立其副本并仅传递值的类型

若是咱们尝试更改类中的任何变量，那就很简单

class Employee {
    var name : String
    var teamName : String
init(name: String, teamName: String) {
        self.name = name
        self.teamName = teamName
    }
func changeTeam(newTeamName : String){
        self.teamName = newTeamName
    }
}
var emp1 = Employee(name : "Suneet", teamName:"Engineering")
print(emp1.teamName)    //Engineering
emp1.changeTeam(newTeamName : "Product")
print(emp1.teamName)    //Product

而若是您尝试在任何值类型中执行相同操做，则会向咱们显示编译错误.

struct Employee {
    var name : String
    var teamName : String
init(name: String, teamName: String) {
        self.name = name
        self.teamName = teamName
    }
func changeTeam(newTeamName : String){
        self.teamName = newTeamName
        //cannot assign to property: 'self' is immutable
    }
}

将会显示下面

cannot assign to property: 'self' is immutable

Struct 是值类型，咱们不可以直接修改属性。所以编译器会提示咱们作以下修改

mark method 'mutating' to make 'self' mutable

它明确指出，在值类型中向任何函数添加突变关键字能够使它们可以修改变量。在内部，当咱们尝试突变值类型时，它不会mutate其值，但它会mutate持有该值的变量。

struct Employee {
    var name : String
    var teamName : String
init(name: String, teamName: String) {
        self.name = name
        self.teamName = teamName
    }
mutating func changeTeam(newTeamName : String){
        self.teamName = newTeamName
    }
    
}
var emp1 = Employee(name : "Suneet", teamName:"Engineering")
print(emp1.teamName)    //Engineering
emp1.changeTeam(newTeamName : "Product")
print(emp1.teamName)    //Product

值类型

• enum
• struct
• Int
• Double
• String
• Array
• Dictionary
• Set
• Tuple

引用类型

• Functions
• Closures
• Class

参考文件

• https://medium.com/@agrawalsuneet/mutating-in-swift-7327d8a1cddd

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