属性观察者 willSet 和 didSet；属性 getter 和 setter [英] Property observers willSet and didSet; Property getters and setters

问题描述

willSet - didSet 和 get - set 有什么区别，在里面使用这个属性?

What is the difference between willSet - didSet, and get - set, when working with this inside a property?

在我看来，它们都可以为属性设置值.何时以及为什么应该使用 willSet - didSet，以及何时使用 get - set?

From my point of view, both of them can set a value for a property. When, and why, should I use willSet - didSet, and when get - set?

我知道对于 willSet 和 didSet，结构如下所示:

I know that for willSet and didSet, the structure looks like this:

var variable1 : Int = 0 {
    didSet {
        println (variable1)
    }
    willSet(newValue) {
    ..
    }
}

var variable2: Int {
    get {
        return variable2
    }
    set (newValue){
    }
}

推荐答案

我何时以及为什么应该使用 willSet/didSet

When and why should I use willSet/didSet

• willSet 在值被存储之前被调用.
• didSet 在存储新值后立即调用.
• willSet is called just before the value is stored.
• didSet is called immediately after the new value is stored.

考虑您的输出示例:

var variable1 : Int = 0 {
        didSet{
            print("didSet called")
        }
        willSet(newValue){
            print("willSet called")
        }
    }

    print("we are going to add 3")

     variable1 = 3

    print("we added 3")

输出:

we are going to add 3
willSet called
didSet called
we added 3

它像前置/后置条件

另一方面，如果你想添加一个只读属性，你可以使用get:

On the other hand, you can use get if you want to add, for example, a read-only property:

var value : Int {
 get {
    return 34
 }
}

print(value)

value = 2 // error: cannot assign to a get-only property 'value'

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