我的理解是set和get是针对计算属性的(没有来自存储属性的支持)

_{如果你是来自Objective-C,请记住命名约定已经改变.在Swift中,iVar或实例变量被命名为stored属性}

示例1(只读属性) - 带警告:

var test : Int {
    get {
        return test
    }
}

这将导致警告,因为这会导致递归函数调用(getter调用自身).在这种情况下的警告是"尝试在其自己的getter中修改'test'".

示例2.条件读/写 - 带警告

var test : Int {
    get {
        return test
    }
    set (aNewValue) {
        //I've contrived some condition on which this property can be set
        //(prevents same value being set)
        if (aNewValue != test) {
            test = aNewValue
        }
    }
}

类似的问题 - 你不能这样做,因为它递归调用setter.此外,请注意,此代码不会抱怨没有初始化者,因为没有存储属性可以初始化.

示例3.读/写计算属性 - 使用后备存储

这是一种允许条件设置实际存储属性的模式

//True model data
var _test : Int = 0

var test : Int {
    get {
        return _test
    }
    set (aNewValue) {
        //I've contrived some condition on which this property can be set
        if (aNewValue != test) {
            _test = aNewValue
        }
    }
}

注意实际数据称为_test(尽管它可以是任何数据或数据组合)注意还需要提供初始值(或者您需要使用init方法),因为_test实际上是一个实例变量

示例4.使用will和did set

//True model data
var _test : Int = 0 {

    //First this
    willSet {
        println("Old value is \(_test), new value is \(newValue)")
    }

    //value is set

    //Finaly this
    didSet {
        println("Old value is \(oldValue), new value is \(_test)")
    }
}

var test : Int {
    get {
        return _test
    }
    set (aNewValue) {
        //I've contrived some condition on which this property can be set
        if (aNewValue != test) {
            _test = aNewValue
        }
    }
}

在这里,我们看到willSet和didSet拦截了实际存储属性的变化.这对于发送通知,同步等非常有用...(参见下面的示例)

示例5.具体示例 - ViewController容器

//Underlying instance variable (would ideally be private)
var _childVC : UIViewController? {
    willSet {
        //REMOVE OLD VC
        println("Property will set")
        if (_childVC != nil) {
            _childVC!.willMoveToParentViewController(nil)
            self.setOverrideTraitCollection(nil, forChildViewController: _childVC)
            _childVC!.view.removeFromSuperview()
            _childVC!.removeFromParentViewController()
        }
        if (newValue) {
            self.addChildViewController(newValue)
        }

    }

    //I can't see a way to 'stop' the value being set to the same controller - hence the computed property

    didSet {
        //ADD NEW VC
        println("Property did set")
        if (_childVC) {
//                var views  = NSDictionaryOfVariableBindings(self.view)    .. NOT YET SUPPORTED (NSDictionary bridging not yet available)

            //Add subviews + constraints
            _childVC!.view.setTranslatesAutoresizingMaskIntoConstraints(false)       //For now - until I add my own constraints
            self.view.addSubview(_childVC!.view)
            let views = ["view" : _childVC!.view] as NSMutableDictionary
            let layoutOpts = NSLayoutFormatOptions(0)
            let lc1 : AnyObject[] = NSLayoutConstraint.constraintsWithVisualFormat("|[view]|",  options: layoutOpts, metrics: NSDictionary(), views: views)
            let lc2 : AnyObject[] = NSLayoutConstraint.constraintsWithVisualFormat("V:|[view]|", options: layoutOpts, metrics: NSDictionary(), views: views)
            self.view.addConstraints(lc1)
            self.view.addConstraints(lc2)

            //Forward messages to child
            _childVC!.didMoveToParentViewController(self)
        }
    }
}


//Computed property - this is the property that must be used to prevent setting the same value twice
//unless there is another way of doing this?
var childVC : UIViewController? {
    get {
        return _childVC
    }
    set(suggestedVC) {
        if (suggestedVC != _childVC) {
            _childVC = suggestedVC
        }
    }
}

注意使用BOTH计算和存储的属性.我已经使用了一个计算属性来防止设置两次相同的值(以避免发生坏事!); 我使用了willSet和didSet将通知转发给viewControllers(请参阅UIViewController文档和viewController容器上的信息)

我希望这会有所帮助,如果我在这里任何地方犯了错误,请有人大声喊叫!

这些被称为Property Observers:

财产观察员观察并回应财产价值的变化.每次设置属性值时都会调用属性观察者,即使新值与属性的当前值相同.

摘录自:Apple Inc."The Swift Programming Language."iBooks.https://itun.es/ca/jEUH0.l

我怀疑这是允许我们传统上使用KVO做的事情,例如与UI元素的数据绑定,或触发更改属性的副作用,触发同步过程,后台处理等等.

注意

willSet和didSet代表团发生前,没有当属性在初始化设置所谓的观察家

关键在于,有时候,您需要一个具有自动存储和某些行为的属性,例如通知其他对象该属性刚刚更改.当你拥有的是get/时set,你需要另一个字段来保存值.使用willSet和didSet,您可以在修改值时执行操作,而无需其他字段.例如,在该示例中:

class Foo {
    var myProperty: Int = 0 {
        didSet {
            print("The value of myProperty changed from \(oldValue) to \(myProperty)")
        }
    }
}

myProperty每次修改时都会打印其旧值和新值.只有吸气剂和二传手,我需要这样做:

class Foo {
    var myPropertyValue: Int = 0
    var myProperty: Int {
        get { return myPropertyValue }
        set {
            print("The value of myProperty changed from \(myPropertyValue) to \(newValue)")
            myPropertyValue = newValue
        }
    }
}

因此willSet,didSet代表了几行的经济,并且在现场列表中的噪音更少.

您还可以使用将didSet变量设置为其他值.这不会导致按照属性指南中的说明再次调用观察者.例如,当您想要将值限制如下时,它非常有用:

let minValue = 1

var value = 1 {
    didSet {
        if value < minValue {
            value = minValue
        }
    }
}

value = -10 // value is minValue now.

许多写得很好的现有答案很好地涵盖了这个问题,但我会详细地提到一个我认为值得报道的补充.

在willSet和didSet财产观察者可以用来打电话的代表,例如,对于通过用户交互永远只能更新类的属性,但要避免在调用对象的初始化委托.

我会引用Klaas对已接受的答案进行评论:

首次初始化属性时,不会调用willSet和didSet观察者.仅在属性的值设置在初始化上下文之外时才调用它们.

这是非常简洁的,因为它意味着例如didSet属性是委托回调和函数的启动点的良好选择,对于您自己的自定义类.

作为示例,考虑一些自定义用户控件对象,具有一些关键属性value(例如,在评级控件中的位置),实现为以下的子类UIView:

// CustomUserControl.swift
protocol CustomUserControlDelegate {
    func didChangeValue(value: Int)
    // func didChangeValue(newValue: Int, oldValue: Int)
    // func didChangeValue(customUserControl: CustomUserControl)
    // ... other more sophisticated delegate functions
}

class CustomUserControl: UIView {

    // Properties
    // ...
    private var value = 0 {
        didSet {
            // Possibly do something ...

            // Call delegate.
            delegate?.didChangeValue(value)
            // delegate?.didChangeValue(value, oldValue: oldValue)
            // delegate?.didChangeValue(self)
        }
    }

    var delegate: CustomUserControlDelegate?

    // Initialization
    required init?(...) { 
        // Initialise something ...

        // E.g. 'value = 1' would not call didSet at this point
    }

    // ... some methods/actions associated with your user control.
}

之后,您的委托函数可用于某些视图控制器,以观察模型中的关键更改CustomViewController,就像您使用UITextFieldDelegatefor UITextField对象的固有委托函数(例如textFieldDidEndEditing(...))一样.

对于这个简单的示例,使用来自didSet类属性的委托回调value来告诉视图控制器其中一个出口已经关联了模型更新:

// ViewController.swift
Import UIKit
// ...

class ViewController: UIViewController, CustomUserControlDelegate {

    // Properties
    // ...
    @IBOutlet weak var customUserControl: CustomUserControl!

    override func viewDidLoad() {
        super.viewDidLoad()
        // ...

        // Custom user control, handle through delegate callbacks.
        customUserControl = self
    }

    // ...

    // CustomUserControlDelegate
    func didChangeValue(value: Int) {
        // do some stuff with 'value' ...
    }

    // func didChangeValue(newValue: Int, oldValue: Int) {
        // do some stuff with new as well as old 'value' ...
        // custom transitions? :)
    //}

    //func didChangeValue(customUserControl: CustomUserControl) {
    //    // Do more advanced stuff ...
    //}
}

这里,value属性已被封装,但通常:在这种情况下,注意不要在视图控制器中更新相关委托函数(此处:)的范围内对象的value属性,否则最终会无限递归.customUserControldidChangeValue()