如果我想为自己分配属性怎么办？

Question

如果我尝试运行以下代码:

photographer = photographer

我收到错误:

为自己分配属性.

---

我想将属性分配给自己以强制photographer didSet块运行.

这是一个真实的例子:在2013年冬季斯坦福iOS课程(13:20)的"16. Segues and Text Fields"讲座中,教授建议编写类似如下的代码:

@IBOutlet weak var photographerLabel: UILabel!

var photographer: Photographer? {
    didSet {
        self.title = photographer.name
        if isViewLoaded() { reload() }
    }
}

override func viewDidLoad() {
    super.viewDidLoad()
    reload()
}

func reload() {
    photographerLabel.text = photographer.name
}

注意:我做了以下更改:(1)代码从Objective-C切换到Swift; (2)因为它在Swift中,我使用didSet属性的块而不是setPhotographer:方法; (3)而不是self.view.window我使用,isViewLoaded因为前者在访问view财产时错误地强迫视图加载; (4)该reload()方法(仅)为了简单起见而更新标签,因为它更接近我的代码; (5)IBOutlet添加了摄影师标签以支持这个更简单的代码; (6)由于我使用的是Swift,isViewLoaded()因为性能原因,检查不再存在,现在需要防止崩溃,因为IBOutlet被定义为UILabel!不UILabel?所以在加载视图之前尝试访问它会使应用程序崩溃; 这在Objective-C中不是强制性的,因为它使用空对象模式.

我们调用reload两次的原因是因为我们不知道是否在创建视图之前或之后设置了属性.例如,用户可能首先设置属性,然后呈现视图控制器,或者它们可能呈现视图控制器,然后更新属性.

我喜欢这个属性对于何时加载视图是不可知的(最好不要对视图加载时间做任何假设),所以我想在我自己的代码中使用相同的模式(仅略微修改):

@IBOutlet weak var photographerLabel: UILabel?

var photographer: Photographer? {
    didSet {
        photographerLabel?.text = photographer.name
    }
}

override func viewDidLoad() {
    super.viewDidLoad()
    photographer = photographer
}

这里不是创建一个从两个地方调用的新方法,而只是想要didSet块中的代码.我想viewDidLoad强制didSet调用,所以我将属性分配给自己.不过,斯威夫特不允许我这样做.我怎么强迫didSet被召唤？

Answer 1

在Swift 3.1之前,您可以将属性分配name给自己:

name = (name)

但这现在给出了同样的错误:"为自己分配一个属性".

还有很多其他方法可以解决这个问题,包括引入一个临时变量:

let temp = name
name = temp

这太有趣了,不能分享.我相信社区可以提出更多方法来做到这一点,越疯狂就越好

class Test: NSObject {
    var name: String? {
        didSet {
            print("It was set")
        }
    }

    func testit() {
        // name = (name)    // No longer works with Swift 3.1 (bug SR-4464)
        // (name) = name    // No longer works with Swift 3.1
        // (name) = (name)  // No longer works with Swift 3.1
        (name = name)
        name = [name][0]
        name = [name].last!
        name = [name].first!
        name = [1:name][1]!
        name = name ?? nil
        name = nil ?? name
        name = name ?? name
        name = {name}()
        name = Optional(name)!
        name = ImplicitlyUnwrappedOptional(name)
        name = true ? name : name
        name = false ? name : name
        let temp = name; name = temp
        name = name as Any as? String
        name = (name,0).0
        name = (0,name).1
        setValue(name, forKey: "name") // requires class derive from NSObject
        name = Unmanaged.passUnretained(self).takeUnretainedValue().name
        name = unsafeBitCast(name, to: type(of: name))
        name = unsafeDowncast(self, to: type(of: self)).name
        perform(#selector(setter:name), with: name) // requires class derive from NSObject
        name = (self as Test).name
        unsafeBitCast(dlsym(dlopen("/usr/lib/libobjc.A.dylib",RTLD_NOW),"objc_msgSend"),to:(@convention(c)(Any?,Selector!,Any?)->Void).self)(self,#selector(setter:name),name) // requires class derive from NSObject
        unsafeBitCast(class_getMethodImplementation(type(of: self), #selector(setter:name)), to:(@convention(c)(Any?,Selector!,Any?)->Void).self)(self,#selector(setter:name),name) // requires class derive from NSObject
        unsafeBitCast(method(for: #selector(setter:name)),to:(@convention(c)(Any?,Selector,Any?)->Void).self)(self,#selector(setter:name),name) // requires class derive from NSObject 
        _ = UnsafeMutablePointer(&name)
        _ = UnsafeMutableRawPointer(&name)
        _ = UnsafeMutableBufferPointer(start: &name, count: 1)
        withUnsafePointer(to: &name) { name = $0.pointee }

        //Using NSInvocation, requires class derive from NSObject
        let invocation : NSObject = unsafeBitCast(method_getImplementation(class_getClassMethod(NSClassFromString("NSInvocation"), NSSelectorFromString("invocationWithMethodSignature:"))),to:(@convention(c)(AnyClass?,Selector,Any?)->Any).self)(NSClassFromString("NSInvocation"),NSSelectorFromString("invocationWithMethodSignature:"),unsafeBitCast(method(for: NSSelectorFromString("methodSignatureForSelector:"))!,to:(@convention(c)(Any?,Selector,Selector)->Any).self)(self,NSSelectorFromString("methodSignatureForSelector:"),#selector(setter:name))) as! NSObject
        unsafeBitCast(class_getMethodImplementation(NSClassFromString("NSInvocation"), NSSelectorFromString("setSelector:")),to:(@convention(c)(Any,Selector,Selector)->Void).self)(invocation,NSSelectorFromString("setSelector:"),#selector(setter:name))
        var localVarName = name
        withUnsafePointer(to: &localVarName) { unsafeBitCast(class_getMethodImplementation(NSClassFromString("NSInvocation"), NSSelectorFromString("setArgument:atIndex:")),to:(@convention(c)(Any,Selector,OpaquePointer,NSInteger)->Void).self)(invocation,NSSelectorFromString("setArgument:atIndex:"), OpaquePointer($0),2) }
        invocation.perform(NSSelectorFromString("invokeWithTarget:"), with: self)
    }
}

let test = Test()
test.testit()

Answer 2

有一些很好的解决方法,但这样做没什么意义.如果程序员(代码的未来维护者)看到这样的代码:

a = a

他们将删除它.

此类声明(或变通方法)不应出现在您的代码中.

如果您的财产看起来像这样:

var a: Int {
   didSet {
      // code
   }
}

那么didSet通过赋值来调用处理程序并不是一个好主意a = a.

如果未来的维护者在didSet这方面增加了性能改进怎么办？

var a: Int {
   didSet {
      guard a != oldValue else {
          return
      }

      // code
   }
}

真正的解决方案是重构:

var a: Int {
   didSet {
      self.updateA()
   }
}

fileprivate func updateA() {
   // the original code
}

而不是a = a直接打电话updateA().

如果我们谈论网点,一个合适的解决方案是在第一次分配之前强制加载视图:

@IBOutlet weak var photographerLabel: UILabel?

var photographer: Photographer? {
    didSet {
        _ = self.view // or self.loadViewIfNeeded() on iOS >= 9

        photographerLabel?.text = photographer.name // we can use ! here, it makes no difference
    }
}

这将使代码变得viewDidLoad不必要.

现在你可能会问"如果我不需要它,我为什么要加载视图呢？我只想将我的变量存储在这里以备将来使用".如果这就是你要问的,那就意味着你使用视图控制器作为模型类,只是为了存储数据.这本身就是一个架构问题.如果您不想使用控制器,请不要实例化它.使用模型类来存储数据.

Answer 3

我希望有一天 #Swift 开发者能够解决这个错误:)

简易拐杖：

func itself<T>(_ value: T) -> T {
    return value
}

使用：

// refresh
style = itself(style)
image = itself(image)
text  = itself(text)

（可选包括）