Question

我在全局范围内声明了一个可能随时更改的变量globalVariable。

当ViewControllers更改时，我的应用中的不同globalVariable需要做出不同的反应。

因此，当ViewController发生变化时，需要在每个globalVariable中添加一个执行所需代码的属性观察器。

我似乎无法通过override或extension 实现这一目标。 这里有什么方法？

Answer 1

如果您的目标是简单地知道您的全局变量何时发生变化，您可以在变更时发布通知：

extension NSNotification.Name {
    static let globalVariableChanged = NSNotification.Name(Bundle.main.bundleIdentifier! + ".globalVariable")
}

var globalVariable: Int = 0 {
    didSet {
        NotificationCenter.default.post(name: .globalVariableChanged, object: nil)
    }
}

然后任何对象都可以为该通知添加观察者：

class ViewController: UIViewController {

    private var observer: NSObjectProtocol!

    override func viewDidLoad() {
        super.viewDidLoad()

        // add observer; make sure any `self` references are `weak` or `unowned`; obviously, if you don't reference `self`, that's not necessary

        observer = NotificationCenter.default.addObserver(forName: .globalVariableChanged, object: nil, queue: .main) { [weak self] notification in
            // do something with globalVariable here
        }
    }

    deinit {
        // remember to remove it when this object is deallocated

        NotificationCenter.default.removeObserver(observer)
    }

}

注意，如果（a）全局变量是引用类型，即didSet，则此class机制将不会检测到更改。（b）它只是改变全局变量引用的对象而不是用新实例替换它。要识别该场景，您需要使用KVO或其他机制来检测突变。

Answer 2

全局变量只能有一个didSet {}函数，它必须属于变量本身。你可以做的是让变量的didSet {}函数调用来自其他对象的函数列表。

您可以为此使用通知，或者您可以构建自己的机制。

以下是如何创建自己的机制的示例：

（请注意，这非常通用，可以适用于任何变量类型或单例实例）

// Container for an observer's function reference
// - will be used to call the observer's code when the variable is set
// - Separates the object reference from the function reference
//   to avoid strong retention cycles.
struct GlobalDidSet<T>
{
   weak var observer:AnyObject?
   var didSetFunction:(AnyObject)->(T)->()
   init(_ observer:AnyObject, didSet function:@escaping (AnyObject)->(T)->())
   {
      self.observer  = observer
      didSetFunction = function
   }
}

// Container for a list of observers to be notified
// - maintains the list of observers
// - automatically clears entries that non longer have a valid object
// - calls all observers when variable changes
// - erases type of observer to allow generic use of GlobalDidSet<>
struct GlobalDidSets<T>
{
   var observers : [GlobalDidSet<T>] = []
   mutating func register<O:AnyObject>(_ observer:O, didSet function:@escaping (O)->(T)->())
   {
      let observer = GlobalDidSet<T>(observer)
      { (object:AnyObject) in function(object as! O) }
      observers.append(observer)  
   }

   mutating func notifyDidSet(_ oldValue:T)
   {
      observers = observers.filter{$0.observer != nil}
      observers.forEach{ $0.didSetFunction($0.observer!)(oldValue) }
   }
}

...

// To use this, you will need a second variable to manage the list of observers
// and your global variable's didSet{} must use that observer list
// to perform the multiple function calls
//
var globalVariableDidSets = GlobalDidSets<String>()
var globalVariable : String = "Initial Value"
{
   didSet { globalVariableDidSets.notifyDidSet(oldValue) }
}

// In your view controllers (or any other class), you need to setup the 
// reaction to the global variable changes by registering to the observer list
//
class MyVC:UIViewController
{
    override func viewDidLoad()
    {
      globalVariableDidSets.register(self){ $0.handleVariableChange }  
      // ...
    }

    func handleVariableChange(_ oldValue:String)
    {
      //...
    }
}

将属性观察者添加到Swift中的类内的全局变量中

2 个答案: