无法从 Class 更新 Published var

Posted 2023-02-24

【中文标题】无法从 Class 更新 Published var

【英文标题】：Unable to update Published var from Class

【发布时间】：2021-08-07 02:43:38

【问题描述】：

我有一个已发布的 var prediction:

class TrainingStatus: ObservableObject 
    @Published var isTrained: Bool = false
    @Published var modelUrl: URL = URL(fileURLWithPath: "hello")
    @Published var prediction:String = ""

我尝试在一个类中更新这个变量：

class ClassificationController: ObservableObject 
...
 func processObservations(for request: VNRequest, error: Error?) 
      DispatchQueue.main.async 
        if let results = request.results as? [VNClassificationObservation] 
          if results.isEmpty 
            print("nothing found")
           else 
              print("got a result: \(results)!")
              let top3 = results.prefix(3).map  observation in
                String(format: "%@ %.1f%%", observation.identifier, observation.confidence * 100)
              
              self.prediction = top3.joined(separator: " ")
              
              // attempt to set environment variable?
              let trainingStatus = TrainingStatus()
              trainingStatus.prediction = self.prediction
              
              print("set prediction: \(trainingStatus.prediction)")
          

并在我的 UIView 中显示这个变量：

struct SidebarView: View 
   
    @EnvironmentObject var trainingStatus: TrainingStatus
 
    
    var body: some View 
        VStack
            NavigationView
                    Section
                        Text("Trained? " + String(trainingStatus.isTrained))
                        Text("Prediction: \(trainingStatus.prediction)")
                    
                    
                
                .navigationTitle("Data")
            
            .environmentObject(modelData)
            .environmentObject(trainingStatus)
        
    

但是，尽管 prediction 的值似乎在 ClassificationController 中更新，但我的带有文本 \(trainingStatus.prediction) 的 UI 视图从未更新。

如何更新视图以获取最新发布的值？

【问题讨论】：

如您所见，您不能这样做：let trainingStatus = TrainingStatus() trainingStatus.prediction = self.prediction。阅读更多关于如何使用 ObservableObject 以及如何传递它们的信息。您想要实现的是仅使用 1 个 TrainingStatus。

@workingdog 你能推荐一些关于如何正确设置可观察对象的资源吗？

【参考方案1】：

您可以有效地利用（几乎）单例。

您首先需要更改 ObservableObject 类并添加一个新常量：

     class TrainingStatus: ObservableObject 
        @Published var isTrained: Bool = false
        @Published var modelUrl: URL = URL(fileURLWithPath: "hello")
        @Published var prediction:String = ""

        public static let shared = TrainingStatus()
    

然后您可以通过声明以下内容在其他地方访问它：

var trainingStatus = TrainingStatus.shared

现在您可以随意修改它的值。这也将在 ObservableObject 类中更新它，因此在任何其他使用它的视图中更改它。

您的用例示例：

 struct SidebarView: View 
   
    var trainingStatus = TrainingStatus.shared
 
    
    var body: some View 
        VStack
            NavigationView
                    Section
                        Text("Trained? " + String(trainingStatus.isTrained))
                        Text("Prediction: \(trainingStatus.prediction)") //This is now referencing your new shared variable of trainingStatus
                    
                    
                
                .navigationTitle("Data")
            
            .environmentObject(modelData)
            .environmentObject(trainingStatus)
        
    

要修改常量的类：

 class ClassificationController: ObservableObject 
 
 var trainingStatus = TrainingStatus.shared

...
 func processObservations(for request: VNRequest, error: Error?) 
      DispatchQueue.main.async 
        if let results = request.results as? [VNClassificationObservation] 
          if results.isEmpty 
            print("nothing found")
           else 
              print("got a result: \(results)!")
              let top3 = results.prefix(3).map  observation in
                String(format: "%@ %.1f%%", observation.identifier, observation.confidence * 100)
              
              self.prediction = top3.joined(separator: " ")
              
              // attempt to set environment variable?
              //let trainingStatus = TrainingStatus() --- no longer need this line
              trainingStatus.prediction = self.prediction //this is now changing your singleton
              
              print("set prediction: \(trainingStatus.prediction)")
          

【讨论】：

已编辑以在您提供的代码中添加示例