SwiftUI .onAppear，只运行一次

Question

在一个SwiftUI应用程序中，我有这样的代码：

var body: some View {
    VStack {
        Spacer()
        ........
    }
    .onAppear {
      .... I want to have some code here ....
      .... to run when the view appears ....
    }
}

我的问题是我想在.onAppear块中运行一些代码，以便在应用程序出现在屏幕上、启动后或在后台一段时间后运行。但似乎这段代码只在应用启动时运行一次，之后再也不运行了。我错过了什么吗？或者我应该使用不同的策略来获得我想要的结果？

Answer 1

当应用程序进入前台时，您必须观察该事件并将其发布@Published到ContentView. 就是这样：

struct ContentView: View {

    @ObservedObject var observer = Observer()

    var body: some View {
        VStack {
            Spacer()
            //...
        }
        .onReceive(self.observer.$enteredForeground) { _ in
            print("App entered foreground!") // do stuff here
        }
    }
}

class Observer: ObservableObject {

    @Published var enteredForeground = true

    init() {
        if #available(iOS 13.0, *) {
            NotificationCenter.default.addObserver(self, selector: #selector(willEnterForeground), name: UIScene.willEnterForegroundNotification, object: nil)
        } else {
            NotificationCenter.default.addObserver(self, selector: #selector(willEnterForeground), name: UIApplication.willEnterForegroundNotification, object: nil)
        }
    }

    @objc func willEnterForeground() {
        enteredForeground.toggle()
    }

    deinit {
        NotificationCenter.default.removeObserver(self)
    }
}

Answer 2

如果您要针对 iOS14 进行链接，那么您可以利用新scenePhase概念：

@Environment(\.scenePhase) var scenePhase

如果注入此属性，您可以针对以下三个条件进行测试：

switch newPhase {
    case .inactive:
        print("inactive")
    case .active:
        print("active")
    case .background:
        print("background")
}

所以，一起来：

struct ContentView: View {
    @Environment(\.scenePhase) var scenePhase

    var body: some View {
        Text("Hello, World!")
            .onChange(of: scenePhase) { newPhase in
                switch newPhase {
                    case .inactive:
                        print("inactive")
                    case .active:
                        print("active")
                    case .background:
                        print("background")
                }
            }
    }
}