SwiftUI - onChange 沒有被調用

Question

首先添加代碼以幫助更好地理解問題。

struct MainSetupView: View {
    @StateObject var viewModel: MainViewModel
    
    var body: some View {
        VStack {
            switch viewModel.state {
            case .idle:
                Color.clear.onAppear(perform: { viewModel.prepare() })
            case .loading:
                ProgressView()
            case .showMessage(let messageType, let messageText):
                ErrorHandlingView(messageType: messageType, messageText: messageText)
            }
        }.onAppear() {
            viewModel.state = .idle
        }
    }
}

struct ErrorHandlingView: View {
    @State var messageType: String
    var messageText: String

    @State private var messageViewImageDetails: (imageName: String, isSystemImage: Bool) = (imageName: "", isSystemImage: false)

    var body: some View {
        VStack {
            MessageViewWithActions(imageName: messageViewImageDetails.imageName, isSystemImage: messageViewImageDetails.isSystemImage, mainMessageText: messageText)
        }.onChange(of: messageType) {_ in
            self.messageViewImageDetails = getImageForError()
            // call more functions
        }
    }
    
    private func getImageForError() -> (imageName: String, isSystemImage: Bool) {
        var imageName = ""
        var isSystemImage = false
        
        switch messageType {
        case "noInternetError":
            imageName = "xmark.icloud"
            isSystemImage = true
        case "userSignedOut":
            imageName = "person.crop.circle.fill.badge.xmark"
            isSystemImage = true
        case "userSignedIn":
            imageName = "person.crop.circle.badge.checkmark"
            isSystemImage = true
        // more cases.
        default:
            imageName = "info.circle"
            isSystemImage = true
        }
        
        return (imageName: imageName, isSystemImage: isSystemImage)
    }
}

如果“MainSetupView”打開並且我收到一條消息，即.showMessage，則會顯示“ErrorHandlingView”，並且在調用ErrorHandlingView 時設置“messageType”和“messageText”。 在顯示第一條消息時，假設我收到另一條消息並且“ErrorHandlingView”再次被調用，現在使用不同的消息類型。 然而，“ErrorHandlingView”中的“onChange(of: messageType)”永遠不會被調用。

“onChange(of: messageType)”調用 function “getImageForError”，它告訴應用程序根據 messageType 的值顯示哪個圖像。 由於 onChange 中的代碼沒有被執行，因此永遠不會調用“getImageForError”，因此永遠不會填充“messageViewImageDetails”，這會導致視圖沒有圖像。

知道為什么 is.onChange() 沒有被執行嗎？ 我究竟做錯了什么？

注意：我有很多代碼，我正在分享一些我認為會有所幫助的代碼。 讓我知道是否需要更多代碼。

編輯：為 MessageViewWithActions 添加代碼。

struct MessageViewWithActions: View {
    // Image properties
    var imageName: String?
    var isSystemImage: Bool = false
    
    // Text properties
    var mainMessageText: String?

    var body: some View {
        ScrollView {
            VStack {
                Text("")
                VStack {
                    if let imageName = imageName {
                        if isSystemImage {
                            Image(systemName: imageName)
                        } else {
                            Image(imageName)
                        }
                    }
                }.frame(maxWidth: .infinity).padding(.bottom, 8)
                
                if let mainMessageText = mainMessageText {
                    Text(mainMessageText).font(.subheadline).fixedSize(horizontal: false, vertical: true)
                }
            }
        }
    }
}

Answer 1

將 messageType 更改為 Binding var 而不是 State var 有效。 共享未來的更新代碼：

struct MainSetupView: View {
    @StateObject var viewModel: MainViewModel
    @State var messageType: String
    
    var body: some View {
        VStack {
            switch viewModel.state {
            case .idle:
                Color.clear.onAppear(perform: { viewModel.prepare() })
            case .loading:
                ProgressView()
            case .showMessage(_, let messageText):
                ErrorHandlingView(messageType: messageType, messageText: messageText)
            }
        }.onAppear() {
            viewModel.state = .idle
        }.onReceive(viewModel.$state, perform: { (value) in
            switch value {
            case .showMessage(let messageType, _):
                self.messageType = messageType
            case .idle, .loading:
                print("")
            }
        })
    }
}

struct ErrorHandlingView: View {
    @Binding var messageType: String
    var messageText: String

    @State private var messageViewImageDetails: (imageName: String, isSystemImage: Bool) = (imageName: "", isSystemImage: false)

    var body: some View {
        VStack {
            MessageViewWithActions(imageName: messageViewImageDetails.imageName, isSystemImage: messageViewImageDetails.isSystemImage, mainMessageText: messageText)
        }.onChange(of: messageType) {_ in
            self.messageViewImageDetails = getImageForError()
            // call more functions
        }
    }
    
    private func getImageForError() -> (imageName: String, isSystemImage: Bool) {
        var imageName = ""
        var isSystemImage = false
        
        switch messageType {
        case "noInternetError":
            imageName = "xmark.icloud"
            isSystemImage = true
        case "userSignedOut":
            imageName = "person.crop.circle.fill.badge.xmark"
            isSystemImage = true
        case "userSignedIn":
            imageName = "person.crop.circle.badge.checkmark"
            isSystemImage = true
        // more cases.
        default:
            imageName = "info.circle"
            isSystemImage = true
        }
        
        return (imageName: imageName, isSystemImage: isSystemImage)
    }
}