@Published 属性内模型更新时的 SwiftUI 触发功能

Question

我从 Firestore 表中获取一个数组。显然是实时的，因此任何新添加的文档或对现有文档的修改都会反映在视图中。这适用于字符串和数字等简单属性，但我也需要显示和刷新其他数据。

我也在存储坐标，如果坐标发生变化，我需要调用geocoder.reverseGeocodeLocation() 以便将地标移出该位置并将地址显示为字符串。为此，我创建了一个名为 PlacemarkService (code provided below).

的服务

问题是这样的。每当修改模型时，数据都来自PackagesViewModel，它有一个@Published var results = [Package]() 这是由Firestore（可编码）动态修改的。然后，对数组造成的任何更改都将导致 PackagesView 内的 List 被刷新，这将导致创建 PackageView(packageService: PackageService(package: package)) 的新实例，并且由于 PackageService 被作为参数传递，这里是一个新实例的PackageService 将被创建。最后，这将触发该服务内部的 init() 被调用。

@Published var results = [Package]() -> List -> PackageView(packageService: PackageService(package: package)) -> init()

所以现在，如果我按照这个流程并在 init 内触发 getPlacemarks()，一切正常（大多数情况下），如果包模型发生更改，此函数将在 init 上调用，并将反转修改位置的 GeocodeLocation。但是这里有一个大问题，这就是原因。我无法将此逻辑放在 init 上，因为我无法对 for 循环内的数百个位置进行地理编码。我在一个列表中显示了多个包，而 MapKit 不允许这样做。

显然，在显示PackageView之后需要触发此逻辑，因此选择其中一个包。

如果我在 PackageView 出现时调用 packageService.getPlacemarks()，这第一次可以正常工作，但是......当 @Published var results = [Package]() 更新时不会触发 onAppear。

所以最后的问题是：

在哪里调用 packageService.getPlacemarks()，以便在 @Published var results = [Package] 中的 Package 更新时调用它

但由于我上面解释的原因，不在 PlacemarkService 的 init 上。

对不起，解释太长了，我只是想清楚一点。

class PackageService: ObservableObject {
    var package: Package
    private var cancellables = Set<AnyCancellable>()
    
    @Published var sourcePlacemarkService = PlacemarkService()
    @Published var destinationPlacemarkService = PlacemarkService()
    
    var cancellable: AnyCancellable? = nil
    
    init(package: Package) {
        self.package = package
        startListening()
    }
    
    func startListening() {
        // Listen to placemark changes.
        cancellable = Publishers.CombineLatest(sourcePlacemarkService.$placemark, destinationPlacemarkService.$placemark).sink(receiveValue: {_ in
            
            // Publish changes manually to the view.
            self.objectWillChange.send()
        })
    }
    
    // Get placemarks from locations
    func getPlacemarks() {
        sourcePlacemarkService.reverseGeocodeLocation(location: package.source.toCLLocation)
        destinationPlacemarkService.reverseGeocodeLocation(location: package.destination.toCLLocation)
    }
}

class PackagesViewModel: ObservableObject {
    @Published var results = [Package]()
    
    func load() {
        query.addSnapshotListener { (querySnapshot, error) in 
            // updates results array when a document is modified.
        }
    }
}


struct PackagesView: View {
    @StateObject var packagessViewModel = PackagesViewModel()

    var body: some View {
        List(packagessViewModel.results, id: \.self) { package in
            NavigationLink(destination: PackageView(packageService: PackageService(package: package))) {
                Text(package.title)
            }
        }
    }
}

struct PackageView: View {
    @ObservedObject var packageService: PackageService

    func onAppear() {
        packageService.getPlacemarks()
    }
    
    var body: some View {
        // show the address from placemark after it is geocoded.
        VStack {
            Text(packageService.sourcePlacemarkService.placemark.title)
            Text(packageService.destinationPlacemarkService.placemark.title)
        }
        .onAppear(perform: onAppear)
    }
}

class PlacemarkService: ObservableObject {
    @Published var placemark: CLPlacemark?
    
    init(placemark: CLPlacemark? = nil) {
        self.placemark = placemark
    }
    
    func reverseGeocodeLocation(location: CLLocation?) {
        if let location = location {
            geocoder.reverseGeocodeLocation(location, completionHandler: { (placemark, error) in
                // some code here
                self.placemark = placemark
            })
        }
    }
}

struct Package: Identifiable, Codable {
    @DocumentID var id: String?
    var documentReference: DocumentReference
    
    var uid: String
    var title: String
    var description: String
    var source, destination: GeoPoint
    var amount: Double
    
    var createdAt: Timestamp = Timestamp()
    var paid: Bool = false
}

Answer 1

首先 - 简化此代码。大部分代码对于重现问题是不必要的，甚至无法编译。下面的代码不是可以工作的代码，而是我们以后必须启动和更改的代码：

内容视图

struct ContentView: View {
    var body: some View {
        NavigationView {
            PackagesView()
        }
    }
}

包服务

class PackageService: ObservableObject {
    let package: Package

    init(package: Package) {
        self.package = package
    }

    // Get placemarks from locations
    func getPlacemarks() {
        print("getPlacements called")
    }
}

PackagesViewModel

class PackagesViewModel: ObservableObject {
    @Published var results = [Package]()
}

包视图

struct PackagesView: View {
    @StateObject var packagesViewModel = PackagesViewModel()

    var body: some View {
        VStack {
            Button("Add new package") {
                let number = packagesViewModel.results.count + 1
                let new = Package(title: "title \(number)", description: "description \(number)")
                packagesViewModel.results.append(new)
            }

            Button("Change random title") {
                guard let randomIndex = packagesViewModel.results.indices.randomElement() else {
                    return
                }

                packagesViewModel.results[randomIndex].title = "new title (\(Int.random(in: 1 ... 100)))"
            }

            List(packagesViewModel.results, id: \.self) { package in
                NavigationLink(destination: PackageView(packageService: PackageService(package: package))) {
                    Text(package.title)
                }
            }
        }
    }
}

包视图

struct PackageView: View {
    @ObservedObject var packageService: PackageService

    var body: some View {
        VStack {
            Text("Title: \(packageService.package.title)")

            Text("Description: \(packageService.package.description)")
        }
    }
}

包

struct Package: Identifiable, Hashable {
    let id = UUID()
    var title: String
    let description: String
}

---

现在，解决问题。我通过检测 results 与 onChange(of:perform:) 的变化来解决此问题。但是，从这里无法访问视图正文中使用的PackageServices。

为了防止这个问题，PackageServices 实际上存储在PackagesViewModel 中，这在逻辑上对数据流更有意义。现在PackageService 也是struct，所以@Published 在数组上为results 工作，现在可以工作了。

请看下面的代码：

PackageService（更新）

struct PackageService: Hashable {
    var package: Package

    init(package: Package) {
        self.package = package
    }

    // Get placemarks from locations
    mutating func getPlacemarks() {
        print("getPlacements called")

        // This function is mutating, feel free to set any properties in here
    }
}

PackagesViewModel（更新）

class PackagesViewModel: ObservableObject {
    @Published var results = [PackageService]()
}

PackagesView（更新）

struct PackagesView: View {
    @StateObject var packagesViewModel = PackagesViewModel()

    var body: some View {
        VStack {
            Button("Add new package") {
                let number = packagesViewModel.results.count + 1
                let new = Package(title: "title \(number)", description: "description \(number)")
                packagesViewModel.results.append(PackageService(package: new))
            }

            Button("Change random title") {
                guard let randomIndex = packagesViewModel.results.indices.randomElement() else {
                    return
                }

                let newTitle = "new title (\(Int.random(in: 1 ... 100)))"
                packagesViewModel.results[randomIndex].package.title = newTitle
            }

            List($packagesViewModel.results, id: \.self) { $packageService in
                NavigationLink(destination: PackageView(packageService: $packageService)) {
                    Text(packageService.package.title)
                }
            }
        }
        .onChange(of: packagesViewModel.results) { _ in
            for packageService in $packagesViewModel.results {
                packageService.wrappedValue.getPlacemarks()
            }
        }
    }
}

PackageView（更新）

struct PackageView: View {
    @Binding var packageService: PackageService

    var body: some View {
        VStack {
            Text("Title: \(packageService.package.title)")

            Text("Description: \(packageService.package.description)")
        }
    }
}

Answer 2

我认为解决您的问题的最简单方法是首先在onAppear 中调用getPlacemarks，然后使用onChange 重新调用它，如下所示：

VStack {
    Text(packageService.sourcePlacemarkService.placemark.title)
    Text(packageService.destinationPlacemarkService.placemark.title)
}
.onChange(of: packageService.package.id) { _ in
    packageService.getPlacemarks()
}
.onAppear {
    packageService.getPlacemarks()
}

根据包结构的哪些字段发生变化，您可能需要将其设为Equtable 并传递整个对象而不是id。