SwiftUI 中的“@EnvironmentObject”属性是什么意思？

Question

正如您在 official tutorial project 中看到的，var 之前有一个奇怪的 @EnvironmentObject。这是什么意思？

struct LandmarkDetail: View {
    @EnvironmentObject var userData: UserData
    var landmark: Landmark

    var landmarkIndex: Int {
        userData.landmarks.firstIndex(where: { [=10=].id == landmark.id })!
    }
    // ...
}

Answer 1

环境对象

我从苹果文档中找到了。

Description from apple:

A dynamic view property that uses a bindable object supplied by an ancestor view to invalidate the current view whenever the bindable object changes.

apple doc

我的理解是当我们设置一个值或对象时被初始化。例如：列表。当我们重新加载一个列表时，一些显示的视图对象被替换和删除。

Answer 2

它是一个 BindableObject，SwiftUI 将使用它来确定如何通过访问该对象来刷新您的视图。查看示例中的 UserData class。声明环境对象的视图的任何子级都可以访问它。在这种情况下 userData.

Answer 3

例如，这是出于网络目的。您有一个 class 符合 BindableObject（Beta 5：ObservableObject）并通过 PassthroughSubject（Beta 5：@Published）发送您的数据。

比如这个案例：

SwiftUI (1.0)

struct Course: Decodable {
    let name: String
}

class NetworkManager: ObservableObject {

    @Published var courses = [Course]()

    func getAllCourses() {
        guard let url = URL(string: "https://api.letsbuildthatapp.com/jsondecodable/courses") else { return }
        URLSession.shared.dataTask(with: url) { (data, response, error) in
            do {
                let courses = try JSONDecoder().decode([Course].self, from: data!)
                DispatchQueue.main.async {
                    self.courses = courses
                }
            } catch {
                print("Failed To decode: ", error)
            }
        }.resume() // VERY IMPORTANT! As the request won't run
    }
}

struct ContentView : View {
    @ObservedObject var networkManager: NetworkManager = NetworkManager()
    // Or this
    // @EnvironmentObject var networkManager: NetworkManager // Don't forget to instaniate it with .environmentObject()

    var body: some View {
        VStack {
            Button(action: {
                self.networkManager.getAllCourses()
            }, label: {
                Text("Get All Courses")
            })

            List(networkManager.courses, id: \.name) {
                Text([=10=].name)
            }
        }
    }
}

测试版

struct Course: Decodable {
    let name: String
}

class NetworkManager: BindableObject {
    let didChange = PassthroughSubject<NetworkManager, Never>()

    var courses = [Course]() {
        didSet {
            didChange.send(self)
        }
    }

    func getAllCourses() {
        guard let url = URL(string: "https://api.letsbuildthatapp.com/jsondecodable/courses") else { return }
        URLSession.shared.dataTask(with: url) { (data, response, error) in
            do {
                let courses = try JSONDecoder().decode([Course].self, from: data!)
                DispatchQueue.main.async {
                    self.courses = courses
                }
            } catch {
                print("Failed To decode: ", error)
            }
        }.resume() // VERY IMPORTANT! As the request won't run
    }
}

struct ContentView : View {
    @EnvironmentObject var networkManager: NetworkManager

    var body: some View {
        VStack {
            Button(action: {
                self.networkManager.getAllCourses()
            }, label: {
                Text("Get All Courses")
            })

            List(networkManager.courses.identified(by: \.name)) {
                Text([=11=].name)
            }
        }
    }
}

Answer 4

它旨在让您的视图自动访问模型，而无需依赖项注入。

在场景委托中设置一次环境对象，然后层次结构中的所有视图都可以通过自己的魔法访问它属性。我相信视图也可以为其子对象覆盖对象，但我还没有测试过。

SwiftUI 中的“@EnvironmentObject”属性是什么意思？

What does the attribute `@EnvironmentObject` mean in the SwiftUI?

ios

swift

swiftui