SwiftUI视图-viewDidLoad（）？

Question

尝试在加载视图后加载图像，驱动视图的模型对象（请参见下面的MovieDetail）具有urlString。由于SwiftUI View元素没有生命周期方法（并且没有视图控制器来驱动事物），处理此问题的最佳方法是什么？

我遇到的主要问题是无论我尝试以哪种方式解决问题（绑定对象或使用State变量），我的View urlString加载后都不会显示...。

// movie object
struct Movie: Decodable, Identifiable {

    let id: String
    let title: String
    let year: String
    let type: String
    var posterUrl: String

    private enum CodingKeys: String, CodingKey {
        case id = "imdbID"
        case title = "Title"
        case year = "Year"
        case type = "Type"
        case posterUrl = "Poster"
    }
}

// root content list view that navigates to the detail view
struct ContentView : View {

    var movies: [Movie]

    var body: some View {
        NavigationView {
            List(movies) { movie in
                NavigationButton(destination: MovieDetail(movie: movie)) {
                    MovieRow(movie: movie)
                }
            }
            .navigationBarTitle(Text("Star Wars Movies"))
        }
    }
}

// detail view that needs to make the asynchronous call
struct MovieDetail : View {

    let movie: Movie
    @State var imageObject = BoundImageObject()

    var body: some View {
        HStack(alignment: .top) {
            VStack {
                Image(uiImage: imageObject.image)
                    .scaledToFit()

                Text(movie.title)
                    .font(.subheadline)
            }
        }
    }
}

提前致谢。

Answer 1

我们可以使用视图修饰符来实现这一点。

1. 创建ViewModifier：

struct ViewDidLoadModifier: ViewModifier {

    @State private var didLoad = false
    private let action: (() -> Void)?

    init(perform action: (() -> Void)? = nil) {
        self.action = action
    }

    func body(content: Content) -> some View {
        content.onAppear {
            if didLoad == false {
                didLoad = true
                action?()
            }
        }
    }

}

2. 创建View扩展：

extension View {

    func onLoad(perform action: (() -> Void)? = nil) -> some View {
        modifier(ViewDidLoadModifier(perform: action))
    }

}

3. 像这样使用：

struct SomeView: View {
    var body: some View {
        VStack {
            Text("HELLO!")
        }.onLoad {
            print("onLoad")
        }
    }
}

Answer 2

我希望这是有帮助的。我找到了一篇博客文章，内容涉及在Appear上进行导航视图操作。

想法是您将服务烘焙到BindableObject中，并在视图中订阅这些更新。

struct SearchView : View {
    @State private var query: String = "Swift"
    @EnvironmentObject var repoStore: ReposStore

    var body: some View {
        NavigationView {
            List {
                TextField($query, placeholder: Text("type something..."), onCommit: fetch)
                ForEach(repoStore.repos) { repo in
                    RepoRow(repo: repo)
                }
            }.navigationBarTitle(Text("Search"))
        }.onAppear(perform: fetch)
    }

    private func fetch() {
        repoStore.fetch(matching: query)
    }
}

import SwiftUI
import Combine

class ReposStore: BindableObject {
    var repos: [Repo] = [] {
        didSet {
            didChange.send(self)
        }
    }

    var didChange = PassthroughSubject<ReposStore, Never>()

    let service: GithubService
    init(service: GithubService) {
        self.service = service
    }

    func fetch(matching query: String) {
        service.search(matching: query) { [weak self] result in
            DispatchQueue.main.async {
                switch result {
                case .success(let repos): self?.repos = repos
                case .failure: self?.repos = []
                }
            }
        }
    }
}

致谢：Majid Jabrayilov

Answer 3

Fully updated for Xcode 11.2, Swift 5.0

I think the viewDidLoad() just equal to implement in the body closure.
SwiftUI gives us equivalents to UIKit’s viewDidAppear() and viewDidDisappear() in the form of onAppear() and onDisappear(). You can attach any code to these two events that you want, and SwiftUI will execute them when they occur.

As an example, this creates two views that use onAppear() and onDisappear() to print messages, with a navigation link to move between the two:

struct ContentView: View {
    var body: some View {
        NavigationView {
            VStack {
                NavigationLink(destination: DetailView()) {
                    Text("Hello World")
                }
            }
        }.onAppear {
            print("ContentView appeared!")
        }.onDisappear {
            print("ContentView disappeared!")
        }
    }
}

ref: https://www.hackingwithswift.com/quick-start/swiftui/how-to-respond-to-view-lifecycle-events-onappear-and-ondisappear

Answer 4

我正在使用init()。我认为onApear()不是替代方案viewDidLoad()。因为 onApear 是在你的视图出现时被调用的。由于您的视图可以出现多次，因此它与viewDidLoad调用一次的视图发生冲突。

想象一下有一个TabView. 通过滑动页面， onApear() 被多次调用。然而 viewDidLoad() 只被调用一次。