SwifUI onAppear被调用两次 [英] SwifUI onAppear gets called twice

问题描述

Q1:为什么onAppears会被调用两次?

Q1: Why are onAppears called twice?

Q2:或者，我可以在哪里拨打网络电话?

Q2: Alternatively, where can I make my network call?

我在代码中的几个不同位置放置了onAppears，它们都被调用了两次.最终，我会尝试在显示下一个视图之前进行网络调用，因此如果您知道不使用 onAppear 的方法，我会全力以赴.

I have placed onAppears at a few different place in my code and they are all called twice. Ultimately, I'm trying to make a network call before displaying the next view so if you know of a way to do that without using onAppear, I'm all ears.

我还尝试过在列表中放置和删除ForEach，但它没有任何改变.

I have also tried to place and remove a ForEach inside my Lists and it doesn't change anything.

Xcode 12 Beta 3->目标iO 14CoreData已启用但尚未使用

Xcode 12 Beta 3 -> Target iOs 14 CoreData enabled but not used yet

struct ChannelListView: View {

@EnvironmentObject var channelStore: ChannelStore
@State private var searchText = ""
@ObservedObject private var networking = Networking()

var body: some View {
    NavigationView {
        VStack {
            SearchBar(text: $searchText)
                .padding(.top, 20)
             
            List() {

                ForEach(channelStore.allChannels) { channel in
                    
                    NavigationLink(destination: VideoListView(channel: channel)
                                    .onAppear(perform: {
                        print("PREVIOUS VIEW ON APPEAR")
                    })) {
                        ChannelRowView(channel: channel)
                    }
                }
                .listStyle(GroupedListStyle())
            }
            .navigationTitle("Channels")
            }
        }
    }
}

---

struct VideoListView: View {

@EnvironmentObject var videoStore: VideoStore
@EnvironmentObject var channelStore: ChannelStore
@ObservedObject private var networking = Networking()

var channel: Channel

var body: some View {
    
    List(videoStore.allVideos) { video in
        VideoRowView(video: video)
    }
        .onAppear(perform: {
            print("LIST ON APPEAR")
        })
        .navigationTitle("Videos")
        .navigationBarItems(trailing: Button(action: {
            networking.getTopVideos(channelID: channel.channelId) { (videos) in
                var videoIdArray = [String]()
                videoStore.allVideos = videos
                
                for video in videoStore.allVideos {
                    videoIdArray.append(video.videoID)
                }
                
                for (index, var video) in videoStore.allVideos.enumerated() {
                    networking.getViewCount(videoID: videoIdArray[index]) { (viewCount) in
                        video.viewCount = viewCount
                        videoStore.allVideos[index] = video
                        
                        networking.setVideoThumbnail(video: video) { (image) in
                            video.thumbnailImage = image
                            videoStore.allVideos[index] = video
                        }
                    }
                }
            }
        }) {
            Text("Button")
        })
        .onAppear(perform: {
            print("BOTTOM ON APPEAR")
        }) 
    }
}

推荐答案

我遇到了同样的问题.

我所做的是以下事情:

struct ContentView: View {

    @State var didAppear = false
    @State var appearCount = 0

    var body: some View { 
       Text("Appeared Count: \(appearrCount)"
           .onAppear(perform: onLoad)
    }

    func onLoad() {
        if !didAppear {
            appearCount += 1
            //This is where I loaded my coreData information into normal arrays
        }
        didAppear = true
    }
}

这通过确保onLoad()的if条件内部的内容仅运行一次来​​解决.

This solves it by making sure only what's inside the the if conditional inside of onLoad() will run once.

更新:Apple Developer论坛上的某人已提交了票证，Apple知道了此问题.我的解决方案是暂时的措施，直到Apple解决问题为止.

Update: Someone on the Apple Developer forums has filed a ticket and Apple is aware of the issue. My solution is a temporary hack until Apple addresses the problem.

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