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Swifui OnAppearは2回呼び出されます

Q1：なぜOnAppearsが2回呼ばれていますか？

Q2：あるいは、私はネットワークコールをどこに呼び出すことができますか？

私は自分のコードの中でいくつかの異なる場所で恩恵を受けました、そしてそれらはすべて2回呼ばれます。最終的には、次のビューを表示する前にネットワークコールを作成しようとしています。

私はまた私のリストの中のforeachを配置して削除しようとしました、そしてそれは何も変わりません。

Xcode 12 Beta 3 - >ターゲットIOS 14 CoreDataが有効になっているが使用されていません

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")
        }) 
    }
}
 _
listnetworkingswiftuinavigationlinkonappear
7
AvsBest

このバグの最初の表示機能を作成できます

extension View {

    /// Fix the SwiftUI bug for onAppear twice in subviews
    /// - Parameters:
    ///   - perform: perform the action when appear
    func onFirstAppear(perform: @escaping () -> Void) -> some View {
        let kAppearAction = "appear_action"
        let queue = OperationQueue.main
        let delayOperation = BlockOperation {
            Thread.sleep(forTimeInterval: 0.001)
        }
        let appearOperation = BlockOperation {
            perform()
        }
        appearOperation.name = kAppearAction
        appearOperation.addDependency(delayOperation)
        return onAppear {
            queue.addOperation(delayOperation)
            queue.addOperation(appearOperation)
        }
        .onDisappear {
            queue.operations
                .first { $0.name == kAppearAction }?
                .cancel()
        }
    }
}
 _
1
Calvin Chang

他の誰かが私のボートにある場合、ここに私が今のところ解決した方法です。

struct ChannelListView: View {

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

var body: some View {
    NavigationView {
        VStack {
            SearchBar(text: $searchText)
                .padding(.top, 20)
            List(channelStore.allChannels) { channel in
                ZStack {
                    NavigationLink(destination: VideoListView(channel: channel)) {
                        ChannelRowView(channel: channel)
                    }
                    HStack {
                        Spacer()
                        Button {
                            isNavLinkActive = true
                            
                            // Place action/network call here
                            
                        } label: {
                            Image(systemName: "arrow.right")
                        }
                        .foregroundColor(.gray)
                    }
                }
                .listStyle(GroupedListStyle())
            }
            .navigationTitle("Channels")
            }
        }
    }
}
 _
0
AvsBest

最初に表示されているかどうかを確認するためのBOOL変数を作成できます。

struct VideoListView: View {
  @State var firstAppear: Bool = true

  var body: some View {
    List {
      Text("")
    }
    .onAppear(perform: {
      if !self.firstAppear { return }
      print("BOTTOM ON APPEAR")
      self.firstAppear = false
    })
  }
}
 _
0
vdotup

.onAppear(perform)でやる必要はありません。これはビューのinitで行うことができます

0
Wasim