iOS14.2中的SwiftUI PageTabView将多次调用ChildView onAppear方法 [英] SwiftUI PageTabView in iOS14.2 will Recall ChildView onAppear method Many times

问题描述

我在SwiftUI中使用TabView PageTabViewStyle来显示网页视图，当我滑动此TabView时，我发现子视图将调用onAppear方法很多次，有人可以告诉我为什么吗?

I use TabView PageTabViewStyle with SwiftUI to display a pageview, when I swipe this TabView I find child view will Recall onAppear method Many times, Can someone tell me why?

这是我的代码

import SwiftUI

struct Pageview: View {
    
    @StateObject var vm = PageViewModel()
    
    var body: some View {
        VStack {
            
            DragViewBar().padding(.top, 14)
            
            TabView(selection: $vm.selectTabIndex) {
                
                TextView(index: "0").tag(0)
                TextView(index: "1").tag(1)
                TextView(index: "2").tag(2)
                TextView(index: "3").tag(3)
                TextView(index: "4").tag(4)
                TextView(index: "5").tag(5)
                
            }
            .tabViewStyle(PageTabViewStyle(indexDisplayMode: .never))
            
        }

    }
}

struct TextView: View {
    
    let index: String
    
    var body: some View {
        VStack {
            Text(index)
        }
        .onAppear { print(index) }
        
    }
}

struct DragViewBar: View {
    var body: some View {
        Rectangle()
            .frame(width:36.0,height:5.0).foregroundColor(Color.blue)
            .cornerRadius(100)
    }
}

class PageViewModel: ObservableObject {
    @Published var selectTabIndex = 0
}

控制台打印的结果

正确的情况是每次滑动只能打印一次

The correct case is to print only once per swipe

它在ios14.2中只是有问题，14.1可以，您可以在Github中加载我的代码: https://github.com/werbhelius/TabViewBug

It just has a problem in ios14.2, 14.1 will be ok, you can load my code in Github: https://github.com/werbhelius/TabViewBug

Xcode版本:12.1(12A7403)

Xcode version: 12.1 (12A7403)

设备:iPhone 6s iOS 14.2

Device: iPhone 6s iOS 14.2

我认为您可以在iOS 14.2中的任何设备上重现此问题

I think you can reproduce this problem on any device in iOS 14.2

我期待着您的帮助解决此问题.谢谢

I look forward to your help to solve this problem. Thank you

推荐答案

View s是由SwiftUI决定预先加载的.有时要比其他设备更多，具体取决于设备的可用资源.即使onAppear出现在视线之外(已预加载)，也会被调用

Views are preloaded at the discretion of SwiftUI. Sometimes more than others depending on the device's available resources. onAppear is called even if it has appeared out of view (pre-loaded)

import SwiftUI

struct PageView: View {
    
    @StateObject var vm = PageViewModel()
    
    var body: some View {
        VStack {
            
            DragViewBar().padding(.top, 14)
            
            TabView(selection: $vm.selectTabIndex) {
                
                TextView(index: "0").tag(0)
                TextView(index: "1").tag(1)
                TextView(index: "2").tag(2)
                TextView(index: "3").tag(3)
                TextView(index: "4").tag(4)
                TextView(index: "5").tag(5)
                
            }
            //This lets you perform an operation when the value has changed
            .onReceive(vm.$selectTabIndex, perform: { idx in
                print("PageView :: body :: onReceive" + idx.description)
            })
            .tabViewStyle(PageTabViewStyle(indexDisplayMode: .never))
            
        }

    }
}

struct TextView: View {
    
    let index: String
    
    var body: some View {
        VStack {
            Text(index)
        }
        //Views are pre-loaded at the discretion of SwiftUI
        .onAppear { print(index) }
        
        .onReceive(index.publisher, perform: { idx in
            print("TextView :: body :: onReceive" + idx.description)
        })
        
    }
}

struct DragViewBar: View {
    var body: some View {
        Rectangle()
            .frame(width:36.0,height:5.0).foregroundColor(Color.blue)
            .cornerRadius(100)
    }
}

class PageViewModel: ObservableObject {
    @Published var selectTabIndex = 0
}


struct PageView_Previews: PreviewProvider {
    static var previews: some View {
        PageView()
    }
} 

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