SwiftUI PageTabView in iOS14.2 will Recall ChildView onAppear method Many times

Question

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?

Answer 1

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()
    }
}