为什么在 SwiftUI 中，当用 .sheet 呈现模态视图时，init() 被调用两次

Question

I am producing the situation on WatchOS with the following code

struct Modal : View {
    @Binding var showingModal : Bool
    
    init(showingModal : Binding<Bool>){
        self._showingModal = showingModal
        print("init modal")
    }
    
    var body: some View {
        Button(action: {
            self.showingModal.toggle()
        }, label: {
            Text("TTTT")
        })
    }
}

struct ContentView: View {
    @State var showingModal = false
    var body: some View {
        Button(action: {
            self.showingModal.toggle()
        }, label: {
            Text("AAAA")
        }).sheet(isPresented: $showingModal, content: {Modal(showingModal: self.$showingModal)})
    }
}

Every time I press the button in the master view to summon the modal with .sheet, Two instances of the modal view are created.

Could someone explain this phenomenon?

Answer 1

I tracked this down in my code to having the following line in my View:

@Environment(\.presentationMode) var presentation

I had been doing it due to https://stackoverflow.com/a/61311279/155186 , but for some reason that problem seems to have disappeared for me so I guess I no longer need it.

I've filed Feedback FB7723767 with Apple about this.

Answer 2

It is probably a bug, as of Xcode 11.4.1 (11E503a). Beware, that if for example initializing view models (or anything else for that matter) like so:

.sheet(isPresented: $isEditingModalPresented) {
    LEditView(vm: LEditViewModel(), isPresented: self.$isEditingModalPresented)
}

the VM will be initialized twice.

Answer 3

Comment out/remove the init() method from Modal with everything else the same. You should be able to solve the issue of two instances of Modal being created, its because you are explicitly initializing the binding (showingModal) in the init() of Modal. Hope this makes sense.

Answer 4

private let uniqueId: String = "uniqueId" 

Button(action: {
    self.showingModal.toggle()
}, label: {
    Text("AAAA")
})
    .sheet(isPresented: $showingModal) {
        Modal(showingModal: self.$showingModal)
          .id("some-unique-id")
    }

Ex:

.id(self.uniqueId)

Add unique id to your .sheet and not't worry :)

But, do not use UUID(), because sheet view will be represented on every touch event