Why is SwiftUI picker in form repositioning after navigation?

问题

After clicking the picker it navigates to the select view. The item list is rendered too far from the top, but snaps up after the animation is finished. Why is this happening?

Demo: https://gfycat.com/idioticdizzyazurevase

I already created a minimal example to rule out navigation bar titles and buttons, form sections and other details:

import SwiftUI

struct NewProjectView: View {

    @State var name = ""

    var body: some View {
        NavigationView {
            Form {
                Picker("Client", selection: $name) {
                    Text("Client 1")
                    Text("Client 2")
                }
            }
        }
    }
}

struct NewProjectView_Previews: PreviewProvider {
    static var previews: some View {
        NewProjectView()
    }
}

This happens in preview mode, simulator and on device (Xcode 11.2, iOS 13.2 in simulator, 13.3 beta 1 on device).

回答1:

The obviously buggy behavior can be worked around when forcing the navigation view style to stacked:

NavigationView {
    …
}.navigationViewStyle(StackNavigationViewStyle())

This is a solution to my problem, but I won‘t mark this as accepted answer (yet).

It seems to be a bug, even if it may be triggered by special circumstances.
My solution won‘t work if you need another navigation view style.
Additionally, it won‘t fix the horizontal repositioning mentioned by DogCoffee in the comments.

回答2:

In my opinion, it has something to do with the navigation bar. In default (no mention of .navigationBarTitle extension), the navigation display mode is set to .automatic, this should be amend to .inline. I came across another post similar to this and use their solution to combine with yours, by using .navigationBarTitle("", displayMode: .inline) should help.

import SwiftUI

struct NewProjectView: View {

    @State var name = ""

    var body: some View {
        NavigationView {
            Form {
                Picker("Client", selection: $name) {
                    Text("Client 1")
                    Text("Client 2")
                }
            }
            .navigationBarTitle("", displayMode: .inline)
        }
    }
}


struct NewProjectView_Previews: PreviewProvider {
    static var previews: some View {
        NewProjectView()
    }
}

回答3:

Until this bug is resolved another way to work around this issue while retaining the DoubleColumnNavigationViewStyle for iPads would be to conditionally set that style:

let navView = NavigationView {
    …
}
if UIDevice.current.userInterfaceIdiom == .pad {
    return AnyView(navView.navigationViewStyle(DoubleColumnNavigationViewStyle()))
} else {
    return AnyView(navView.navigationViewStyle(StackNavigationViewStyle()))
}

来源：https://stackoverflow.com/questions/58773687/why-is-swiftui-picker-in-form-repositioning-after-navigation

标签

ios

forms

swiftui

picker