[Bug][iOS] Binding to TabView.SelectedIndex cause that TabView doesn't work

[SeRgI1982]

Description

I would like to have a chance preselect different TabItem than the first on when navigate to page with TabView. Unfortunately, it doesn't work when you bind SelectedIndex to your ViewModel and set a proper index inside ViewModel. The control stops working - there is no chance to navigate to different TabItems when this Binding exists.

Steps to Reproduce

1. Create Page with TabView and a few TabViewItem's
2. Bind TabView.SelectedIndex to your ViewModel.SelectedTabIndex
3. Try to set ViewModel.SelectedTabIndex to correct value

Expected Behavior

TabView will navigate automatically to an appropriate TabViewItem

Actual Behavior

On iOS TabView stuck - there is no chance to navigate to other tabs - even by tapping other TabViewItem icons. It worked in version 1.0.0

Basic Information

• Version with issue: 1.0.3 like also 1.1.0-pre1
• Last known good version: 1.0.0
• IDE: VS for Mac
• Platform Target Frameworks:
  • iOS: Xcode Version 12.4 (12D4e)
  • Android: 10.0

Sample app below:

TabViewTest.zip

[Cfun1]

Seems the same issue as https://github.com/xamarin/XamarinCommunityToolkit/issues/870 isn't supposed to be fixed @jfversluis ?

[jfversluis]

@jsuarezruiz

[Kapusch]

Actually, I can provide the same comment as for #1012

I also have noticed this bug many times, but I cannot say why exactly. Anytime I sign out / sign in again to my app, then I can see my tab well positioned on the value I have initialized in the ViewModel.

In my XAML file, the property binding is set as followed :

        SelectedIndex="{Binding SelectedTopTabsIndex, Mode=TwoWay}"

And the value is configured in the ViewModel as followed :

        private int _selectedTopTabsIndex = 1;
        public int SelectedTopTabsIndex
        {
            get => _selectedTopTabsIndex;
            set
            {
                _selectedTopTabsIndex = value;
                RaisePropertyChanged(() => SelectedTopTabsIndex);
            }
        }

I guess it could be something related to events ordering, BindingContext initializations... I'll comment more in case I get it.

[Kapusch]

TL;DR;

Check about your async methods which occured before your BindingContext is initialized !

---

I know that's not a concrete indication, but I guess this could help to understand the source of this issue.

In my case, it was clearly about async methods. Indeed, I have a NavigationService which navigates from page to page thanks to async method. And actually, once I sign in to my app, the async method for navigating to the MainView works fine.

But if I restart the app, and I am still authenticated, the NavigationService redirects me directly to the MainView. And this is where the Tabs are not well initialized, probably because OnResume / OnStart methods from "App.xaml.cs" are not async ?

I don't know, but the first thing I've tried by chance was to add this delay task in the NavigationService initialization, and everything worked properly...

            await Task.Delay(1);

But I have continued to investigate. I have a platform-specific implementation for the Firebase RemoteConfig service, which I call every time the application starts. And methods I call in this service are async. What I did is calling .Wait() method to the last awaitable Task, and it fixed all my issues.

                long cacheExpiration = 3600; // 1 hour in seconds.

                //Fetch remote values
                var status = await RemoteConfig.SharedInstance.FetchAsync(cacheExpiration);
                if (status == RemoteConfigFetchStatus.Success)
                {
                    //Activate new values
                    RemoteConfig.SharedInstance.ActivateAsync().Wait();
                }

[vecalion]

We're also experiencing this bug. Here is minimal repro changes: https://github.com/vecalion/XamarinCommunityToolkit/commit/829479e01a2a568b8f81488976f824b7fad88623 (Start the sample project -> Views -> TabView -> Getting Started)

Here is a video demonstration: https://1drv.ms/v/s!AvwfTJGXo3Kxg5oX-HRPnqZKynvh-g?e=AchHNL notice that the index is indefinitely changed from 0 to 1 and back to 0

@jfversluis, @jsuarezruiz I believe the issue was introduced in this changeset: https://github.com/xamarin/XamarinCommunityToolkit/commit/3abf5fa7eeaa29e76e9b1bfeb17575b75bccc799

[InquisitorJax]

... just to mention a work-around: I am still able to use swiping to access the tabs, but the tab selection for a particular tab becomes unresponsive.

[bricefriha]

Hi everyone, If this can helps, I'm facing the same issue. However, I'm not binding TabView.SelectedIndex. It works perfectly with Android but not with iOS.

I hope this will help you 😉

[LeoJHarris]

We're also experiencing this bug. Here is minimal repro changes: vecalion@829479e (Start the sample project -> Views -> TabView -> Getting Started)

Here is a video demonstration: https://1drv.ms/v/s!AvwfTJGXo3Kxg5oX-HRPnqZKynvh-g?e=AchHNL notice that the index is indefinitely changed from 0 to 1 and back to 0

@jfversluis, @jsuarezruiz I believe the issue was introduced in this changeset: 3abf5fa

Puling the community toolkit and reverting the changes resolves the issue. @jfversluis @jsuarezruiz can we revert these changes? Seems to resolve the issue with toggling the tab view items

[luwei888]

same problem，how to fix ？

[Kapusch]

A possible workaround is to set the SelectedIndex in the view "OnAppearing" event (for ContentPage views only).

[LeoJHarris]

@Kapusch

Try this as a work around

 <xct:TabView
        SelectedIndex="{Binding SelectedTabViewIndex, Mode=TwoWay}">
        <xct:TabView.TabIndicatorView>
            <BoxView WidthRequest="{Binding Source={x:Reference tabItemBase}, Path=Width}" />
        </xct:TabView.TabIndicatorView>
        <xct:TabViewItem
            x:Name="tabItemBase"

[nk-alex]

Any update on this topic? Still not working for me on version 2.0.5