ViewModel closing when navigate to new page

[okunevdm]

Catel.MVVM.Xamarin.Forms
After navigate to new page previous ViewModel is closing. And when press back button and return to first page viewmodel for this page create new instance.
is catel:ContentPage contains property like CloseViewModelOnUnloaded?

Expected behaviour

when NavigateForward ViewModel continues to work on background.
when NavigateBack ViewModel closing.

Actual behaviour

ViewModel closing when NavigateForward

[GeertvanHorrik]

@alexfdezsauco care to join the conversation? You wrote this part.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[GreenKn1ght]

This is a critical bug in Xamarin.Android due to the fact that Page.OnAppearing is called every time the page appears on the phone screen: this may be caused by navigation or by the OS when the app/phone comes out of sleep.
CloseViewModelOnUnloaded can't help because then you will have to handle the ViewModel lifecycle on your own.

[GeertvanHorrik]

We definitely want to fix this, but might need a bit of help here, interested?

[okunevdm]

I fix this bug in my project by create my own ContentPage with needed behaviour of _userControlLogic

[GreenKn1ght]

@okunevdm did you use the UserControlLogic.CloseViewModelOnUnloaded property or came up with another solution?

[GreenKn1ght]

I solved this by overriding the OnParentSet method and firing Loaded and Unloaded events depending on whether the Parent is null.
But before creating PR I need to test it at least on UWP and iOS.

[GreenKn1ght]

By the way, we still want to know when the view appears or disappears in associated view model. Any thoughts? Can we extend ViewModelBase with OnAppearing and OnDisappearing methods?

[GeertvanHorrik]

Why would you want Appearing / Disappearing methods, aren't they equal to Loaded/ Unloaded? In that case, you can (should) implement this in the iView implementation so Catel can deal with this.

[GreenKn1ght]

I solved this by overriding the OnParentSet method and firing Loaded and Unloaded events depending on whether the Parent is null.
But before creating PR I need to test it at least on UWP and iOS.

With this they won't longer be equal. Unloaded will be fired when our page leaves the navigation stack.

[GreenKn1ght]

I'll prepare sample to show this behavior.

[okunevdm]

@okunevdm did you use the UserControlLogic.CloseViewModelOnUnloaded property or came up with another solution?

this is my ContentPage class
` public class ContentPage : global::Xamarin.Forms.ContentPage, IView
{
private readonly UserControlLogic _userControlLogic;
private object _oldbindingContext;

    public ContentPage()
        : this(null)
    {
        BindingContextChanged += OnBindingContextChanged;
    }

    private void OnBindingContextChanged(object o, EventArgs eventArgs)
    {
        DataContextChanged?.Invoke(this, new DataContextChangedEventArgs(_oldbindingContext, BindingContext));
        _oldbindingContext = BindingContext;
        RaiseViewModelChanged();
    }

    public ContentPage(IViewModel viewModel)
    {
        _userControlLogic = new UserControlLogic(this, null, viewModel);
        _userControlLogic.TargetViewPropertyChanged += (sender, e) =>
        {
            OnPropertyChanged(e.PropertyName);
        };

        _userControlLogic.ViewModelClosedAsync += OnViewModelClosedAsync;

        _userControlLogic.ViewModelChanged += (sender, args) =>
        {
            if (!ObjectHelper.AreEqual(BindingContext, _userControlLogic.ViewModel))
            {
                BindingContext = _userControlLogic.ViewModel;
            }
        };

        _userControlLogic.ViewModelPropertyChanged += (sender, e) =>
        {
            OnViewModelPropertyChanged(e);
            ViewModelPropertyChanged?.Invoke(this, e);
        };

        Loaded += (sender, e) =>
        {
        };

        Unloaded += (sender, e) =>
        {
        };

        DataContextChanged += OnDataContextChanged;

        this.CustomBackButtonAction = OnBackButtonActionAsync;
    }

    public IViewModel ViewModel => DataContext as IViewModel;

    public event EventHandler<EventArgs> ViewModelChanged;

    public event EventHandler<PropertyChangedEventArgs> ViewModelPropertyChanged;

    public object DataContext
    {
        get => BindingContext;
        set => BindingContext = value;
    }

    public object Tag { get; set; }

    public event EventHandler<EventArgs> Loaded;

    public event EventHandler<EventArgs> Unloaded;

    public event EventHandler<DataContextChangedEventArgs> DataContextChanged;


    private void OnViewModelPropertyChanged(PropertyChangedEventArgs e)
    {
    }

    private void RaiseViewModelChanged()
    {
        OnViewModelChanged();
        ViewModelChanged?.Invoke(this, EventArgs.Empty);
    }

    protected virtual void OnViewModelChanged()
    {
    }

    protected virtual Task OnViewModelClosedAsync(object sender, ViewModelClosedEventArgs e)
    {
        return TaskHelper.Completed;
    }

    public event EventHandler<EventArgs> BackButtonPressed;

    protected override void OnAppearing()
    {
        Loaded?.Invoke(this, EventArgs.Empty);

        base.OnAppearing();
    }

    protected override void OnDisappearing()
    {
        base.OnDisappearing();

        Unloaded?.Invoke(this, EventArgs.Empty);
    }

    protected sealed override bool OnBackButtonPressed()
    {
        /*
        BackButtonPressed.SafeInvoke(this);
        var popupLayout = Content as PopupLayout;
        //// TODO: Lookup for top most popup layout.
        return popupLayout != null && popupLayout.IsPopupActive || base.OnBackButtonPressed();
        */

        BackButtonPressed?.Invoke(this, EventArgs.Empty);

        return base.OnBackButtonPressed();
    }

    private void OnDataContextChanged(object sender, DataContextChangedEventArgs eventArgs)
    {
        ViewModelChanged?.Invoke(this, EventArgs.Empty);
    }

    public Action CustomBackButtonAction { get; set; }
    protected async void OnBackButtonActionAsync()
    {
        if (await _userControlLogic.CancelAndCloseViewModelAsync())
        {
            await Navigation.PopAsync(true);
        }
    }
}

`

[okunevdm]

for hardware back button in android i override in MainActivity
public override void OnBackPressed() { // retrieve the current xamarin forms page instance var currentPage = Xamarin.Forms.Application.Current.MainPage.Navigation.NavigationStack.LastOrDefault(); // check if the page has subscribed to the custom back button event if (currentPage != null && currentPage.GetType().IsSubclassOf(typeof(DmMobile.Controls.ContentPage)) && ((DmMobile.Controls.ContentPage)currentPage).CustomBackButtonAction != null) { // invoke the Custom back button action ((DmMobile.Controls.ContentPage)currentPage).CustomBackButtonAction.Invoke(); // and disable the default back button action } else { base.OnBackPressed(); } }

[GreenKn1ght]

Probably I'm blind, but there is only one difference: CustomBackButtonAction. I do not see how this changes the current behavior.

[GreenKn1ght]

Please take a look at my sample.

[okunevdm]

Probably I'm blind, but there is only one difference: CustomBackButtonAction. I do not see how this changes the current behavior.

CustomBackButtonAction uses in concrete instance of contentPage:
`
[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class SettingsView
{
public SettingsView()
{
InitializeComponent ();

        CustomBackButtonAction = async () =>
        {
            var result = await this.DisplayAlert(Properties.Resources.Warning, Properties.Resources.ConfirmWithoutSaving, Properties.Resources.Yes, Properties.Resources.No);

            if (result)
            {
                OnBackButtonActionAsync();
            }
        };
    }
}

`

[okunevdm]

now i can not use ViewModel for control of Navigation behavior because of issue https://github.com/Catel/Catel/issues/1252 - i can not show message with buttons yes/no, but DisplayAlert works fine.

[GreenKn1ght]

Thus you prevent navigation. We should make it work out of the box, but initial issue is "After navigate to new page previous ViewModel is closing". How did you fix it?

[okunevdm]

default CustomBackButtonAction is protected async void OnBackButtonActionAsync()
in constructor: this.CustomBackButtonAction = OnBackButtonActionAsync;
when back button pressed (hardware or software) it call CustomBackButtonAction. and then:

    protected async void OnBackButtonActionAsync()
    {
        if (await _userControlLogic.CancelAndCloseViewModelAsync())
        {
            await Navigation.PopAsync(true);
        }
    }

[GreenKn1ght]

But if you navigate forward, your view model will still be closed and then recreated.

[okunevdm]

oh, yes of course.. )) I forgot that I really used CloseViewModelOnUnloaded and when copy paste code of ContentPage i cut it... sorry.
I add this property to ContentPage and set it to false:

    public class ContentPage : global::Xamarin.Forms.ContentPage, IView
    {
     ...
        public ContentPage(IViewModel viewModel)
        {
            ....
            _userControlLogic = new UserControlLogic(this, null, viewModel);
            CloseViewModelOnUnloaded = false;
           ....
        }
        public bool CloseViewModelOnUnloaded
        {
            get { return (bool)GetValue(CloseViewModelOnUnloadedProperty); }
            set { SetValue(CloseViewModelOnUnloadedProperty, value); _userControlLogic.CloseViewModelOnUnloaded = CloseViewModelOnUnloaded; }
        }
        public static readonly BindableProperty CloseViewModelOnUnloadedProperty = BindableProperty.Create(nameof(CloseViewModelOnUnloaded), typeof(bool), typeof(ContentPage), false);
    }

[GreenKn1ght]

Now it makes sense!
But you should be wary of navigating backwards using the NavigationPage and NavigationService.

[okunevdm]

why? i use NavigationPage/NavigationService and it work fine.

[GeertvanHorrik]

Maybe we can do type forwarding in https://docs.microsoft.com/en-us/dotnet/framework/app-domains/type-forwarding-in-the-common-language-runtime, but I don't want to lose Windows Phone (windows 10 mobile) support on UAP (use this myself) so updating to the min version of UAP that implements .net standard is not an option.

[GeertvanHorrik]

Now I think of it, this might be caused because Catel.Core doesn't explicitly use the .net standard 2.0 implementation, but the platform-specific one (that is also included there). Maybe we can force it to use the .net standard one instead.

[GreenKn1ght]

Can we support two versions of UAP? Since the Xamarin.Forms common library implements .NETStandard, we can't use the old version of UAP here anyway.

[GreenKn1ght]

AFAIK, XF 3+ doesn't support WM10. Thus, we can only support it as a non-XF library

[GeertvanHorrik]

The issue here is Catel.Core, that supports any platform (and the UWP version picks the UAP, not the .net standard implementation).

[GreenKn1ght]

I understood that. I propose to target Catel.Core to uap10.0 (will use local types) and uap10.0.16299 (will use .NETStandard2.0 types). That should solve the problem, isn't it?

[GeertvanHorrik]

Might work, let me try that.

[GeertvanHorrik]

[GeertvanHorrik]

Just pushed the first bits, need to leave now, but will try to build it on the build server.

[GeertvanHorrik]

Build - 5.10.0-beta.15

[GreenKn1ght]

It works!

[GeertvanHorrik]

Awesome, thanks for pointing me in the right direction, it was a brilliant idea. You got sufficient stuff working to complete this ticket?

[GreenKn1ght]

At first glance, yes. I still need to set up the environment for iOS. I will try to finish ticket at this weekend.

[GeertvanHorrik]

Do you mind if we put this ticket for 5.11? Then we can close the beta and release 5.10 early next week.

[GreenKn1ght]

Yes, let's put it for 5.11.

[GeertvanHorrik]

@GreenKn1ght any updates?

[GeertvanHorrik]

Since we removed support for all frameworks exception .NET 6 (WPF) for 6.0, I am closing this issue.