Xamarin Forms best way to force display of one page from another

Question

This is my first Xamarin Forms app. It's not MVVM; I am tackling one learning curve at a time. VS 2019 Windows; project is Android/UWP and hopefully iOS.

I started from the Flyout menu template. The app is pretty much all working.

Under certain circumstances rather than loading the page the user requested from the Flyout Menu I want to display an "authentication" page to force the user to enter a password (or on Android scan his or her fingerprint if reader is available).

So what I am doing is in the Appearing method of the various pages I am checking for the need to authenticate and if so doing a PushModalAsync(new AuthenticationPage) and then when authentication completes doing a PopModalAsync(). I disable the Back button and so forth to keep the user on the password page until the password is entered successfully.

The problem is that PopModalAsync() seems to load a new instance of the original page? Is that correct? I seem to have a mess that I don't quite understand. Am I doing this the best way? I took a look at https://social.msdn.microsoft.com/Forums/en-US/904d1d72-8d14-417f-875f-0a2f3a73ea11/how-to-cancel-a-page-loading-during-onappearing?forum=xamarinforms but that got me down a rabbit hole. Is there a better way?

Answer 1

Hi CharlesMills-5302,

Welcome to our Microsoft Q&A platform!

Since login is not a frequent operation, creating a new instance will not take up much memory.

If you want to avoid creating new instance every time, here is a workaround that you can create a custom login ContentView and add it into the Page, then set its "IsVisible" property.

Here is a simple example you can refer to.
LoginView.xaml

<ContentView ...  
             xmlns:local="clr-namespace:WayToPopLoginPage.Views">  
    <ContentView.Content>  
        <StackLayout>  
            <Entry Placeholder="User Name"/>  
            <Entry Placeholder="Password"/>  
            <Button Text="login" Command = "{Binding Source={RelativeSource AncestorType={x:Type local:ItemsPage}}, Path=ShowCommand}"/>  
        </StackLayout>  
    </ContentView.Content>  
</ContentView>  

ItemPage.xaml (Show info in this page)

<ContentPage ...>  
    <Grid RowDefinitions="*">  
        <StackLayout IsVisible="{Binding Login}" HorizontalOptions="CenterAndExpand" VerticalOptions="CenterAndExpand">  
                <Label Text="Content here"/>  
        </StackLayout>  
  
        <localview:LoginView IsVisible="{Binding Notlogin}" HorizontalOptions="CenterAndExpand" VerticalOptions="CenterAndExpand"/>  
    </Grid>  
</ContentPage>  

ItemsPage.xaml.cs

public partial class ItemsPage : ContentPage, INotifyPropertyChanged  
{  
    public ItemsPage()  
    {  
        InitializeComponent();  
        ShowCommand = new Command(Show);  
        BindingContext = this;  
    }  
  
    protected override void OnAppearing()  
    {  
        base.OnAppearing();  
        Login = Preferences.Get("logged", false);  
        Notlogin = !Preferences.Get("logged", false);  
    }  
  
    public ICommand ShowCommand { get; }  
  
    bool notlogin;  
    public bool Notlogin  
    {  
        get => notlogin;  
        set  
        {  
            notlogin = value;  
            OnPropertyChanged("Notlogin");  
        }  
    }  
  
    bool login;  
    public bool Login  
    {  
        get => login;  
        set  
        {  
            login = value;  
            OnPropertyChanged("Login");  
        }  
    }  
  
    private void Show()  
    {  
        Login = true;  
        Notlogin = false;  
        Preferences.Set("logged", true);  
    }  
  
    public event PropertyChangedEventHandler PropertyChanged;  
  
    protected void OnPropertyChanged(string propertyName)  
    {  
        var handler = PropertyChanged;  
        if (handler != null)  
            handler(this, new PropertyChangedEventArgs(propertyName));  
    }  
}  

Regards,
Kyle

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