如何使用MVVM从其他xaml页面进行属性更改？

Question

I'm newbie in MVVM . I have two xaml pages. the second one cannot be accessed ( Locked ) unless the button from the first page (Introduction page) has been clicked. How can I do that?

The lock page has this code.

<Frame Grid.Row="0" Grid.Column="1" BackgroundColor="LightGray" IsVisible="{Binding LockPage}">
                <Frame.GestureRecognizers>
                    <TapGestureRecognizer Tapped="Tap_Lock" />
                </Frame.GestureRecognizers>
                <Label Text="Locked"/>
            </Frame>

The Introduction page has this code.

<StackLayout>
        <Label Text="This is only a simple Introduction Text."/>
        <Label Text=""/>
        <Button Text="Lets Go!!" Command="{Binding UnlockPageCommand}" Clicked="Tap_Next"/>
</StackLayout>

This is the the class LockModule.cs

 public class LockModule : INotifyPropertyChanged
{

    public LockModule()
    {
        UnlockPageCommand = new Command(UnlockPage);
    }

    bool lockPage = true;
    public event PropertyChangedEventHandler PropertyChanged;

    void OnPropertyChanged(string lockpage)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(lockpage));
    }

    public bool LockPage
    {
        get { return lockPage; }
        set
        {
                lockPage = value;

            OnPropertyChanged(nameof(LockPage));
        }
    }

    public Command UnlockPageCommand { get; }

    void UnlockPage()
    {
        if (lockPage == true)
        {
            lockPage = false;
        }
        else
        {
            lockPage = true;
        }
        OnPropertyChanged(nameof(LockPage));
    }
}

and it's not working......

Answer 1

I'm assuming both of your pages are bound to the same instance of your LockModule ViewModel, which at least the first misstep I see in your provided code is the UnlockPage() method isn't setting the correct variable. It would prevent OnPropertyChanged from firing and keep the bindings from updating automatically. Your method for your command should at least be something more like this:

void UnlockPage()
{
    if (lockPage == true)
    {
        LockPage = false;
    }
    else
    {
        LockPage = true;
    }
    OnPropertyChanged(nameof(LockPage));
}

Answer 2

One way of communicating between 2 viewmodels would be using the same instance of same object across viewmodels.

Else have your LockPage property in viewmodels base.

However I would suggest you to have such values stored in app's shared preferences namely app setting. You can use this plugin to make your life simpler around app settings. Ref Link: https://github.com/jamesmontemagno/SettingsPlugin/blob/master/README.md

Last option would be to consider using messageing centre to communicate between pages and pass values to viewmodels. Ref: https://docs.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/messaging-center

However the easiest and finest would be, update the value of lock on page one and pass it via navigation parameters to page two.