Nick's .NET Travels

Continually looking for the yellow brick road so I can catch me a wizard....

Getting Started with Xamarin.Forms and Presentation Attributes in MvvmCross

Previous posts in this sequence on Getting Started with Xamarin.Forms:
Multi-Targeting in Visual Studio, SDK Versions, MvvmCross, Authenticating with ADAL
Refactoring MvvmCross and Services, Effects, Navigation with MvvmCross

The great thing about MvvmCross is that it provides a minimal effort approach to things like navigation. However, there are times when you want to override the navigation model. For example you might want a page to appear as a modal page instead of a regular page. MvvmCross allows you to do this by overriding the default way that a page is presented, which is controlled by a presentation attribute applied to the page. Out of the box, MvvmCross comes with a number of presentation attributes that you can use to control how a page is rendered. By default, if you attempt to navigate to a page, MvvmCross assumes that you just want to navigate to that page using the Xamarin.Forms PushAsync method. This is the same as if you had attributed the page with the MvxContentPagePresentation attribute e.g.

[MvxContentPagePresentation]
public partial class MainPage

Note: By default you do not need to attribute each page with an attribute if you just want to navigate to the page. You can use the MvxContentPagePresentation attribute to override the back stack (i.e. using a combination of the NoHistory and WrapInNavigation properties)

I’m going to change the ProfilePage to appear as a modal page. The only change I need to make is to add the MvxModalPresentation attribute to the ProfilePage class.

[MvxModalPresentation]
public partial class ProfilePage

Now when I run the application the Profile Page will appear as a modal page. Unfortunately there is currently no way to dismiss the modal page, so the application will get stuck with the ProfilePage displayed. To fix this, let’s add a button to the ProfilePage that will be used to close the modal page. Again, I’ll start by showing how to close the page in code behind using the Xamarin.Forms navigation construct:

<Button Text="Close" Clicked="CloseClicked"/>

private async void CloseClicked(object sender, System.EventArgs e)
{
    await Navigation.PopModalAsync();
}

Note: Xamarin.Forms distinguishes between closing a modal page using PopModalAsync vs closing a regular page using PopAsync.

Of course, we’d prefer to have the button linked to a command in the view model that can be used to close the corresponding page. The code for the Button and the ProfileViewModel are as follows.

<Button Text="Close" Command="{Binding CloseCommand}"/>


public class ProfileViewModel: MvxNavigationViewModel
{
     private IMvxCommand closeCommand;

    public ProfileViewModel(IMvxLogProvider logProvider, IMvxNavigationService navigationService) : base(logProvider, navigationService)
     {
     }

    public IMvxCommand CloseCommand => closeCommand ?? (closeCommand = new MvxAsyncCommand(()=>NavigationService.Close(this)));
}

Note: The ProfileViewModel inherits from the MvxNavigationViewModel so that it can access the NavigationService.

The NavigationService exposes a Close method which will close the current view model, and the corresponding page. This code is the same irrespective of whether the ProfilePage is shown as a regular or a modal page.

There you have it – you can easily override how pages are displayed in your Xamarin.Forms application using presentation attributes in MvvmCross. There are other attributes that come with MvvmCross to allow you easily display pages in master-detail, tabbed and carousel layouts.

image

In addition to the built in attributes, you can override the MvxPagePresentation attribute and add your own custom layout logic for how pages are presented.

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