MvvmCross MvxWindowsPage compilation e

问题:

I am writing a Xamarin.iOS, Xamarin.Android and UWP cross-platform application with the MvvmCross framework.

I am making a LoginPage which has a LoginViewModel. In the Xamarin.iOS, Xamarin.Android projects, the binding of the ViewModel and the View with below works just fine

public class LoginActivity : MvxAppCompatActivity<LoginViewModel> 

public partial class LoginViewController : MvxViewController<LoginViewModel>  

Trying to do the same as above on UWP project, I get some error.

In XAML:

<views:MvxWindowsPage
x:TypeArguments="viewModels:LoginViewModel" x:Class="MyApp.UWP.Views.LoginView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:views="using:MvvmCross.WindowsUWP.Views"
xmlns:viewModels="using:MyApp.PresentationCore.ViewModels"
mc:Ignorable="d">

And my C# code is

public sealed partial class LoginView : MvxWindowsPage<LoginViewModel>

But I get compilation errors. How can I resolve them?

• `Unknown member 'TypeArguments' on element 'MvxWindowsPage'
• The name "LoginViewModel" does not exist in the namespace "using:MyApp.PresentationCore.ViewModels".
• GenericArguments[0], 'System.Object', on 'MvvmCross.WindowsUWP.Views.MvxWindowsPage`1[TViewModel]' violates the constraint of type 'TViewModel'.

I think the errors are a little ambiguous because at the first error there is no templated version, but the third error is about a template constraint violation.

I know there is an option binding the ViewModel and the View with naming convention or attributes, but I would like to use this strongly typed solution.

回答1:

Unfortunately I believe UWP does not support TypeArguments and subsequently generic types parameters for a base pages. You can check out the Prism GitHub thread where they discuss it. So you will have to go with one of the other registration options.

---

Mvvmcross offers various alternative approaches for registering your View to a ViewModel. During initialisation of Mvvmcross it will attempt to register your ViewModel to your View using MvxViewModelViewTypeFinder in the following order:

Attribute based registration:

You can add MvxViewFor attribute to your page class.

[MvxViewFor(typeof(FirstViewModel))]
public sealed partial class FirstView : MvxWindowsPage
{
    public FirstView()
    {
        this.InitializeComponent();
    }
}

Concrete type based registration:

You can register your View to your ViewModel by specifying the concrete type of the ViewModel.

public sealed partial class FirstView : MvxWindowsPage
{
    public new FirstViewModel ViewModel => base.ViewModel as FirstViewModel;

    public FirstView()
    {
        this.InitializeComponent();
    }
}

Or in the case of Android and iOS where generic base class can be used:

// Android
public class FirstActivity : MvxAppCompatActivity<FirstViewModel>

// iOS
public class FirstViewController : MvxViewController<FirstViewModel>

Convention based registration:

You can have your view and ViewModel follow the same naming convention and Mvvmcross will map them for you (xxxView and xxxViewModel)

View

public sealed partial class FirstView : MvxWindowsPage
{
    public FirstView()
    {
        this.InitializeComponent();
    }
}

ViewModel

public class FirstViewModel : MvxViewModel