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首页 > 解决方案 > 如何使用 MvvmCross 6 在 Xamarin.iOS 中的代码中正确创建根视图控制器？

xamarin - 如何使用 MvvmCross 6 在 Xamarin.iOS 中的代码中正确创建根视图控制器？

问题描述

我正在尝试用代码编写 Xamarin.iOS UI，我正在关注这个 MS 教程。不幸的是，我无法将我的MvxTabBarViewController( MainViewController) 设置为根控制器。使用 NullReferenceException 实例化控制器结果。

在 MvvmCross 中将控制器设置为根控制器的正确方法是什么？

我的 AppDelegate 如下：

[Register(nameof(AppDelegate))]
public class AppDelegate : MvxApplicationDelegate<Setup, App>
{
    public override void FinishedLaunching(UIApplication application)
    {
        base.FinishedLaunching(application);
    }

    public override bool FinishedLaunching(UIApplication application, NSDictionary launchOptions)
    {
        Window = new UIWindow(UIScreen.MainScreen.Bounds);
        var controller = new MainViewController();
        Window.RootViewController = controller;
        Window.MakeKeyAndVisible();
        Debug.WriteLine("Set root view to MainViewController");
        return true;
    }
}

并且视图控制器是根：

[MvxRootPresentation]
public partial class MainViewController : MvxTabBarViewController<MainViewModel>
{
    private UILabel label;
    //public MainViewController(IntPtr handle) : base(handle)
    //{
    //}

    public override void ViewDidLoad()
    {
        base.ViewDidLoad();
        Debug.WriteLine("Main View Controller loaded");
        AddLabel();
    }

    private void AddLabel()
    {
        label = new UILabel
        {
            Text = "testing",
            TranslatesAutoresizingMaskIntoConstraints = false
        };
        var lblConstraints = new[]
        {
            label.LeadingAnchor.ConstraintEqualTo(anchor:this.View.SafeAreaLayoutGuide.LeadingAnchor, constant:20.0f),
            label.WidthAnchor.ConstraintEqualTo(label.IntrinsicContentSize.Width),
            label.TopAnchor.ConstraintEqualTo(anchor:this.View.SafeAreaLayoutGuide.TopAnchor, constant:20.0f),
            label.HeightAnchor.ConstraintEqualTo(label.IntrinsicContentSize.Height)
            };
        View.AddSubview(label);
    }
}

异常堆栈跟踪：

  at MvvmCross.Mvx.Resolve[TService] () [0x00006] in <8a077b300d9c484ab0471c2d21c3cb26>:0 
  at MvvmCross.Platforms.Ios.Views.MvxBindingViewControllerAdapter..ctor (MvvmCross.Platforms.Ios.Views.Base.IMvxEventSourceViewController eventSource) [0x00034] in <8a077b300d9c484ab0471c2d21c3cb26>:0 
  at MvvmCross.Platforms.Ios.Views.MvxViewControllerAdaptingExtensions.AdaptForBinding (MvvmCross.Platforms.Ios.Views.Base.IMvxEventSourceViewController view) [0x00007] in <8a077b300d9c484ab0471c2d21c3cb26>:0 
  at MvvmCross.Platforms.Ios.Views.MvxBaseTabBarViewController..ctor () [0x00006] in <8a077b300d9c484ab0471c2d21c3cb26>:0 
  at MvvmCross.Platforms.Ios.Views.MvxTabBarViewController..ctor () [0x00000] in <8a077b300d9c484ab0471c2d21c3cb26>:0 
  at MvvmCross.Platforms.Ios.Views.MvxTabBarViewController`1[TViewModel]..ctor () [0x00000] in <8a077b300d9c484ab0471c2d21c3cb26>:0 
  at PushNotifTest.iOS.Views.Main.MainViewController..ctor () <0x13d7f4970 + 0x0004a> in <1d11ff58113e46f6a5a9245eccb8c13f>:0 
  at PushNotifTest.iOS.AppDelegate.FinishedLaunching (UIKit.UIApplication application, Foundation.NSDictionary launchOptions) [0x00017] in /Users/dominik/Projekty/PushNotifTest/src/PushNotifTest.iOS/AppDelegate.cs:28 
  at (wrapper managed-to-native) UIKit.UIApplication.UIApplicationMain(int,string[],intptr,intptr)
  at UIKit.UIApplication.Main (System.String[] args, System.IntPtr principal, System.IntPtr delegate) [0x00005] in /Library/Frameworks/Xamarin.iOS.framework/Versions/11.11.0.280/src/Xamarin.iOS/UIKit/UIApplication.cs:79 
  at UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x0002c] in /Library/Frameworks/Xamarin.iOS.framework/Versions/11.11.0.280/src/Xamarin.iOS/UIKit/UIApplication.cs:63 
  at PushNotifTest.iOS.Application.Main (System.String[] args) [0x00001] in /Users/dominik/Projekty/PushNotifTest/src/PushNotifTest.iOS/Main.cs:9

标签： xamarinxamarin.iosmvvmcross

解决方案

你应该让 MvvmCross 为你做这件事。通常，您会指定应首先在IMvxAppStart派生类中显示哪个 ViewModel。

public class AppStart : IMvxAppStart
{
    private readonly IMvxNavigationService _navigationService;

    public AppStart(IMvxNavigationService navigationService)
    {
        _navigationService = navigationService;
    }

    public void Start(object hint = null)
    {
        try
        {
            _navigationService.Navigate<MainViewModel>().GetAwaiter().GetResult();
        }
        catch (System.Exception e)
        {
        }
    }
}

MainViewModel与您的 ViewController 关联的 ViewModel 将在哪里，即MvxTabBarViewController.

然后只需在 AppDelegate 中执行 MvvmCross 期望您执行的常规仪式：

public override bool FinishedLaunching(UIApplication application, NSDictionary launchOptions)
{
    Window = new UIWindow(UIScreen.MainScreen.Bounds);

    var setup = new Setup(this, Window);
    setup.Initialize();

    var startup = Mvx.Resolve<IMvxAppStart>();
    startup.Start();

    Window.MakeKeyAndVisible();

    return true;
}

因此Setup启动了 IoC 容器和其他相关的 MvvmCross 服务。然后IMvxAppStart导航到第一个 ViewController。

根据您的属性MvxTabBarViewController，它将被包装在 NavigationViewController 中。你可以用它来控制它MvxRootPresentationAttribute。

[MvxRootPresentation(WrapInNavigationController = true)]
public partial class MainViewController : MvxTabBarViewController<MainViewModel>

编辑：

MvvmCross 6.0 没有做常规的仪式，而是让这部分变得更容易。所以删除 AppDelegate 的主体并像这样创建它：

[Register("AppDelegate")]
public partial class AppDelegate : MvxApplicationDelegate<MvxIosSetup<App>, App>
{
}

在您的核心中，您将拥有一个App实现MvxApplication. 这里调用RegisterAppStart<TViewModel>()覆盖Initialize()：

public class App : MvxApplication
{
    public override void Initialize()
    {
        RegisterAppStart<RootViewModel>();
    }

然后不要创建自己的AppStart类。您可以Startup()在App课堂上进行 UI Bound 启动。

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