Question

我正在测试现有的Monotouch项目中的MvvmCross。我想开始将项目迁移到Mvvm。

我可以启动应用程序并显示我的第一个（mvvm）屏幕。在那之后，我正在做不同的（非mvvmcross）事情（加载视图等），我想稍后移植。

我现在正尝试将我的一个观点移植到mvvmcross。我创建了一个View“公共类TestView：MvxViewController”和一个ViewModel“公共类TestViewModel：MvxViewModel”

加载该视图时，应用程序崩溃并出现以下错误：

---Message:
Object reference not set to an instance of an object
---Stack Trace:
  at Cirrious.MvvmCross.ViewModels.MvxViewModelLoader.LoadViewModel (Cirrious.MvvmCross.ViewModels.MvxViewModelRequest request, IMvxBundle savedState) [0x00000] in <filename unknown>:0 
  at Cirrious.MvvmCross.Touch.Views.MvxViewControllerExtensionMethods.LoadViewModel (IMvxTouchView touchView) [0x00000] in <filename unknown>:0 
  at Cirrious.MvvmCross.Touch.Views.MvxViewControllerExtensionMethods+<OnViewCreate>c__AnonStorey3.<>m__9 () [0x00000] in <filename unknown>:0 
  at Cirrious.MvvmCross.Views.MvxViewExtensionMethods.OnViewCreate (IMvxView view, System.Func`1 viewModelLoader) [0x00000] in <filename unknown>:0 
  at Cirrious.MvvmCross.Touch.Views.MvxViewControllerExtensionMethods.OnViewCreate (IMvxTouchView touchView) [0x00000] in <filename unknown>:0 
  at Cirrious.MvvmCross.Touch.Views.MvxViewControllerAdapter.HandleViewDidLoadCalled (System.Object sender, System.EventArgs e) [0x00000] in <filename unknown>:0 
  at (wrapper delegate-invoke) <Module>:invoke_void__this___object_EventArgs (object,System.EventArgs)
  at Cirrious.CrossCore.Touch.Views.MvxDelegateExtensionMethods.Raise (System.EventHandler eventHandler, System.Object sender) [0x00000] in <filename unknown>:0 
  at Cirrious.CrossCore.Touch.Views.MvxEventSourceViewController.ViewDidLoad () [0x00000] in <filename unknown>:0 
  at CatalogMVVMTest.Touch.Views.TestView.ViewDidLoad () [0x0001a] in /Users/matthiasvalcke/Projects/MVVMDemos/CatalogMVVMTest/CatalogMVVMTest.Touch/Views/TestView.cs:20

...

调用“base.ViewDidLoad（）;”

时似乎崩溃了

不确定这是否很重要，但在加载第一个视图后，我正在以导航堆栈的根目录加载一个新的（非mvvmcross）视图。

所以现在我正在尝试将MvvmCross视图加载到普通的UIView（控制器）

我也正在关注Mvx诊断输出：

mvx: Diagnostic:   1.08 Request is null - assuming this is a TabBar type situation where ViewDidLoad is called during construction... patching the request now - but watch out for problems with virtual calls during construction

编辑：

如果我只是从MvvmCross编辑一个简单的样本，我也会得到它。例如，如果我只使用一个“FirstView”和“FirstViewModel”来获取基本样本，并且我做了类似的事情，我就会收到错误：

var secondv = new SecondView ();
secondv.View.Frame = new RectangleF (0,250,320,100);
Add (secondv.View);

SecondView只是一个MvxViewController。好像我无法在代码中将MvxViewController中的View添加到另一个MvxViewController。这是对的吗？

任何有想法的人？

Answer 1

好的，而不是这样做：

var secondv = new SecondView (); //(this is a MvxViewController
secondv.View.Frame = new RectangleF (0,250,320,100);
Add (secondv.View);

我必须这样做（我在tabbar示例中找到它）：

var viewModel = new SecondViewModel ();
var secondv = this.CreateViewControllerFor(viewModel) as UIViewController;
secondv.View.Frame = new RectangleF (0,250,320,10);
secondv.View.AutoresizingMask = UIViewAutoresizing.None; //was needed because mvvm is enabling autostretch automatically? 
Add (secondv.View);

对不起，我在其他UIViewControllers上使用了很多UIViewControllers，并且很难理解完整的MvvmCross框架： - ）

我仍然在跟踪诊断输出，但是没有nullpointer，它可以工作：

mvx: Diagnostic:   0.11 Request is null - assuming this is a TabBar type situation where ViewDidLoad is called during construction... patching the request now - but watch out for problems with virtual calls during construction

感谢您的帮助！

Answer 2

您最有可能在Xamarin的最新版本中遇到错误。

https://github.com/slodge/MvvmCross/commit/5786d4964e4262cbf91661f04427e19648acf7df

解决了这个问题

尝试升级到mvvmcross 3.0.11-beta1版本或更高版本 - 或者回滚您的xamarin版本。

有关http://forums.xamarin.com/discussion/comment/26049/#Comment_26049

的更多信息

MvvmCross - 加载ViewModel时出错

2 个答案: