ios - MvvmCross iOS : NullReference when using a Custom Presenter for Transitions

Question

我遵循了 Stuart 关于 MVVMCross iOS: how to bind a command with Custom transition when switching to different View model 的建议

当我切换到使用从 MvxModalNavSupportTouchViewPresenter 派生的自定义演示者时，调用 .Start() 时，我的应用程序会在 AppDelegate::FinishedLaunching 中崩溃(System.NullReferenceException:对象引用未设置为对象的实例):

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

调用堆栈是:

        0x7 in Cirrious.MvvmCross.Touch.Views.Presenters.MvxTouchViewPresenter.get_CurrentTopViewController C#
        0x2 in Cirrious.MvvmCross.Touch.Views.Presenters.MvxTouchViewPresenter.PresentModalViewController   C#
        0x4E in Cirrious.MvvmCross.Touch.Views.Presenters.MvxModalNavSupportTouchViewPresenter.Show C#
        0xB in Cirrious.MvvmCross.Touch.Views.Presenters.MvxTouchViewPresenter.Show C#
        0x28 in Cirrious.MvvmCross.Touch.Views.MvxTouchViewDispatcher.  C#
        0x28 in Cirrious.MvvmCross.Touch.Views.MvxTouchUIThreadDispatcher.RequestMainThreadAction   C#
        0x24 in Cirrious.MvvmCross.Touch.Views.MvxTouchViewDispatcher.ShowViewModel C#
        0x30 in Cirrious.MvvmCross.ViewModels.MvxNavigatingObject.ShowViewModelImpl C#
        0x6 in Cirrious.MvvmCross.ViewModels.MvxNavigatingObject.ShowViewModel  C#
        0xE in Cirrious.MvvmCross.ViewModels.MvxNavigatingObject.ShowViewModel<TestCustomPresenter.Core.ViewModels.FirstViewModel>  C#
        0x17 in Cirrious.MvvmCross.ViewModels.MvxAppStart<TestCustomPresenter.Core.ViewModels.FirstViewModel>.Start C#
    >   0x3A in TestCustomPresenter.iOS.AppDelegate.FinishedLaunching at e:\Src\test\TestCustomPresenter\TestCustomPresenter.iOS\AppDelegate.cs:54  C#
        0x9F in MonoTouch.UIKit.UIApplication.UIApplicationMain C#
        0x52 in MonoTouch.UIKit.UIApplication.Main at /Developer/MonoTouch/Source/monotouch/src/UIKit/UIApplication.cs:38   C#
        0x8 in TestCustomPresenter.iOS.Application.Main at e:\Src\test\TestCustomPresenter\TestCustomPresenter.iOS\Main.cs:23   C#

据我所知，当前没有顶 View Controller ，但我不知道如何设置它，因为我遵循了斯图尔特答案中的所有步骤。 (我无法回复他的答案，因为我的 9 次代表太少了:( )

我已上传一个测试项目，该项目将错误重现到 http://www.cliffcawley.net/TestCustomPresenter.zip

我需要做什么才能使示例正常工作并在单击按钮时看到自定义转换？

Answer 1

模态视图 Controller 显示在其他东西之上。请参阅https://developer.apple.com/library/ios/featuredarticles/ViewControllerPGforiPhoneOS/ModalViewControllers/ModalViewControllers.html

我还没有下载您的示例，但从您的堆栈跟踪中我猜测您正在将模式显示为第一个 View - 因此没有任何内容可以在其上显示。

对此的一个简单解决方案是首先尝试显示另一个 View - 即使它只是占位符或背景。

除了简单的解决方案之外，如果您是 uikit 的新手，值得花一些时间来破解一个演示应用程序(非 mvx)，您可以在导航 Controller 等中尝试将 View Controller 推送和弹出作为模式，等等 - 一旦您熟悉了有了这个，您就可以在任何 mvx 应用程序中快速编写自己的导航/演示逻辑。