android - 契约(Contract)失败 - fragment 标签为空

Question

在一些 Fragment 导航上遇到了很多困难，并且已经进行了几天，现在没有任何进展。

我有一个名为“NewsViewModel”的 ViewModel，其中包含属性 NewsItemWraps。这是包装模型中的新闻列表。这只是在列表中显示在我的 View 中，我希望用户能够标记新闻项目并查看该新闻的详细信息。

只要用户只看到相同的 NewsDetail，它就可以正常工作。但是，如果您选择第 1 项，导航回列表，选择第 2 项查看详细信息，然后再次导航回列表，我会收到以下错误:契约(Contract)失败 - fragment 标签为空!默认情况下不设置 fragment 标签，您应该在 FragmentTransaction 期间添加标签或在您的 fragment 类中覆盖 UniqueImmutableCacheTag。

    public override void OnBackPressed()
    {
        var currentFragment = SupportFragmentManager.FindFragmentById(Resource.Id.content_frame) as MvxFragment;
        if (currentFragment != null && SupportFragmentManager.BackStackEntryCount >= 1)
        {
            SupportFragmentManager.PopBackStackImmediate(); //<-- This guy tosses the error
            return;
        }

        if (DrawerLayout != null && DrawerLayout.IsDrawerOpen(GravityCompat.Start))
            DrawerLayout.CloseDrawers();
        else
            base.OnBackPressed();
    }

当我在 OnFragmentCreated 中将 Fragment 添加到 BackStack 时，我尝试修改 fragmentInfo 上的标签，但这并没有改变任何东西。无论我是像在 OnFragmentCreated 中那样将它添加到后台堆栈，还是像示例中那样让“addToBackstack”bool 处理，都没有关系。它也不关心它是否被缓存。

 public override void OnFragmentCreated(IMvxCachedFragmentInfo fragmentInfo, Android.Support.V4.App.FragmentTransaction transaction)
        {

            // You can do fragment + transaction based configurations here.
            // Note that, the cached fragment might be reused in another transaction afterwards.

            //Adding the fragment to the stack manually. 
            if (fragmentInfo.Tag.Contains(typeof(NewsDetailsViewModel).Name))
            {
                transaction.AddToBackStack(typeof(NewsDetailsViewModel).Name);
            }

            base.OnFragmentCreated(fragmentInfo, transaction);
            var myCustomInfo = fragmentInfo as CustomFragmentInfo; //How the sample handles the backstack in the factory MainActivityFragmentCacheInfoFactory. 
        }

我在 cacheFactory 中的类型:

            {
                typeof (NewsDetailsViewModel).ToString(),
                new CustomFragmentInfo(typeof (NewsDetailsViewModel).Name,
                                       typeof (NewsDetailsFragment),
                                       typeof (NewsDetailsViewModel), cacheFragment: false, addToBackstack: true, isRoot:false )
            }

NewsViewModel 中的属性:

        private List<NewsItemWrap> _newsItemWraps;
        public List<NewsItemWrap> NewsItemWraps
        {
            get { return _newsItemWraps ?? (_newsItemWraps = new List<NewsItemWrap>()); }
            set { _newsItemWraps = value; RaisePropertyChanged(() => NewsItemWraps); }
        }

我一直在尝试和修改来自 Mvvmcross 的示例:https://github.com/MvvmCross/MvvmCross-AndroidSupport/tree/master/Samples

如有任何指点和建议，我们将不胜感激

完整的堆栈:

System.InvalidOperationException: Contract failed - Fragment tag is null! Fragment tags are not set by default, you should add tag during FragmentTransaction or override UniqueImmutableCacheTag in your Fragment class.

02-19 09:38:56.795 I/MonoDroid(30813): UNHANDLED EXCEPTION: 02-19 09:38:56.795 I/MonoDroid(30813): System.InvalidOperationException: Contract failed - Fragment tag is null! Fragment tags are not set by default, you should add tag during FragmentTransaction or override UniqueImmutableCacheTag in your Fragment class. 02-19 09:38:56.795 I/MonoDroid(30813): at MvvmCross.Droid.Support.V7.Fragging.Fragments.MvxFragmentExtensions.RegisterFragmentViewToCacheIfNeeded (IMvxFragmentView fragmentView) [0x0005a] in :0 02-19 09:38:56.795 I/MonoDroid(30813): at MvvmCross.Droid.Support.V7.Fragging.Fragments.MvxBindingFragmentAdapter.HandleCreateCalled (System.Object sender, MvvmCross.Platform.Core.MvxValueEventArgs1
bundleArgs) [0x00024] in <filename unknown>:0 02-19 09:38:56.795
I/MonoDroid(30813): at
MvvmCross.Platform.Core.MvxDelegateExtensionMethods.Raise[T]
(System.EventHandler1 eventHandler, System.Object sender, MvvmCross.Platform.Core.T value) [0x00000] in V:\Xamarin\MvvmCross\MvvmCross\Platform\Platform\Core\MvxDelegateExtensionMethods.cs:21 02-19 09:38:56.795 I/MonoDroid(30813): at MvvmCross.Droid.Support.V7.Fragging.Fragments.EventSource.MvxEventSourceFragment.OnCreate (Android.OS.Bundle savedInstanceState) [0x00014] in :0 02-19 09:38:56.795 I/MonoDroid(30813): at Android.Support.V4.App.Fragment.n_OnCreate_Landroid_os_Bundle_ (IntPtr jnienv, IntPtr native__this, IntPtr native_savedInstanceState) [0x00011] in :0 02-19 09:38:56.795 I/MonoDroid(30813): at (wrapper dynamic-method) System.Object:136761ed-d521-468a-9257-5d08a4df776b (intptr,intptr,intptr)

Answer 1

这是当前 Nuget 中的一个错误，并已在此提交中修复:https://github.com/MvvmCross/MvvmCross-AndroidSupport/commit/72356791714795c9d711a0c491367fd5c5ebdb82

我们将很快发布更新。