RxUI application as a target for "sharing content"

[ghuntley]

ghuntley [10:31 PM] @flagbug: I'm doing UWP currently, as is @moswald so
between us we got that covered.

flagbug [10:34 PM] @ghuntley: Beware, there are some major issues with RxUI
if your app is a target for a share operation: 

https://msdn.microsoft.com/en-us/library/windows/apps/hh465251.aspx
because of the way that RxApp.MainThreadScheduler and friends work.

ghuntley [10:39 PM] @flagbug: can you give some examples of what you have
observed? Thanks for the heads up, not using sharing in this app fortunately.

flagbug [10:41 PM] @ghuntley: You have to set `RxApp.MainThreadScheduler`
when opening the app as share target, since the share target UI gets its own UI
thread and then, when you close the share UI, you have to set
`RxApp.MainThreadScheduler` back to its previous value, or else your main
application will crash.

See also #965

[flagbug]

As a workaround, I'm doing something like this in the OnShareTargetActivated method:

// Capture the UI thread scheduler of the main application, so we can reset it, when we
// exit the share flyout
IScheduler realMainThreadScheduler = RxApp.MainThreadScheduler;
RxApp.MainThreadScheduler = new WaitForDispatcherScheduler(() => CoreDispatcherScheduler.Current);

// Pretty much the same goes for the DefaulltItemTemplate, since the template is
// generated on a different UI thread, we have to reset it here and change it back when
// closing the share target
// 
// See https://github.com/reactiveui/ReactiveUI/issues/965 for more information
var previousTemplate = AutoDataTemplateBindingHook.DefaultItemTemplate;
AutoDataTemplateBindingHook.DefaultItemTemplate = new Lazy<DataTemplate>(() =>
{
    const string template = "<DataTemplate xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation' xmlns:xaml='using:ReactiveUI'>" +
        "<xaml:ViewModelViewHost ViewModel=\"{Binding}\" VerticalContentAlignment=\"Stretch\" HorizontalContentAlignment=\"Stretch\" IsTabStop=\"False\" />" +
    "</DataTemplate>";
    var assemblyName = "";

    return (DataTemplate)XamlReader.Load(template.Replace("__ASSEMBLYNAME__", assemblyName));
});

Window.Current.Activate();
Window.Current.VisibilityChanged += (sender, ev) =>
{
    if (!ev.Visible)
    {
        RxApp.MainThreadScheduler = realMainThreadScheduler;
        AutoDataTemplateBindingHook.DefaultItemTemplate = previousTemplate;
    }
};