All the MvvmCross platforms require that UI-actions get marshalled back on to the UI Thread/Apartment - but each platform does this differently....
To work around this, MvvmCross provides a cross-platform way to do this - using an IMvxViewDispatcherProvider
injected object.
For example, on WindowsPhone IMvxViewDispatcherProvider
is provided ultimately by MvxMainThreadDispatcher
in https://github.com/slodge/MvvmCross/blob/vnext/Cirrious/Cirrious.MvvmCross.WindowsPhone/Views/MvxMainThreadDispatcher.cs
This implements the InvokeOnMainThread
using:
private bool InvokeOrBeginInvoke(Action action)
{
if (_uiDispatcher.CheckAccess())
action();
else
_uiDispatcher.BeginInvoke(action);
return true;
}
For code in ViewModels:
- your
ViewModel
inherits from MvxViewModel
- the
MvxViewModel
inherits from an MvxApplicationObject
- the
MvxApplicationObject
inherits from an MvxNotifyPropertyChanged
- the
MvxNotifyPropertyChanged
object inherits from an MvxMainThreadDispatchingObject
MvxMainThreadDispatchingObject
is https://github.com/slodge/MvvmCross/blob/vnext/Cirrious/Cirrious.MvvmCross/ViewModels/MvxMainThreadDispatchingObject.cs
public abstract class MvxMainThreadDispatchingObject
: IMvxServiceConsumer<IMvxViewDispatcherProvider>
{
protected IMvxViewDispatcher ViewDispatcher
{
get { return this.GetService().Dispatcher; }
}
protected void InvokeOnMainThread(Action action)
{
if (ViewDispatcher != null)
ViewDispatcher.RequestMainThreadAction(action);
}
}
So... your ViewModel can just call InvokeOnMainThread(() => DoStuff());
One further point to note is that MvvmCross automatically does UI thread conversions for property updates which are signalled in a MvxViewModel
(or indeed in any MvxNotifyPropertyChanged
object) through the RaisePropertyChanged()
methods - see:
protected void RaisePropertyChanged(string whichProperty)
{
// check for subscription before going multithreaded
if (PropertyChanged == null)
return;
InvokeOnMainThread(
() =>
{
var handler = PropertyChanged;
if (handler != null)
handler(this, new PropertyChangedEventArgs(whichProperty));
});
}
in https://github.com/slodge/MvvmCross/blob/vnext/Cirrious/Cirrious.MvvmCross/ViewModels/MvxNotifyPropertyChanged.cs
This automatic marshalling of RaisePropertyChanged()
calls works well for most situations, but can be a bit inefficient if you Raise a lot of changed properties from a background thread - it can lead to a lot of thread context switching. It's not something you need to be aware of in most of your code - but if you ever do find it is a problem, then it can help to change code like:
MyProperty1 = newValue1;
MyProperty2 = newValue2;
// ...
MyProperty10 = newValue10;
to:
InvokeOnMainThread(() => {
MyProperty1 = newValue1;
MyProperty2 = newValue2;
// ...
MyProperty10 = newValue10;
});
If you ever use ObservableCollection
, then please note that MvvmCross does not do any thread marshalling for the INotifyPropertyChanged
or INotifyCollectionChanged
events fired by these classes - so it's up to you as a developer to marshall these changes.
The reason: ObservableCollection
exists in the MS and Mono code bases - so there is no easy way that MvvmCross can change these existing implementations.