Tuesday, April 23, 2013
Using Mvx Messenger for loosely coupled communication
A ViewModel often needs to know of some status change in a service class. For instance you may have a network connection that you keep open to your platform. The ViewModel may want to know when this connection goes up and down.
When your ViewModel is created it subscribes to the event you interested in monitoring.
var connection = Mvx.Resolve<INetworkConnection>();
connection.OnChange += () => RaisePropertyChanged(“IsConnecting”)
This works just fine. When the connection state changes you get notified and raise the fact that the IsConnecting property on your ViewModel has changed. Any UI that is bound to this property can update accordingly.
The problem with this approach is that the INetworkConnection outlives the ViewModel. Whats supposed to happen is when the View leaves the screen it gets garbage collected along with the ViewModel it owns. Now the INetworkConnection has a reference to the ViewModel (created by the event subscription) effectively preventing the ViewModel from getting GC’d. Of course we can make sure we unsubscribe from the event at some point but this has its caveats and becomes messy. We need a better way…
Enter the MvxMessenger
OK so these Message Aggregators have been around for a while. I’ve used TinyMessenger in the past when using V2 of MvvmCross. V3 introduces an MvxMessenger in the form of a Plugin. To get started make sure you register the plugin in your Setup class.
public override void LoadPlugins (IMvxPluginManager pluginManager)
In this example my ViewModel registers interest in a message inside its constructor.
public HomeVM ()
token = MessageHub.Subscribe<CoreStatusMessage> ((d) => CoreStatusChanged(d.CoreStatus));
MessageHub is a property on my ViewModel base class that just returns an instance of the IMvxMessenger plugin we registered in the beginning. Something worth noting is that the token returned is stored in a field within the ViewModel. MvxMessenger uses a WeakReference to the Action<TMessage> when you subscribe. If you don’t hold on to the token the reference gets lost and you wont receive your message. Something else to consider is the thread where your Action delegate (the one you passed to IMvxMessenger.Subscribe) gets called. In this example it will be called on whatever thread the INetworkConnection calls the IMvxMessenger.Publish() method on. Checkout IMvxMessenger.SubscribeOnMainThread which is especially important if your Action code calls into the UI. Using RaisePropertyChanged in your Action code is a prime example.
The Publish() method (already mentioned above) is used to send a message to subscribers. In this example its called by the INetworkConnection when its state changes. CoreStatusMessage is a MvxMessage that you can customise as you please.
void ConnectionChanged (CoreStatusType coreStatus)
IMvxMessenger messenger = Mvx.Resolve(IMvxMessenger);
messenger.Publish (new CoreStatusMessage(sender, coreStatus));
The great thing about this is you don’t have to be concerned with holding on to objects and causing memory leaks. Consider using this instead of regular events in your MvvmCross app’s. Especially where short lived Views need event’s from some of your long living service classes.