One of the problems with MVVM designs is the inability of the ViewModel to singularly effect change(s) within the View; yes, you can use data-changes through data-binding as a crude-bludgeon, but I'd rather have the right tools for the right job. And that is where Reverse ICommands come in, they allow you to execute an ICommand in your ViewModel and have it trigger a set of specified action(s) in the View - the reverse taxonomy speaks to the fact that reverse ICommands targets your View rather than the ViewModel.
View the sample application here and the source code is also available at Codeplex.
Design: Extends ICommand
In working terms reverse commands are ICommands that acknowledge when they execute - the acknowledgement gives us the ability to respond to them from within the View. Implementation wise, see the IReverseCommand interface, we only add a CommandExecuted event to the ICommand definition.
The other part to reverse commands is the ReverseCommandTrigger behavior, to which we can bind our reverse command and associate one or more Trigger Actions to invoke when the associated command is executed. For example, in the Silverlight Console Project the Console definition has a "Beep" method to which we have to make a beeping sound (obviously) - however, the beeping sound playing control lies in our View and we have to trigger it from our ViewModel. So using reverse commands, first in our ViewModel we declare the reverse command property, initialize it to do nothing in our ViewModel (though you can have it do something else), and execute it when asked to beep (the OnBeep method), have a look:
1: // we define our reverse command as a property in our ViewModel
2: public IReverseCommand BeepReverseCommand { get; private set; }
3:
4: // and we set it up like this in our ViewModel
5: BeepReverseCommand = new ActionCommand(() => { });
6:
7: // we handle Beep call request in our ViewModel and Execute the reverse command
8: protected override void OnBeep() {
9: BeepReverseCommand.Execute();
10: }
Then in our View, the XAML features the Media Element control and a reverse command Trigger which plays the media:
1: <!-- We define the sound playing Media Element -->
2: <MediaElement x:Name="beepAudioMedia"
3: Source="/Resources/beep.wma" Volume="1" AutoPlay="False"/>
4:
5: <!-- And we set up Reverse Command Trigger to play the media -->
6: <nTriggers:ReverseCommandTrigger ReverseCommandBinding="{Binding BeepReverseCommand}">
7: <Behaviors:PlayMediaElementAction TargetName="beepAudioMedia"/>
8: </nTriggers:ReverseCommandTrigger>
As you can see, the workflow of reverse commands is very similar to regular ICommands, except we reverse the receiver/sender roles - the ViewModel here explicitly triggers the Command by calling execute, and the View hooks up Actions to go with the execution of the Command. Also note, all the ICommand implementations in nRoute are reversible because they all implement IReverseCommand - so you can use them as regular and/or reversed commands.
Usage: An Example
The attached sample application is really very simple, it basically has three screens and we control the flow and interaction between them from our ViewModel using ICommands and IReverseCommands. The three screens are realized as Visual States on the View and, as shown below, the Home Screen helps get a user's response to a question, the Trailer Screen shows a movie trailer and the Message Screen self-descriptively shows a message.
Within our ViewModel, the Home command navigates to the Home Screen, the PlayPause command toggles the pause-state of the trailer, the Response command gets the user's response from the Home Screen. The reverse commands help navigate between the screens by changing the Visual State of the View, and the play/pause reverse commands do as they suggest. The idea here is to show how from within the ViewModel we can, without direct dependencies, control the View - like when we navigate away from the Trailer Screen we pause the movie and when we enter into the Trailer Screen we resume the movie all from the ViewModel. Even the playing state of the trailer is kept within the ViewModel, similarly when we get the initial question's response the ViewModel decides which screen to navigate to, and if appropriate which message to show. All this builds on the separation of concerns principle, and though the functionality here is petty, the application of ICommands and IReverseCommands helps us manage the View-ViewModel interaction in both directions.
Summary: ICommands Reversed
Per the principles of MVVM, in SL, we have two primary ways of communication between the View and the ViewModel - one, data-changes signifiers that rely on data-binding, and two, ICommands based invocation of actions from the View. Data-changes as a mechanism for inter-communication works for some cases, but in other cases it is too implicit and imprecise, and this is where ICommands weigh in - they are good for explicit, loosely-coupled, and strongly-typed invoking of actions. However ICommands are one-way creatures designed to be invoked from the View, and handled within the ViewModel.
On the other end, invoking of actions from the ViewModel-to-View is the functional gap that IReverseCommands fulfill, and just like ICommands they are explicit, precise and strongly-typed. Further, as with ICommands you also get ICommands-associated functionality like can-execute checks, enabling-disabling of ICommands, and explicitly defining of dependencies like in nRoute. Now, technically there might be many ways to achieve what reverse commands do but for me the fact that here we have a loosely-coupled setup which makes use of the binding infrastructure in SL and extends one the primary communication mechanism to work in reverse is a handsomely meriting solution.
You can view the Sample App here,
you can download the Sample App source-code here,
and read about the rich support for ICommands in nRoute here.
Posted by Rishi on 07-Jan-10 5:13 AM, 45 Comments