MVVM - what is the ideal way for usercontrols to talk to each other

Question

I have a a user control which contains several other user controls. I am using MVVM. Each user control has a corresponding VM. How do these user controls send information to

Answer 1

There are many differenct mechanisms for this, but you should first find out in what layer of your architecture this communication belongs.

One of the purposes of the MVVM framework is that different views can be made over the same viewmodel. Would those usercontrols talk to each other only in the view you are currently implementing, or would they have to talk to each other in other possible views? In the latter case, you want to implement it below the view level, either in the viewmodel or the model itself.

An example of the first case may be if your application is running on a very small display surface. Maybe your user controls have to compete for visual space. If the user clicks one usercontrol to maximize, the others must minimize. This would have nothing to do with the viewmodel, it's just an adaption to the technology.

Or maybe you have different viewmodels with different usercontrols, where things can happen without changing the model. An example of this could be navigation. You have a list of something, and a details pane with fields and command buttons that are connected to the selected item in the list. You may want to unit test the logic of which buttons are enabled for which items. The model isn't concerned with which item you're looking at, only when button commands are pressed, or fields are changed.

The need for this communication may even be in the model itself. Maybe you have denormalized data that are updated because other data are changed. Then the various viewmodels that are in action must change because of ripples of changes in the model.

So, to sum up: "It depends...."

Answer 2

As others have said you have a couple of options.

Exposing DepedencyProperties on your user controls and binding to those properties provides a pure XAML solution in most cases but can introduce some UI dependencies in order for the bindings to see each other

The other option is a decoupled messaging pattern to send messages between ViewModels. I would have your user controls bind to properties on thier own VM's and then on the property change inside that VM it can "publish" a message that notifies other "subscribers" that something has happened and they can react to that message however they want to.

I have a blog post on this very topic if it helps: http://www.bradcunningham.net/2009/11/decoupled-viewmodel-messaging-part-1.html

Answer 3

You can communicate between elements on the UI by using element binding, so assuming a user control you created exposes a property, the other user controls could bind to it. You can configure the binding, use dependency properties instead of basic properties / implement INotifyPropertyChanged but it is in theory possible, but does require some forethought to enable to communication this way.

You will probably find it far easier using a combination of events, code and properties than try a pure declarative way, but in theory possible.