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 each other? I want to avoid writing any code in the xaml code behind. Particularly I am interested in how the controls (inside the main user control) will talk to each other and how will they talk to the container user control.

EDIT: I know that using events-delegates will help me solve this issue. But, I want to avoid writing any code in xaml code-behind.

Answer 1

Typically, it's best to try to reduce the amount of communication between parts, as each time two user controls "talk" to each other, you're introducing a dependency between them.

That being said, there are a couple of things to consider:

• UserControls can always "talk" to their containing control via exposing properties and using DataBinding. This is very nice, since it preserves the MVVM style in all aspects.
• The containing control can use properties to "link" two properties on two user controls together, again, preserving clean boundaries

If you do need to have more explicit communication, there are two main approachs.

1. Implement a service common to both elements, and use Dependency Injection to provide the implementation at runtime. This lets the controls talk to the service, which can in turn, keep the controls synchronized, but also keeps the dependency to a minimum.
2. Use some form of messaging to pass messages between controls. Many MVVM frameworks take this approach, as it decouples sending the message from receiving the message, again, keeping the dependencies to a minimum.