MVVM DataContext Question

Question

Hello,

I'm new to MVVM and I'm trying to understand how the relationships between user controls work.  Let's say I have user control A, which has a dataContext of it's VM (call it A_VM). Serval ui elements on A are bond to A_VM.  Now I have user control B, which contains an instance of A.  I want to bind some of A's properties to B using xaml.  How would I do this if they are all in A_VM?  Do I need to then set the datacontext of A_VM to B?

Thanks,

Eric

Answer 1

You'd typically expose them as dependency properties in the "A" user control.

That being said, if it's purely a matter of "B" owning "A", the B's VM can own the A VM, and use the properties directly (without binding), as another option.

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Reed Copsey, Jr. - http://reedcopsey.com
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Answer 2

If I put the dependency properties in A, won't A need two DataContext's,  B and A_VM?

Eric

Answer 3

No - you can still bind to a DP even if the data context is set.

---

Reed Copsey, Jr. - http://reedcopsey.com
If a post answers your question, please click "Mark As Answer" on that post and "Mark as Helpful".

Answer 4

Binding uses DataContext Property as the Root object. Path experssion in the Binding is relative to this root object  empty or . points the  datacontext. In a control hierarchy if most inner control datacontext is NULL the binding look the  parent controls datacontext, if it s  NULL again then parent and parent  and so until find the first  non null datacontext.  In your example  B contains A;   if you set datacontext  to B then A can also use it if you dont set datacontext for a. So you can acess B model from A then  you can bind the same property on the model both on A or B.

 For example a control in B is a label  another control in A is textbox if they  both have same binding  then both are syncronized when a change occurs on any where   on controls or model. 

Another option is to bind your inner user control(A) datacontext in outcontrol(B) markup  this time AModel instance will be property of B model.