Wednesday, June 30, 2010

event handlers s/d be defined in VM not code-behind

Update - this is the pre-Commanding perspective. In the Commanding paradigm, everything changes.

Event handling logic should ideally be in VM, but VM source code should not mention view objects. See post on model depending on view.

Many xaml developers put event handlers in the code behind the xaml. This is the "lazy" alternative. The code behind is an integral part of the view layer IMO. Its source code mentions numerous view objects.

Code behind also makes liberal use of vm objects (V is over VM). Therefore this class is the perfect melting pot of V and VM. Event handlers and other glue logic can be implemented more easily in code behind than elsewhere.

However, what is too easy is often too easy to be good. Throwing every bit of glue logic into one big class is poor modularization, poor separation of concern, poor layering. As an alternative, it's probably wise to
move as many event handlers as possible out of code behind into the VM, esp if the handler action affects only the VM.

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