How can WPF Converters be used in an MVVM pattern?

 

Let's say I have a View that is bound to ViewModel A which has an observable collection Customers .

An advantage of this MVVM pattern is that I can also bind the View to ViewModel B which fills it with different data.

But what if in my View converter Converters to display my customers, eg I have a "ContractToCustomerConverter" that accepts a Contract and returns the appropriate Customer to be displayed.

The problem with this is that the converter exists outside the MVVM pattern and thus doesn't know that my ViewModel has another source for customers.

  • is there a way for the View to pass the ViewModel into the Converter so that it participates in the decoupling that the MVVM pattern provides?
  • is there a way for me to somehow include the Converter in my ViewModel so that the converter uses the current dependencies which ViewModel has available?
  • or are converters just glorified code-behind and thus not used in the MVVM pattern, so if you are using MVVM then you just create your own "converters" (methods on your ViewModel class) which return things like Image objects, Visibility objects, FlowDocuments, etc. to be used on the view, instead of using converters at all?

    • (I came upon these questions after seeing the use of Converters in the WPF demo application that comes with the MVVM Template Toolkit download, see the "Messenger Sample" after unpacking it.)

    I usually don't use converters at all in MVVM, except for pure UI tasks (like BooleanToVisibilityConverter for instance). IMHO you should rather declare a Customer property of type CustomerViewModel in your ContractViewModel, rather than use a ContractToCustomerConverter

    In this conversation there is a comment that agrees with Kent's position, not to use Converters at all, interesting:

    A ViewModel is basically a value converter on steroids. It takes "raw" data and converts it into something presentation-friendly, and vice-versa. If you ever find yourself binding an element's property to a ViewModel's property, and you're using a value converter, stop! Why not just create a property on the ViewModel that exposes the "formatted" data, and then drop the value converter altogether?

    And in this conversation:

    The only place I can see a use for value converters in an MVVM architecture is cross-element bindings. If I'm binding the Visibility of a panel to the IsChecked of a CheckBox, then I will need to use the BooleanToVisibilityConverter.

    Converters should rarely be used with MVVM. In fact, I strive not to use them at all. The VM should be doing everything the view needs to get its job done. If the view needs a Customer based on a Contract , there should be a Customer property on the VM that is updated by VM logic whenever the Contract changes.

    An advantage of this MVVM pattern is that I can also bind the View to ViewModel B which fills it with different data.

    I dispute that claim. In my experience, views are not shared across different VM types, and nor is that a goal of MVVM.

    链接地址: http://www.djcxy.com/p/56210.html

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