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POMVVM: How to handle interaction between nested ViewModels?
 

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    <p>I'm been experimenting with the oft-mentioned MVVM pattern and I've been having a hard time defining clear boundaries in some cases. In my application, I have a dialog that allows me to create a Connection to a Controller. There is a ViewModel class for the dialog, which is simple enough. However, the dialog also hosts an additional control (chosen by a <code>ContentTemplateSelector</code>), which varies depending on the particular type of Controller that's being connected. This control has its own ViewModel.</p> <p>The issue I'm encountering is that, when I close the dialog by pressing OK, I need to actually create the requested connection, which requires information captured in the inner Controller-specific ViewModel class. It's tempting to simply have all of the Controller-specific ViewModel classes implement a common interface that constructs the connection, but should the inner ViewModel really be in charge of this construction?</p> <p>My general question is: are there are any generally-accepted design patterns for how ViewModels should interact with eachother, particularly when a 'parent' VM needs help from a 'child' VM in order to know what to do?</p> <hr> <p>EDIT:</p> <p>I did come up with a design that's a bit cleaner than I was originally thinking, but I'm still not sure if it's the 'right' way to do this. I have some back-end services that allow a ContentTemplateSelector to look at a Controller instance and pseudo-magically find a control to display for the connection builder. What was bugging me about this is that my top-level ViewModel would have to look at the <code>DataContext</code> for the generated control and cast it to an appropriate interface, which seems like a bad idea (why should the View's <code>DataContext</code> have anything to do with creating the connection?)</p> <p>I wound up with something like this (simplifying):</p> <pre><code>public interface IController { IControllerConnectionBuilder CreateConnectionBuilder(); } public interface IControllerConnectionBuilder { ControllerConnection BuildConnection(); } </code></pre> <p>I have my inner ViewModel class implement <code>IControllerConnectionBuilder</code> and the Controller returns the inner ViewModel. The top-level ViewModel then visualizes this <code>IControllerConnectionBuilder</code> (via the pseudo-magical mechanism). It still bothers me a little that it's my inner ViewModel performing the building, but at least now my top-level ViewModel doesn't have to know about the dirty details (it doesn't even know or care that the visualized control is using a ViewModel).</p> <p>I welcome additional thoughts if there are ways to clean this up further. It's still not clear to me how much responsibility it's 'okay' for the ViewModel to have.</p>
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    1. USDan Bryant
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    1. COWe ask ourselves this type of question constantly at my job. You've worded this question very well, so I hope you get some good feedback here.
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      1. USSteve Danner
    2. COThankfully this is a pet project, so I have the luxury of exploring different designs. My shop hasn't adopted WPF or MVVM because the overhead and awkwardness at this early stage isn't acceptable for our current schedules. I strongly believe this is a technology that will pay big dividends in productivity once we understand how to use it, but it's such a shift in perspective that it's hard to know where to draw the lines in a design.
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      1. USDan Bryant
 

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