CODEWITHSUNDEEP
mvvmmvvmcross
WickedW
WickedW

Reputation: 2601

How to pass a View to my ViewModel so an I*Service can use that View to do something

https://github.com/brianchance/MvvmCross-UserInteraction is a very nice plugin for showing cross platform Alerts!

But for this question, can we assume it can not use a UIAlertView (or some other top level MessageBox type call on other platforms) but needs to show a Message within a given subsection of the screen (i.e. on IPhone you would need to supply a UIView to the plugin which it will use to show the message within).

So, how would you set this up so the ViewModel knows what View to use as its display container?

As a specific example, if I wanted an Error Service, as so -

public interface IErrorPFService
{
    void Show();
    void Hide();
    void SetErrors(List<Error> errors);
}

and I create a platform specific implementation for it.

If I inject this into my ViewModel so it can control Error Show/Hide/Set how do I tell it the UIView (or equivalent) that I want my Errors to show within?

Can I just expose the IErrorPFService field as a public property and do -

MyViewModel.ErrorPFService = new ErrorPFService(View);

in my ViewDidLoad ...

Or is this coupled incorrectly vs Mvvm Practice?

Upvotes: 1

Views: 97

Answers (2)

WriteEatSleepRepeat
WriteEatSleepRepeat

Reputation: 3143

It would help if you could give an example for the scenario you are describing.

Sometimes, the way you visually want to display something might not be the best way, so if it's possible for you, you might find a different and simpler way, which spares you from having to find a solution regarding what you are describing.

Generally, I always do the best I can to avoid the idea of having to actually pass a 'view' or an abstraction of it, from the view-model to view. Also, cross-platform wise, things can work very different in terms of UI interaction. You can find yourself in a situation when things are complicated just because UI works differently than what you expected.

But let's try find another perspective:

At any given point, the view knows what data \ feature it's displaying. So when you are calling from the view-model an user interaction action (by a service, property change, event, etc) the view should 'expect' it.

For example, the platform specific user interaction implementation is able to get the currently displayed top-view and interact it in a platform specific manner or based a relationship. In your example, the message-box can be displayed in a specific sub-view of the top level view.

In advanced scenarios, I guess you could try to create a cross-platform approach for this, but you should try to put in balance all the abstraction you want to create just for that. Think about doing this as a plan ... Z. If possible. Again, giving an example might help.

Upvotes: 2

Emond
Emond

Reputation: 50682

I would expect the ViewModel to subscribe itself to the ErrorService.

When receiving a message it would expose it in a collection(?) and the View would bind to that collection.

This way the View is unknown to the service and the ViewModel has the chance to influence the View contrary to your solution.

Upvotes: 2

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