CODEWITHSUNDEEP
c#xamarinmvvmcross
Mark Silver
Mark Silver

Reputation: 432

Best approach for calling a web service (or similar) with MvvmCross

I've been giving this some thought lately and I was hoping someone who has better knowledge of MvvmCross than myself can shed some light on this. Given the nuances between each mobile platform there are probably a few different factors that can affect this problem. But for this scenario let's assume we want the best approach for a cross platform solution.

So let's say we have a basic View and a ViewModel class setup. Here's an iOS example.

View

public partial class FirstView : MvxViewController<FirstViewModel>
{
    public FirstView(IntPtr handle) : base(handle)
    {
    }

    public override void ViewDidLoad()
    {
        Request = new MvxViewModelInstanceRequest(FirstViewModel.NewInstance());
        base.ViewDidLoad();
    }
}

View Model

public class FirstViewModel : MvxViewModel
{
    public static FirstViewModel NewInstance()
    {
        return Mvx.IocConstruct<FirstViewModel>();
    }

    public FirstViewModel()
    {
    }
}

Now at the loading of this View or at some point just before the view is created we want to fetch some data from the web using a service that we inject using dependency injection; because the displaying of the view depends on that data. Here lies the problem.. at which point from a platform perspective and in the MvvmCross lifecycle would be the most appropriate place to call the web fetch function in the service.

With regards to platform I would assume that we should do it once the view loads. Because if the fetched data is anything other than simple data types it will be inconvenient to work with on Android, as one would have to persist the data to disk and retrieve it after the navigation, due to serialization between activities.

So assuming we called the web fetch during the view loading process. Where is the best place in the MvvmCross architecture to fire it off, that most closely follows the design paradigms. e.g. The View Model. Is there any lifecycle methods that someone could recommend to call it inside as well. Something like the Start method, called after the view model has been created.

Upvotes: 0

Views: 168

Answers (1)

Cheesebaron
Cheesebaron

Reputation: 24470

First of all, I don't understand why you won't let the platform itself instantiate and do it's ViewModel lifecycle instead of creating a new instance of the ViewModel using Mvx.IocConstruct. That method does not invoke the ViewModel lifecycle and will not call neither Init or Start on the ViewModel.

If you let the platform do this for you, first the Init method will be called with the arguments that you set when using ShowViewModel<T>(args).

When ViewDidLoad invokes the Start method will subsequently be called.

This gives you two places to invoke the Service that you inject in the ctor of the ViewModel.

If you want more control over when to download the data, you could create some ICommand's, which you invoke on your ViewModel in any of the ViewController lifecycle methods. This could be in the ViewWillDisappear/ViewDidDisappear method, or you could fetch the data.

There are so many ways you can do this. In the end it is entirely up to you, and you can't possibly know when a user decides to change to another View. However, you can make qualified guesses and try fetch data before the user actually wants it.

There is a nice article for you to read here, by Rob Gibbens on how you could do Resilient network services. It describes how you could speculatively fetch resources based on what the user is doing, and this way have something ready for the user to see when he enters the View. This could be cached data or fresh data that you are fetching after showing the cached version.

In any case, I would suggest you stop loading your ViewModel's with Mvx.IocConstruct and let MvvmCross handle this for you in order to get lifecycle methods invoked.

Upvotes: 4

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