c# - ViewModel 中的构造函数

Answer 1

One of the unusual (opinionated) features of MvvmCross is that by default it uses ViewModel constructor parameters as part of the navigation mechanism.

This is explained with an example in my answer to Passing on variables from ViewModel to another View (MVVMCross)

The basic idea is that when a HomeViewModel requests a navigation using:

private void DoSearch()
{
    RequestNavigate<TwitterViewModel>(new { searchTerm = SearchText });
}

then this will cause a TwitterViewModel to be constructed with the searchTerm passed into the constructor:

public TwitterViewModel(string searchTerm)
{
    StartSearch(searchTerm);
}

At present, this means that every ViewModel must have a public constructor which has either no parameters or which has only string parameters.

So the reason your ViewModel isn't loading is because the MvxDefaultViewModelLocator can't find a suitable constructor for your ViewModel.

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For "services", the MvvmCross framework does provide a simple ioc container which can be most easily accessed using the GetService<IServiceType>() extension methods. For example, in the Twitter sample one of the ViewModel contains:

public class TwitterViewModel
    : MvxViewModel
    , IMvxServiceConsumer<ITwitterSearchProvider>
{
    public TwitterViewModel(string searchTerm)
    {
        StartSearch(searchTerm);
    }

    private ITwitterSearchProvider TwitterSearchProvider
    {
        get { return this.GetService<ITwitterSearchProvider>(); }
    }

    private void StartSearch(string searchTerm)
    {
        if (IsSearching)
            return;

        IsSearching = true;
        TwitterSearchProvider.StartAsyncSearch(searchTerm, Success, Error);
    }

    // ...
}

Similarly, you can see how the conference service data is consumed in the Conference BaseViewModel

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If your preference is to use some other IoC container or some other construction mechanism for your ViewModels, then you can override the ViewModel construction within MvvmCross.

Take a look at this question (and answers) for ideas on how to do this - How to replace MvxDefaultViewModelLocator in MVVMCross application

e.g. if you want to, then it should be fairly easy for you to adjust the MyViewModelLocator example in that question to construct your ViewModel with your service.