MVVMCross Messenger插件Android

Question

Reffering to N=9 what I would like to do is in place of ILocationService I have my own INavigationService and a Navigation Service class.

  public class NavigationService :INavigationService
{
    private readonly IMvxMessenger _navigator;

    public NavigationService(IMvxMessenger navigator)
    {
        _navigator = navigator;
    }

    public void OnNavigation(PurchasesDataEntryViewModel vm)
    {
        var navigationMessage = new NavigationMessage(this,vm);
        _navigator.Publish(navigationMessage);
    }

}

And this is my Messenger class:

   public class NavigationMessage : MvxMessage
{
    public string ShopID { private set; get; }
    public int TotalStock { private set; get; }
    public int TotalPurchases { private set; get; }
    public int Stock1 { private set; get; }
    public int Stock2 { private set; get; }
    public int Stock3 { private set; get; }
    public int Purch1 { private set; get; }
    public int Purch2 { private set; get; }
    public int Purch3 { private set; get; }
    public string Name { private set; get; }
    public string CPrice { private set; get; }
    public int CSales { private set; get; }
    public string BrandID { private set; get; }
    public string CatID { private set; get; }
    public int LadgePur { private set; get; }
    public decimal LPrice { private set; get; }
    public int LPurch { private set; get; }
    public int LSales { private set; get; }
    public int LStock { private set; get; }
    public int LStock1 { private set; get; }
    public string MeasureID { private set; get; }


    public NavigationMessage(object sender, PurchasesDataEntryViewModel vm)
        : base(sender)
    {
        ShopID = vm.ShopID;
        TotalStock = vm.TotalStock;
        TotalPurchases = vm.TotalPurchases;
        Stock1 = vm.Stock1;
        Stock2 = vm.Stock2;
        Stock3 = vm.Stock3;
        //Purch1 = vm.Purch1;
        //Purch2 = vm.Purch2;
        //Purch3 = vm.Purch3;
        Name = vm.Name;
        CPrice = vm.CPrice;
      //  CSales = vm.CSales;
        BrandID = vm.BrandID;
        CatID = vm.CatID;
        LadgePur = vm.LadgePur;
        LPrice = vm.LPrice;
        LPurch = vm.LPurch;
        LSales = vm.LSales;
        LStock = vm.LStock;
        LStock1 = vm.LStock1;
        MeasureID = vm.MeasureID;
    }

What I want to achieve is to send the above values to my subscriber PurchasesDataEntryViewModel

 private readonly IDataService _dataService;
    private readonly MvxSubscriptionToken _token;

    public PurchasesDataEntryViewModel(IDataService dataService,INavigationService service, IMvxMessenger navigator)
    {
        _dataService = dataService;
        _token = navigator.Subscribe<NavigationMessage>(OnNavigationMessage);
    }

    private void OnNavigationMessage(NavigationMessage navigationMessage)
    {
        ShopID = navigationMessage.ShopID;
        .
        .
        .

    }

}

I cannot make it to work. I need to understand the interaction and lifecycle of Publish/Subscribe. How do the two viewmodels will understand how to communicate. I need somehow to tell my Messenger service that I want to send parameters from ViewModel A to ViewModel B through the messenger class. This is the first time I use the plugin so forgive me for the stupid questions.

Answer 1

The use of the Messenger class Publish and Subscribe methods is introduced in https://github.com/MvvmCross/MvvmCross/wiki/MvvmCross-plugins#Messenger

Your use of the calls in the code in the question looks good to me - any message published by your navigation service should be received by all existing PurchasesDataEntryViewModel s

---

Update After your comment about:

when I publish the message I'am expecting my debugger to fire the constructor of the PurchasesDataEntryViewModel class

This is incorrect.

Any messenger (MvvmCross or not) can only pass messages between existing objects - it can't create new listeners dynamically.

If you do want a new object created in response to a message, then you will need to subscribe to that message type from a Factory object - and that Factory object will then need to create the new objects in its message handler.

private readonly MvxSubscriptionToken _token;

public Factory(IMvxMessenger navigator)
{
    _token = navigator.Subscribe<NavigationMessage>(OnNavigationMessage);
}

private void OnNavigationMessage(NavigationMessage navigationMessage)
{
    switch (navigationMessage.NavType)
    {
         case NavType.One:
             var newOne = new One(navigationMessage.Args);
             // use newOne;
             // ...

         // ...
    }
}

For UI navigation, you will need to make sure that whatever Factory objects you write will also create the View as well as the ViewModel .

In practice, if you want to write your own navigation service, then you will need to write a navigation service specific to each platform you support - as each platform has different navigation techniques and lifecycles (Android has Intent s, WP has Uri s, iOS has code-based transitions, etc).

Since MvvmCross already provides a cross-platform navigation service, you may find it easier to use this and to adapt it using a custom presenter - this is discussed in:

• https://github.com/MvvmCross/MvvmCross/wiki/ViewModel--to-ViewModel-navigation
• https://github.com/MvvmCross/MvvmCross/wiki/Customising-using-App-and-Setup#custom-presenters