How can I communicate in my app with ViewModel?

问题

How communicate in my app with my ViewModel?

I have this code, sleep and resume of my app

protected override void OnSleep()
{
    MessagingCenter.Send<App, string>(this, "gotosleep", "savedata");
}

in my ViewModel I subscribe to the message, but it does not work. My message is never displayed.

public MyViewModel()
{
    MessagingCenter.Subscribe<App, string>(this, "gotosleep", async (obj, item) =>
    {
        Console.WriteLine("HERE"); 
    });
}

回答1:

You should use the MyViewModel in ContentPage , then Subscribe of MessagingCenter will work .

public MainPage()
{
    InitializeComponent();

    // Use model in Content Page 
    MyViewModel viewModel = new MyViewModel();
}

However ,I find this not works in iOS device , but works in Android .

Here is the Workaround to solve this , you can pass the ContentPageas an attribute for ViewModel when initialiation.

public MyViewModel(MainPage mainPage)
{
    MessagingCenter.Subscribe<App, string>(mainPage, "gotosleep", async (obj, item) =>
    {
        Console.WriteLine("HERE"); 
        await mainPage.DisplayAlert("Message received", "arg=" + item, "OK");
    });
}

In ContentPage , modify as follow :

public MainPage()
{
    InitializeComponent();

    // Use model in Content Page 
    MyViewModel viewModel = new MyViewModel(this);
}

iOS effect :

Android effect :

来源：https://stackoverflow.com/questions/61333329/how-can-i-communicate-in-my-app-with-viewmodel

标签

xamarin.forms