c# - MessageCenter 订阅方法没有发生

Question

我以前从未使用过消息中心，并且在应用程序之间传递消息时可能会犯一些荒谬的错误。我尝试从 Xamarin.Android 向 Xamarin.Forms 发送消息。

Xamarin.Android 项目代码：

MessagingCenter.Send(Xamarin.Forms.Application.Current, "MessageReceived", "new message");

Xamarin.Forms：

public partial class App : Application
{

    public App()
    {

        InitializeComponent();

        MainPage = (new MasterDetailPage()
        {
            Master = new MenuPage { Title = "Title" },
            Detail = new NavigationPage(new XxmsApp.MainPage()) { BarBackgroundColor = Color.Black }
        });

        MessagingCenter.Subscribe<App>(
            this,                                                     
            "MessageReceived",        
            (sender) => 
            {
                var StartPage = (((MainPage as MasterDetailPage).Detail as NavigationPage).RootPage as XxmsApp.MainPage);

                StartPage.DisplayAlert(
                    "message",
                    "message received",
                    "ok");
            });    
    }

    // ...

}

但是指定为第三个参数的Subscribe<App>方法永远不会被调用。我也试过这个：

        Device.BeginInvokeOnMainThread(() =>
        {
            MessagingCenter.Send(Xamarin.Forms.Application.Current, "MessageReceived", "new message");
        });

但什么都没有。

我究竟做错了什么？

Answer 1

在您的 android 项目代码中

MessagingCenter.Send<object, object>(this, "MessageReceived", "new message");

在您的共享代码中

 MessagingCenter.Subscribe<object, object>(this, "MessageReceived", (sender, args) =>
    {

var StartPage = (((MainPage as MasterDetailPage).Detail as NavigationPage).RootPage as XxmsApp.MainPage);

                StartPage.DisplayAlert(
                    "message",
                    "message received",
                    "ok");
    });