How to stop messaging center to stop subscribe multiple time in Xamarin.Forms?

Question

I am creating an application and in that it need to send argument to the other page and send argument using the Messagingcenter when I send using MessagingCenter it call more than one time.

If I am using unscribe than in next time it is not receive next time.

MessagingCenter.Send(this, "Invoke", "Invokedtrue");

MessagingCenter.Subscribe<MyPage, string>(this, "Invoke", async (sender, arg) =>
{
    await Process(arg);
});
MessagingCenter.Unsubscribe<MyPage>(this,"Invoke");


**ListPage**

    private void ViewCell_Tapped(object sender, EventArgs e)
    {
        try
        {
            MessagingCenter.Send(this, "Invoke", "Invokedtrue");
        }
        catch (Exception ex)
        {
            Debug.Write(ex);
        }
    }

**Detail Page**

    MessagingCenter.Subscribe<ListPage, string>(this, "Invoke", async (sender, arg) =>
    {
        await Process(arg);
    });

    private async Task Process(string arg)
    {
        //Here is api call to view detail of particular record

        //Here I unsubscribe the MessagingCenter.
        MessagingCenter.Unsubscribe<ListPage>(this,"Invoke");
    }

I want to send only one time using subscribe and send only one time.

Can anyone look into this and suggest me what should I have to do in that?

Answer 1

Do you want to send and receive message only one time? Then you can use Unsubscribe method after you receive the message. For example, you can do like this:

 MessagingCenter.Subscribe<MyPage, string>(this, "Invoke", async (sender, 
   arg) =>
{
await Process(arg); 
MessagingCenter.Unsubscribe<MyPage>(this,"Invoke");
});

Updated:

On DetailPage, you can call MessagingCenter.Subscribe in method OnAppearing() and call MessagingCenter.Unsubscribe in method OnDisappearing , just as follows:

protected override void OnAppearing()
    {
        base.OnAppearing();
       MessagingCenter.Subscribe<ListPage, string>(this, "Invoke", async (sender, arg) =>
        {
            Debug.Write("123456---->  get one msg");
            DisplayAlert("Alert", "We have received a message.", "OK");
        });
    }

    protected async override void OnDisappearing()
    {
        base.OnDisappearing();
        MessagingCenter.Unsubscribe<ListPage,string>(this, "Invoke");
    }
}

On ListPage

async void OnTap (object sender, EventArgs e)  
    {
        await Navigation.PushAsync(new DetailPage());
        try
        {

            MessagingCenter.Send(this, "Invoke", "Invokedtrue");

            Debug.Write("123456---->  send one msg");

        }
        catch (Exception ex)
        {
            Debug.Write(ex);
        }

    }

Note: when you enter into ListPage, you can try the following code:

  MainPage = new  NavigationPage( new ListPage());

The effect in IOS is :

The link of full demo is here, you can check it.