在多个Xamarin.Forms视图上显示时间

Question

I have a Xamarin.Forms application with pages that all use ControlTemplate to implement a custom header. In the header, some pages (and therefore ControlTemplates) have a label for time, that gets updated with a timer in the ViewModel (with a Binding).

What I am currently doing is implementing the time functionality on each ViewModel. Is there a good way to implement this at one place and use it everywhere needed with minimal boilerplate code? I thought about implementing the timer in App.xaml.cs, but I would still have to notify each viewmodel somehow. I just cant come up with an elegant solution.

Answer 1

Because there is no code it is kind of hard to say what a proper solution would be, but you could either work with a base ViewModel and inherit from that?

Or, like you said yourself, have one in your App.xaml.cs, you could update it by the means of the Messaging Center , or implement your own event that fires at each interval and hook into that from your ViewModels.

Answer 2

Here is my solution. It uses a .NET Standard Library instead of PCL. You need .NET Standard for System.Threading.Timer or otherwise you need to use the Xamarin.Forms Timer or 3rd party implementations.

public partial class App : Application
{
    private Timer timer;
    private AutoResetEvent autoEvent = new AutoResetEvent(false); // Configures the state of the event

    public App() 
    {
        this.InitializeComponent();

        // Start timer
        this.timer = new Timer(this.CheckTime, this.autoEvent, 1000, 60000);
    }

    // ViewModels will subscribe to this
    public static event EventHandler<TimeEventArgs> TimeEvent;

    // The TimerCallback needed for the timer. The parameter is not practically needed but needed for the TimerCallback signature.
    private void CheckTime(object state) =>
        this.OnRaiseTimeEvent(new TimeEventArgs(DateTime.Now.ToString("HH:mm")));

    // Invokes the event
    private void OnRaiseTimeEvent(TimeEventArgs e) => 
        TimeEvent?.Invoke(this, e);
}

In the ViewModel

public class ViewModel : BaseViewModel
{
    private string time;

    public ViewModel()
    {
        // Subscribes to the event
        App.TimeEvent += (object o, TimeEventArgs e) =>
        {
            this.Time = e.Time;
        };
    }

    // Bind to this in your view
    public string Time
    { 
        get => this.time;
        set => this.SetProperty(ref this.time, value);
    }
}