Xamarin.Forms : Using NSTimer and MessagingCenter

AppDelegate.cs

using System;
using System.Collections.Generic;
using System.Linq;

using MonoTouch.Foundation;
using MonoTouch.UIKit;

using Xamarin.Forms;
using System.Net;

using MyNamespace.Models;

namespace MyNamespace.iOS
{
    [Register("AppDelegate")]
    public partial class AppDelegate : UIApplicationDelegate
    {
        UIWindow window;
        RootPage _root;
        NSTimer _realtimPowerChartTimer;

        public override bool FinishedLaunching(UIApplication app, NSDictionary options)
        {
            window = new UIWindow(UIScreen.MainScreen.Bounds);

            Forms.Init();
            window.RootViewController = BuildView();
            window.MakeKeyAndVisible();
           return true;
        }
        
        private UINavigationController BuildView()
        {
            _root = new RootPage();
            var controller = _root.CreateViewController();
            UINavigationController nav = new UINavigationController(controller);

            nav.SetNavigationBarHidden(true, false);

            return nav;
        }
        void StartTimers()
        {
            StartRealtimePowerChartTimer();
            // More timers can be started here
        }

        void StopTimers()
        {
            StopRealtimePowerChartTimer();
            // More timers can be stopped here
        }

    
        void StartRealtimePowerChartTimer()
        {
            StopRealtimePowerChartTimer();

            int powerInterval = 15;//seconds
            if (powerInterval > 0)
            {
                _realtimPowerChartTimer = NSTimer.CreateRepeatingScheduledTimer(powerInterval, new NSAction(() =>
                    {
                        MessagingCenter.Send<MessagingCenterMessage<DateTime>>(new MessagingCenterMessage<DateTime>{ Data = DateTime.Now},"REALTIME_POWERCHART_TIMER_MESSAGE");
                    }));
                _realtimPowerChartTimer.Fire();
            }
        }

        void StopRealtimePowerChartTimer()
        {
            if (_realtimPowerChartTimer != null)
            {
                _realtimPowerChartTimer.Invalidate();
                _realtimPowerChartTimer.Dispose();
                _realtimPowerChartTimer = null;
            }
        }

        public override void OnActivated(UIApplication application)
        {
            StartTimers();
        }

        public override void OnResignActivation(UIApplication application)
        {
            StopTimers();
        }
       
    }
}

MessagingCenterMessage.cs

using System;

namespace MyNamespace
{
    public class MessagingCenterMessage<T>
    {
        public MessagingCenterMessage()
        {
        }

        public T Data
        {
            get;
            set;
        }

    }
}

Sample.cs

namespace MyNamespace
{
    public class MyPage:ContentPage
    {
        protected override void OnAppearing()
        {
            base.OnAppearing();
        
            MessagingCenter.Subscribe<MessagingCenterMessage<DateTime>>(this,"REALTIME_POWERCHART_TIMER_MESSAGE", (message) =>
                {
                    //TODO: Do whatever you need to do here
                });
        }
        
        protected override void OnDisappearing()
        {
            MessagingCenter.Unsubscribe<MessagingCenterMessage<DateTime>>(this, "REALTIME_POWERCHART_TIMER_MESSAGE");
            base.OnDisappearing();
        }
    }
    
}

Do not forget

• To stop your timers when your app is send to background,
• To restart the timers when your app is back in the foreground.

Please note, detection method and events of whether your app is active or in the background is platform specific

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