Make core Ticker abstract and public (#116)

* Make core Ticker abstract and public
Make the core Ticker abstract and public so it can be inherited by platform
implementations; remove now-unused parts of original Ticker; add CreateTicker
to IPlatformServices; remove unused CreateTimer methods from IPlatformServices

* Add docs for Ticker

* Remove unnecessary Ticker.Default set

* Move Ticker into Internals

* Update Ticker docs

* Remove old Ticker docs

* Remove commented code

5 files changed, 11 insertions, 34 deletions

diff --git a/Xamarin.Forms.Core/AnimationExtensions.cs b/Xamarin.Forms.Core/AnimationExtensions.cs
index 957d002e..2256cb73 100644
--- a/Xamarin.Forms.Core/AnimationExtensions.cs
+++ b/Xamarin.Forms.Core/AnimationExtensions.cs
@@ -26,6 +26,7 @@
 
 using System;
 using System.Collections.Generic;
+using Xamarin.Forms.Internals;
 
 namespace Xamarin.Forms
 {
diff --git a/Xamarin.Forms.Core/IPlatformServices.cs b/Xamarin.Forms.Core/IPlatformServices.cs
index e4a3ce0b..a01baa02 100644
--- a/Xamarin.Forms.Core/IPlatformServices.cs
+++ b/Xamarin.Forms.Core/IPlatformServices.cs
@@ -3,6 +3,7 @@ using System.IO;
 using System.Reflection;
 using System.Threading;
 using System.Threading.Tasks;
+using Xamarin.Forms.Internals;
 
 namespace Xamarin.Forms
 {
@@ -12,12 +13,7 @@ namespace Xamarin.Forms
 
 		void BeginInvokeOnMainThread(Action action);
 
-		//this will go once Timer is included in Pcl profiles
-		ITimer CreateTimer(Action<object> callback);
-		ITimer CreateTimer(Action<object> callback, object state, int dueTime, int period);
-		ITimer CreateTimer(Action<object> callback, object state, long dueTime, long period);
-		ITimer CreateTimer(Action<object> callback, object state, TimeSpan dueTime, TimeSpan period);
-		ITimer CreateTimer(Action<object> callback, object state, uint dueTime, uint period);
+		Ticker CreateTicker();
 
 		Assembly[] GetAssemblies();
 
diff --git a/Xamarin.Forms.Core/Ticker.cs b/Xamarin.Forms.Core/Internals/Ticker.cs
index ed828f8a..7fe60cc0 100644
--- a/Xamarin.Forms.Core/Ticker.cs
+++ b/Xamarin.Forms.Core/Internals/Ticker.cs
@@ -2,26 +2,21 @@ using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
-using System.Threading;
 
-namespace Xamarin.Forms
+namespace Xamarin.Forms.Internals
 {
-	internal class Ticker
+	public abstract class Ticker
 	{
 		static Ticker s_ticker;
 		readonly Stopwatch _stopwatch;
-		readonly SynchronizationContext _sync;
 		readonly List<Tuple<int, Func<long, bool>>> _timeouts;
 
-		readonly ITimer _timer;
 		int _count;
 		bool _enabled;
 
 		internal Ticker()
 		{
-			_sync = SynchronizationContext.Current;
 			_count = 0;
-			_timer = Device.PlatformServices.CreateTimer(HandleElapsed, null, Timeout.Infinite, Timeout.Infinite);
 			_timeouts = new List<Tuple<int, Func<long, bool>>>();
 
 			_stopwatch = new Stopwatch();
@@ -30,7 +25,7 @@ namespace Xamarin.Forms
 		public static Ticker Default
 		{
 			internal set { s_ticker = value; }
-			get { return s_ticker ?? (s_ticker = new Ticker()); }
+			get { return s_ticker ?? (s_ticker =  Device.PlatformServices.CreateTicker()); }
 		}
 
 		public virtual int Insert(Func<long, bool> timeout)
@@ -61,16 +56,10 @@ namespace Xamarin.Forms
 			});
 		}
 
-		protected virtual void DisableTimer()
-		{
-			_timer.Change(Timeout.Infinite, Timeout.Infinite);
-		}
-
-		protected virtual void EnableTimer()
-		{
-			_timer.Change(16, 16);
-		}
+		protected abstract void DisableTimer();
 
+		protected abstract void EnableTimer();
+		
 		protected void SendSignals(int timestep = -1)
 		{
 			long step = timestep >= 0 ? timestep : _stopwatch.ElapsedMilliseconds;
@@ -103,15 +92,5 @@ namespace Xamarin.Forms
 			_stopwatch.Start();
 			EnableTimer();
 		}
-
-		void HandleElapsed(object state)
-		{
-			if (_timeouts.Count > 0)
-			{
-				_sync.Post(o => SendSignals(), null);
-				_stopwatch.Reset();
-				_stopwatch.Start();
-			}
-		}
 	}
 }
\ No newline at end of file
diff --git a/Xamarin.Forms.Core/Tweener.cs b/Xamarin.Forms.Core/Tweener.cs
index 73ed7853..b90e7a8a 100644
--- a/Xamarin.Forms.Core/Tweener.cs
+++ b/Xamarin.Forms.Core/Tweener.cs
@@ -25,6 +25,7 @@
 // THE SOFTWARE.
 
 using System;
+using Xamarin.Forms.Internals;
 
 namespace Xamarin.Forms
 {
diff --git a/Xamarin.Forms.Core/Xamarin.Forms.Core.csproj b/Xamarin.Forms.Core/Xamarin.Forms.Core.csproj
index b1925578..c9f7a5bc 100644
--- a/Xamarin.Forms.Core/Xamarin.Forms.Core.csproj
+++ b/Xamarin.Forms.Core/Xamarin.Forms.Core.csproj
@@ -232,7 +232,7 @@
     <Compile Include="TextChangedEventArgs.cs" />
     <Compile Include="TextKeyboard.cs" />
     <Compile Include="ThicknessTypeConverter.cs" />
-    <Compile Include="Ticker.cs" />
+    <Compile Include="Internals\Ticker.cs" />
     <Compile Include="ToggledEventArgs.cs" />
     <Compile Include="ToolbarItemEventArgs.cs" />
     <Compile Include="ToolbarItemOrder.cs" />