slider fix (#1756)

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Github1625.cs

@@ -0,0 +1,45 @@
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+#if UITEST
+using Xamarin.UITest;
+using NUnit.Framework;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 1625, "Slider value is not changed for the first position change", PlatformAffected.Android)]
+	public class Github1625 : TestContentPage // or TestMasterDetailPage, etc ...
+	{
+		protected override void Init()
+		{
+			var slider = new Slider();
+			slider.Maximum = 10;
+			slider.Minimum = 1;
+			slider.Value = 5;
+
+			var valueLabel = new Label();
+			var stack = new StackLayout { Orientation = StackOrientation.Vertical, Spacing = 15 };
+
+			valueLabel.SetBinding(Label.TextProperty, new Binding("Value", source: slider));
+			stack.Children.Add(valueLabel);
+			stack.Children.Add(slider);
+
+			var button = new Button
+			{
+				Text = "Set to 7",
+				Command = new Command(() => slider.Value = 7)
+			};
+			stack.Children.Add(button);
+
+			var label = new Label
+			{
+				Text = "On start, slider value should show 5 even though SeekBar is 4.996. Sliding back and forth should update the label and the tracker. Tapping on a slider position should do the same. Tapping on the button should show 7 even though SeekBar is 6.994."
+			};
+			stack.Children.Add(label);
+
+			Content = stack;
+		}
+	}
+}

Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems

@@ -261,6 +261,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)GitHub1331.xaml.cs">
       <DependentUpon>GitHub1331.xaml</DependentUpon>
     </Compile>
+    <Compile Include="$(MSBuildThisFileDirectory)Github1625.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)InputTransparentTests.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue1614.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)IsInvokeRequiredRaceCondition.cs" />

Xamarin.Forms.Platform.Android/Renderers/SliderRenderer.cs

@@ -10,7 +10,7 @@ namespace Xamarin.Forms.Platform.Android
 	public class SliderRenderer : ViewRenderer<Slider, SeekBar>, SeekBar.IOnSeekBarChangeListener
 	{
 		double _max, _min;
-		bool _progressChangedOnce;
+		bool _isTrackingChange;
 
 		public SliderRenderer(Context context) : base(context)
 		{
@@ -31,21 +31,18 @@ namespace Xamarin.Forms.Platform.Android
 
 		void SeekBar.IOnSeekBarChangeListener.OnProgressChanged(SeekBar seekBar, int progress, bool fromUser)
 		{
-			if (!_progressChangedOnce)
-			{
-				_progressChangedOnce = true;
-				return;
-			}
-
-			((IElementController)Element).SetValueFromRenderer(Slider.ValueProperty, Value);
+			if (_isTrackingChange)
+				((IElementController)Element).SetValueFromRenderer(Slider.ValueProperty, Value);
 		}
 
 		void SeekBar.IOnSeekBarChangeListener.OnStartTrackingTouch(SeekBar seekBar)
 		{
+			_isTrackingChange = true;
 		}
 
 		void SeekBar.IOnSeekBarChangeListener.OnStopTrackingTouch(SeekBar seekBar)
 		{
+			_isTrackingChange = false;
 		}
 
 		protected override SeekBar CreateNativeControl()