added method to set thumb image

Summary: this change will allow the slider to have different thumb images .

Sets an image for the thumb. It only supports static images
Closes https://github.com/facebook/react-native/pull/3849

Reviewed By: svcscm

Differential Revision: D2665699

Pulled By: nicklockwood

fb-gh-sync-id: 3a767e43170074e2419067d5c8eae61668ebb5e9

React/Views/RCTSlider.h

@@ -20,5 +20,7 @@
 @property (nonatomic, assign) float lastValue;
 
 @property (nonatomic, strong) UIImage *trackImage;
+@property (nonatomic, strong) UIImage *thumbImage;
+
 
 @end

React/Views/RCTSlider.m

@@ -47,4 +47,14 @@
   }
 }
 
+- (void)setThumbImage:(UIImage *)thumbImage
+{
+  [self setThumbImage:thumbImage forState:UIControlStateNormal];
+}
+
+- (UIImage *)thumbImage
+{
+  return [self thumbImageForState:UIControlStateNormal];
+}
+
 @end

React/Views/RCTSliderManager.m

@@ -83,6 +83,7 @@ RCT_EXPORT_VIEW_PROPERTY(minimumTrackTintColor, UIColor);
 RCT_EXPORT_VIEW_PROPERTY(maximumTrackTintColor, UIColor);
 RCT_EXPORT_VIEW_PROPERTY(onValueChange, RCTBubblingEventBlock);
 RCT_EXPORT_VIEW_PROPERTY(onSlidingComplete, RCTBubblingEventBlock);
+RCT_EXPORT_VIEW_PROPERTY(thumbImage, UIImage);
 RCT_CUSTOM_VIEW_PROPERTY(disabled, BOOL, RCTSlider)
 {
   if (json) {