[SliderIOS] Apply value after minimum/maximumValue in order to ensure it is properly set

Summary:
`value` is clamped between min/max and so order of prop application matters - `value` always ended up being set first in my tests, and consequently a value outside of the default range 0-1 would not work. So this applies the value when the min/max are set.

[Gist of broken example](https://gist.github.com/brentvatne/fc637b3e21d012966f3a)

![screenshot](http://url.brentvatne.ca/SQPC.png)
^ the second slider here should have it's cursor in the middle

/cc @tadeuzagallo
Closes facebook/react-native#835
Github Author: Brent Vatne <[email protected]>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Author: james-watkin

RCTSlider.h

@@ -0,0 +1,14 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+#import <UIKit/UIKit.h>
+
+@interface RCTSlider : UISlider
+
+@end

RCTSlider.m

@@ -0,0 +1,35 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+#import "RCTSlider.h"
+
+@implementation RCTSlider
+{
+  float _unclippedValue;
+}
+
+- (void)setValue:(float)value
+{
+  _unclippedValue = value;
+  super.value = value;
+}
+
+- (void)setMinimumValue:(float)minimumValue
+{
+  super.minimumValue = minimumValue;
+  super.value = _unclippedValue;
+}
+
+- (void)setMaximumValue:(float)maximumValue
+{
+  super.maximumValue = maximumValue;
+  super.value = _unclippedValue;
+}
+
+@end

RCTSliderManager.m

@@ -11,6 +11,7 @@
 
 #import "RCTBridge.h"
 #import "RCTEventDispatcher.h"
+#import "RCTSlider.h"
 #import "UIView+React.h"
 
 @implementation RCTSliderManager
@@ -19,32 +20,31 @@ @implementation RCTSliderManager
 
 - (UIView *)view
 {
-  UISlider *slider = [[UISlider alloc] init];
+  RCTSlider *slider = [[RCTSlider alloc] init];
   [slider addTarget:self action:@selector(sliderValueChanged:) forControlEvents:UIControlEventValueChanged];
   [slider addTarget:self action:@selector(sliderTouchEnd:) forControlEvents:UIControlEventTouchUpInside];
   return slider;
 }
 
-- (void)sliderValueChanged:(UISlider *)sender
+static void RCTSendSliderEvent(RCTSliderManager *self, UISlider *sender, BOOL continuous)
 {
   NSDictionary *event = @{
     @"target": sender.reactTag,
     @"value": @(sender.value),
-    @"continuous": @YES,
+    @"continuous": @(continuous),
   };
 
   [self.bridge.eventDispatcher sendInputEventWithName:@"topChange" body:event];
 }
 
-- (void)sliderTouchEnd:(UISlider *)sender
+- (void)sliderValueChanged:(UISlider *)sender
 {
-  NSDictionary *event = @{
-    @"target": sender.reactTag,
-    @"value": @(sender.value),
-    @"continuous": @NO,
-  };
+  RCTSendSliderEvent(self, sender, YES);
+}
 
-  [self.bridge.eventDispatcher sendInputEventWithName:@"topChange" body:event];
+- (void)sliderTouchEnd:(UISlider *)sender
+{
+  RCTSendSliderEvent(self, sender, NO);
 }
 
 RCT_EXPORT_VIEW_PROPERTY(value, float);