iOS: Add onScroll event to TextInput

Summary:
Corresponding Android PR: #11001

This adds an onScroll event to TextInput which is useful when a multiline TextInput has so much content that it is scrollable.

**Test plan (required)**

Verified the event works properly in a test app. Also, my team uses this event in our app.

Adam Comella
Microsoft Corp.
Closes #11002

Differential Revision: D4203565

Pulled By: ericvicenti

fbshipit-source-id: 7cb5e10325c3b03c6b395cce0f1bacb0528db40a

Libraries/Components/TextInput/TextInput.js

@@ -364,6 +364,12 @@ const TextInput = React.createClass({
      * Invoked on mount and layout changes with `{x, y, width, height}`.
      */
     onLayout: PropTypes.func,
+    /**
+     * Invoked on content scroll with `{ nativeEvent: { contentOffset: { x, y } } }`.
+     * May also contain other properties from ScrollEvent but on Android contentSize
+     * is not provided for performance reasons.
+     */
+    onScroll: PropTypes.func,
     /**
      * The string that will be rendered before text input has been entered.
      */
@@ -652,6 +658,7 @@ const TextInput = React.createClass({
           onSelectionChangeShouldSetResponder={emptyFunction.thatReturnsTrue}
           text={this._getText()}
           dataDetectorTypes={this.props.dataDetectorTypes}
+          onScroll={this._onScroll}
         />;
     }
 
@@ -811,6 +818,10 @@ const TextInput = React.createClass({
   _onTextInput: function(event: Event) {
     this.props.onTextInput && this.props.onTextInput(event);
   },
+
+  _onScroll: function(event: Event) {
+    this.props.onScroll && this.props.onScroll(event);
+  },
 });
 
 var styles = StyleSheet.create({

Libraries/Text/RCTTextView.h

@@ -32,6 +32,7 @@
 @property (nonatomic, copy) RCTDirectEventBlock onContentSizeChange;
 @property (nonatomic, copy) RCTDirectEventBlock onSelectionChange;
 @property (nonatomic, copy) RCTDirectEventBlock onTextInput;
+@property (nonatomic, copy) RCTDirectEventBlock onScroll;
 
 - (instancetype)initWithEventDispatcher:(RCTEventDispatcher *)eventDispatcher NS_DESIGNATED_INITIALIZER;
 

Libraries/Text/RCTTextView.m

@@ -106,6 +106,7 @@ - (instancetype)initWithEventDispatcher:(RCTEventDispatcher *)eventDispatcher
 #if !TARGET_OS_TV
     _scrollView.scrollsToTop = NO;
 #endif
+    _scrollView.delegate = self;
     [_scrollView addSubview:_textView];
 
     [self addSubview:_scrollView];
@@ -141,6 +142,11 @@ - (void)insertReactSubview:(UIView *)subview atIndex:(NSInteger)index
   }
 }
 
+- (void)dealloc
+{
+  _scrollView.delegate = nil;
+}
+
 - (void)removeReactSubview:(UIView *)subview
 {
   [super removeReactSubview:subview];
@@ -694,4 +700,31 @@ - (UIColor *)defaultPlaceholderTextColor
   return [UIColor colorWithRed:0.0/255.0 green:0.0/255.0 blue:0.098/255.0 alpha:0.22];
 }
 
+- (void)scrollViewDidScroll:(UIScrollView *)scrollView
+{
+  if (_onScroll) {
+    _onScroll(@{
+      @"contentOffset": @{
+        @"x": @(scrollView.contentOffset.x),
+        @"y": @(scrollView.contentOffset.y)
+      },
+      @"contentInset": @{
+        @"top": @(_scrollView.contentInset.top),
+        @"left": @(_scrollView.contentInset.left),
+        @"bottom": @(_scrollView.contentInset.bottom),
+        @"right": @(_scrollView.contentInset.right)
+      },
+      @"contentSize": @{
+        @"width": @(_scrollView.contentSize.width),
+        @"height": @(_scrollView.contentSize.height)
+      },
+      @"layoutMeasurement": @{
+        @"width": @(_scrollView.frame.size.width),
+        @"height": @(_scrollView.frame.size.height)
+      },
+      @"zoomScale": @(_scrollView.zoomScale ?: 1),
+    });
+  }
+}
+
 @end

Libraries/Text/RCTTextViewManager.m

@@ -38,6 +38,7 @@ - (UIView *)view
 RCT_EXPORT_VIEW_PROPERTY(onChange, RCTBubblingEventBlock)
 RCT_EXPORT_VIEW_PROPERTY(onContentSizeChange, RCTBubblingEventBlock)
 RCT_EXPORT_VIEW_PROPERTY(onSelectionChange, RCTDirectEventBlock)
+RCT_EXPORT_VIEW_PROPERTY(onScroll, RCTDirectEventBlock)
 RCT_EXPORT_VIEW_PROPERTY(onTextInput, RCTDirectEventBlock)
 RCT_EXPORT_VIEW_PROPERTY(placeholder, NSString)
 RCT_EXPORT_VIEW_PROPERTY(placeholderTextColor, UIColor)