rename and extend new maintain visible content position feature

Summary:
Builds off of cae7179

- Make the prop a dictionary for more configuration options
- Rename `maintainPositionAtOrBeyondIndex` -> `maintainVisibleContentPosition` + `minIndexForVisible`
- Add autoscroll threshold feature

Given the async native of RN JS and background layout, there is no way to trigger the scrollTo from JS without risking a delay, so we add the feature in native code.

== Test Plan ==
ScrollViewExample:
https://youtu.be/pmY8pxC9PRs

Reviewed By: shergin

Differential Revision: D6729160

fbshipit-source-id: 70f9bae460ce84567857a4f696da78ce9b3b834c

Libraries/Components/ScrollView/ScrollView.js

@@ -234,18 +234,33 @@ const ScrollView = createReactClass({
      */
     keyboardShouldPersistTaps: PropTypes.oneOf(['always', 'never', 'handled', false, true]),
     /**
-     * When non-null, the scroll view will adjust the scroll position so that the content at or
-     * beyond the specified index that is currently visible will not change position. This is useful
-     * for lists that are loading content in both directions, e.g. a chat thread, where new messages
-     * coming in might otherwise cause the scroll position to jump. A value of 1 can be used to skip
-     * a spinner that does not need to maintain position. The default value is null.
+     * When set, the scroll view will adjust the scroll position so that the first child that is
+     * currently visible and at or beyond `minIndexForVisible` will not change position. This is
+     * useful for lists that are loading content in both directions, e.g. a chat thread, where new
+     * messages coming in might otherwise cause the scroll position to jump. A value of 0 is common,
+     * but other values such as 1 can be used to skip loading spinners or other content that should
+     * not maintain position.
      *
-     * Caveat: reordering elements in the scrollview with this enabled will probably cause jumpiness
-     * and jank. It can be fixed, but there are currently no plans to do so.
+     * The optional `autoscrollToTopThreshold` can be used to make the content automatically scroll
+     * to the top after making the adjustment if the user was within the threshold of the top before
+     * the adjustment was made. This is also useful for chat-like applications where you want to see
+     * new messages scroll into place, but not if the user has scrolled up a ways and it would be
+     * disruptive to scroll a bunch.
+     *
+     * Caveat 1: Reordering elements in the scrollview with this enabled will probably cause
+     * jumpiness and jank. It can be fixed, but there are currently no plans to do so. For now,
+     * don't re-order the content of any ScrollViews or Lists that use this feature.
+     *
+     * Caveat 2: This simply uses `contentOffset` and `frame.origin` in native code to compute
+     * visibility. Occlusion, transforms, and other complexity won't be taken into account as to
+     * whether content is "visible" or not.
      *
      * @platform ios
      */
-    maintainPositionAtOrBeyondIndex: PropTypes.number,
+    maintainVisibleContentPosition: PropTypes.shape({
+      minIndexForVisible: PropTypes.number.isRequired,
+      autoscrollToTopThreshold: PropTypes.number,
+    }),
     /**
      * The maximum allowed zoom scale. The default value is 1.0.
      * @platform ios

RNTester/js/ScrollViewExample.js

@@ -131,7 +131,7 @@ if (Platform.OS === 'ios') {
   exports.examples.push({
     title: '<ScrollView> smooth bi-directional content loading\n',
     description:
-      'The `maintainPositionAtOrBeyondIndex` prop allows insertions to either end of the content ' +
+      'The `maintainVisibleContentPosition` prop allows insertions to either end of the content ' +
       'without causing the visible content to jump. Re-ordering is not supported.',
     render: function() {
       let itemCount = 6;
@@ -146,7 +146,10 @@ if (Platform.OS === 'ios') {
             <View>
               <ScrollView
                 automaticallyAdjustContentInsets={false}
-                maintainPositionAtOrBeyondIndex={1}
+                maintainVisibleContentPosition={{
+                  minIndexForVisible: 1,
+                  autoscrollToTopThreshold: 10,
+                }}
                 style={styles.scrollView}>
                 <ActivityIndicator style={{height: 40}} />
                 {this.state.items.map(item =>
@@ -156,9 +159,12 @@ if (Platform.OS === 'ios') {
               <ScrollView
                 horizontal={true}
                 automaticallyAdjustContentInsets={false}
-                maintainPositionAtOrBeyondIndex={1}
+                maintainVisibleContentPosition={{
+                  minIndexForVisible: 1,
+                  autoscrollToTopThreshold: 10,
+                }}
                 style={[styles.scrollView, styles.horizontalScrollView]}>
-                <ActivityIndicator style={{height: 40}} />
+                <ActivityIndicator style={{width: 40}} />
                 {this.state.items.map(item =>
                   React.cloneElement(item, {key: item.props.msg, style: null}),
                 )}

React/Views/ScrollView/RCTScrollView.h

@@ -45,7 +45,7 @@
 @property (nonatomic, assign) BOOL DEPRECATED_sendUpdatedChildFrames;
 @property (nonatomic, assign) NSTimeInterval scrollEventThrottle;
 @property (nonatomic, assign) BOOL centerContent;
-@property (nonatomic, copy) NSNumber *maintainPositionAtOrBeyondIndex;
+@property (nonatomic, copy) NSDictionary *maintainVisibleContentPosition;
 @property (nonatomic, assign) int snapToInterval;
 @property (nonatomic, copy) NSString *snapToAlignment;
 

React/Views/ScrollView/RCTScrollView.m

@@ -911,16 +911,16 @@ - (void)updateContentOffsetIfNeeded
   }
 }
 
-// maintainPositionAtOrBeyondIndex is used to allow seamless loading of content from both ends of
+// maintainVisibleContentPosition is used to allow seamless loading of content from both ends of
 // the scrollview without the visible content jumping in position.
-- (void)setMaintainPositionAtOrBeyondIndex:(NSNumber *)maintainPositionAtOrBeyondIndex
+- (void)setMaintainVisibleContentPosition:(NSDictionary *)maintainVisibleContentPosition
 {
-  if (maintainPositionAtOrBeyondIndex != nil) {
+  if (maintainVisibleContentPosition != nil && _maintainVisibleContentPosition == nil) {
     [_eventDispatcher.bridge.uiManager.observerCoordinator addObserver:self];
-  } else {
+  } else if (maintainVisibleContentPosition == nil && _maintainVisibleContentPosition != nil) {
     [_eventDispatcher.bridge.uiManager.observerCoordinator removeObserver:self];
   }
-  _maintainPositionAtOrBeyondIndex = maintainPositionAtOrBeyondIndex;
+  _maintainVisibleContentPosition = maintainVisibleContentPosition;
 }
 
 #pragma mark - RCTUIManagerObserver
@@ -930,7 +930,7 @@ - (void)uiManagerWillPerformMounting:(RCTUIManager *)manager
   RCTAssertUIManagerQueue();
   [manager prependUIBlock:^(RCTUIManager *uiManager, NSDictionary<NSNumber *, UIView *> *viewRegistry) {
     BOOL horz = [self isHorizontal:self->_scrollView];
-    NSUInteger minIdx = [self->_maintainPositionAtOrBeyondIndex integerValue];
+    NSUInteger minIdx = [self->_maintainVisibleContentPosition[@"minIndexForVisible"] integerValue];
     for (NSUInteger ii = minIdx; ii < self->_contentView.subviews.count; ++ii) {
       // Find the first entirely visible view. This must be done after we update the content offset
       // or it will tend to grab rows that were made visible by the shift in position
@@ -946,9 +946,10 @@ - (void)uiManagerWillPerformMounting:(RCTUIManager *)manager
     }
   }];
   [manager addUIBlock:^(RCTUIManager *uiManager, NSDictionary<NSNumber *, UIView *> *viewRegistry) {
-    if (self->_maintainPositionAtOrBeyondIndex == nil) {
+    if (self->_maintainVisibleContentPosition == nil) {
       return; // The prop might have changed in the previous UIBlocks, so need to abort here.
     }
+    NSNumber *autoscrollThreshold = self->_maintainVisibleContentPosition[@"autoscrollToTopThreshold"];
     // TODO: detect and handle/ignore re-ordering
     if ([self isHorizontal:self->_scrollView]) {
       CGFloat deltaX = self->_firstVisibleView.frame.origin.x - self->_prevFirstVisibleFrame.origin.x;
@@ -957,15 +958,27 @@ - (void)uiManagerWillPerformMounting:(RCTUIManager *)manager
           self->_scrollView.contentOffset.x + deltaX,
           self->_scrollView.contentOffset.y
         );
+        if (autoscrollThreshold != nil) {
+          // If the offset WAS within the threshold of the start, animate to the start.
+          if (self->_scrollView.contentOffset.x - deltaX <= [autoscrollThreshold integerValue]) {
+            [self scrollToOffset:CGPointMake(0, self->_scrollView.contentOffset.y) animated:YES];
+          }
+        }
       }
     } else {
       CGRect newFrame = self->_firstVisibleView.frame;
       CGFloat deltaY = newFrame.origin.y - self->_prevFirstVisibleFrame.origin.y;
-      if (ABS(deltaY) > 0.1 || deltaY != 0.0) {
+      if (ABS(deltaY) > 0.1) {
         self->_scrollView.contentOffset = CGPointMake(
           self->_scrollView.contentOffset.x,
           self->_scrollView.contentOffset.y + deltaY
         );
+        if (autoscrollThreshold != nil) {
+          // If the offset WAS within the threshold of the start, animate to the start.
+          if (self->_scrollView.contentOffset.y - deltaY <= [autoscrollThreshold integerValue]) {
+            [self scrollToOffset:CGPointMake(self->_scrollView.contentOffset.x, 0) animated:YES];
+          }
+        }
       }
     }
   }];

React/Views/ScrollView/RCTScrollViewManager.m

@@ -62,7 +62,7 @@ - (UIView *)view
 RCT_EXPORT_VIEW_PROPERTY(bouncesZoom, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(canCancelContentTouches, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(centerContent, BOOL)
-RCT_EXPORT_VIEW_PROPERTY(maintainPositionAtOrBeyondIndex, NSNumber)
+RCT_EXPORT_VIEW_PROPERTY(maintainVisibleContentPosition, NSDictionary)
 RCT_EXPORT_VIEW_PROPERTY(automaticallyAdjustContentInsets, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(decelerationRate, CGFloat)
 RCT_EXPORT_VIEW_PROPERTY(directionalLockEnabled, BOOL)