Add pinWindowTo prop for iOS.

Author: calebmer

Libraries/Components/ScrollView/ScrollView.js

@@ -248,6 +248,15 @@ type IOSProps = $ReadOnly<{|
    * @platform ios
    */
   snapToAlignment?: ?('start' | 'center' | 'end'),
+  /**
+   * Pins the currently scrolled window to either the top of its content or
+   * the bottom. The default behavior is "top". That means when the content in
+   * the scroll view changes we will keep the offset of the scroll view from
+   * the top of the content the same. If we set this prop to "bottom" then
+   * when the content of the scroll view changes we will keep the offset from
+   * the bottom of the content the same.
+   */
+  pinToButton?: ?('top' | 'bottom'),
   /**
    * The current scale of the scroll view content. The default value is 1.0.
    * @platform ios

React/Views/ScrollView/RCTScrollView.h

@@ -50,6 +50,7 @@
 @property (nonatomic, assign) BOOL snapToStart;
 @property (nonatomic, assign) BOOL snapToEnd;
 @property (nonatomic, copy) NSString *snapToAlignment;
+@property (nonatomic, copy) NSString *pinWindowTo;
 
 // NOTE: currently these event props are only declared so we can export the
 // event names to JS - we don't call the blocks directly because scroll events

React/Views/ScrollView/RCTScrollView.m

@@ -966,20 +966,29 @@ - (CGPoint)calculateOffsetForContentSize:(CGSize)newContentSize
   CGSize oldContentSize = _scrollView.contentSize;
   CGSize viewportSize = [self _calculateViewportSize];
 
+  BOOL updatedOffsetY = false;
   BOOL fitsinViewportY = oldContentSize.height <= viewportSize.height && newContentSize.height <= viewportSize.height;
   if (newContentSize.height < oldContentSize.height && !fitsinViewportY) {
     CGFloat offsetHeight = oldOffset.y + viewportSize.height;
     if (oldOffset.y < 0) {
       // overscrolled on top, leave offset alone
+      updatedOffsetY = true;
     } else if (offsetHeight > oldContentSize.height) {
       // overscrolled on the bottom, preserve overscroll amount
       newOffset.y = MAX(0, oldOffset.y - (oldContentSize.height - newContentSize.height));
+      updatedOffsetY = true;
     } else if (offsetHeight > newContentSize.height) {
       // offset falls outside of bounds, scroll back to end of list
       newOffset.y = MAX(0, newContentSize.height - viewportSize.height);
+      updatedOffsetY = true;
     }
   }
 
+  if (!updatedOffsetY && [self.pinWindowTo isEqualToString:@"bottom"]) {
+    CGFloat oldOffsetBottom = oldContentSize.height - (oldOffset.y + viewportSize.height);
+    newOffset.y = newContentSize.height - viewportSize.height - oldOffsetBottom;
+  }
+
   BOOL fitsinViewportX = oldContentSize.width <= viewportSize.width && newContentSize.width <= viewportSize.width;
   if (newContentSize.width < oldContentSize.width && !fitsinViewportX) {
     CGFloat offsetHeight = oldOffset.x + viewportSize.width;

React/Views/ScrollView/RCTScrollViewManager.m

@@ -86,6 +86,7 @@ - (UIView *)view
 RCT_EXPORT_VIEW_PROPERTY(snapToStart, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(snapToEnd, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(snapToAlignment, NSString)
+RCT_EXPORT_VIEW_PROPERTY(pinWindowTo, NSString)
 RCT_REMAP_VIEW_PROPERTY(contentOffset, scrollView.contentOffset, CGPoint)
 RCT_EXPORT_VIEW_PROPERTY(onScrollBeginDrag, RCTDirectEventBlock)
 RCT_EXPORT_VIEW_PROPERTY(onScroll, RCTDirectEventBlock)