react-native/React/Fabric/Mounting/ComponentViews/ScrollView/RCTEnhancedScrollView.mm

/*
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */

#import "RCTEnhancedScrollView.h"
#import <React/RCTUtils.h>

@interface RCTEnhancedScrollView () <UIScrollViewDelegate>
@end

@implementation RCTEnhancedScrollView {
  __weak id<UIScrollViewDelegate> _publicDelegate;
  BOOL _isSetContentOffsetDisabled;
}

+ (BOOL)automaticallyNotifiesObserversForKey:(NSString *)key
{
  if ([key isEqualToString:@"delegate"]) {
    // For `delegate` property, we issue KVO notifications manually.
    // We need that to block notifications caused by setting the original `UIScrollView`s property.
    return NO;
  }

  return [super automaticallyNotifiesObserversForKey:key];
}

- (instancetype)initWithFrame:(CGRect)frame
{
  if (self = [super initWithFrame:frame]) {
    // We set the default behavior to "never" so that iOS
    // doesn't do weird things to UIScrollView insets automatically
    // and keeps it as an opt-in behavior.
    self.contentInsetAdjustmentBehavior = UIScrollViewContentInsetAdjustmentNever;

    // We intentionally force `UIScrollView`s `semanticContentAttribute` to `LTR` here
    // because this attribute affects a position of vertical scrollbar; we don't want this
    // scrollbar flip because we also flip it with whole `UIScrollView` flip.
    self.semanticContentAttribute = UISemanticContentAttributeForceLeftToRight;

    __weak __typeof(self) weakSelf = self;
    _delegateSplitter = [[RCTGenericDelegateSplitter alloc] initWithDelegateUpdateBlock:^(id delegate) {
      [weakSelf setPrivateDelegate:delegate];
    }];
    [_delegateSplitter addDelegate:self];
  }

  return self;
}

- (void)preserveContentOffsetWithBlock:(void (^)())block
{
  if (!block) {
    return;
  }

  _isSetContentOffsetDisabled = YES;
  block();
  _isSetContentOffsetDisabled = NO;
}

/*
 * Automatically centers the content such that if the content is smaller than the
 * ScrollView, we force it to be centered, but when you zoom or the content otherwise
 * becomes larger than the ScrollView, there is no padding around the content but it
 * can still fill the whole view.
 */
- (void)setContentOffset:(CGPoint)contentOffset
{
  if (_isSetContentOffsetDisabled) {
    return;
  }

  if (_centerContent && !CGSizeEqualToSize(self.contentSize, CGSizeZero)) {
    CGSize scrollViewSize = self.bounds.size;
    if (self.contentSize.width <= scrollViewSize.width) {
      contentOffset.x = -(scrollViewSize.width - self.contentSize.width) / 2.0;
    }
    if (self.contentSize.height <= scrollViewSize.height) {
      contentOffset.y = -(scrollViewSize.height - self.contentSize.height) / 2.0;
    }
  }

  super.contentOffset = CGPointMake(
      RCTSanitizeNaNValue(contentOffset.x, @"scrollView.contentOffset.x"),
      RCTSanitizeNaNValue(contentOffset.y, @"scrollView.contentOffset.y"));
}

- (BOOL)touchesShouldCancelInContentView:(UIView *)view
{
  if ([_overridingDelegate respondsToSelector:@selector(touchesShouldCancelInContentView:)]) {
    return [_overridingDelegate touchesShouldCancelInContentView:view];
  }

  return [super touchesShouldCancelInContentView:view];
}

#pragma mark - RCTGenericDelegateSplitter

- (void)setPrivateDelegate:(id<UIScrollViewDelegate>)delegate
{
  [super setDelegate:delegate];
}

- (id<UIScrollViewDelegate>)delegate
{
  return _publicDelegate;
}

- (void)setDelegate:(id<UIScrollViewDelegate>)delegate
{
  if (_publicDelegate == delegate) {
    return;
  }

  if (_publicDelegate) {
    [_delegateSplitter removeDelegate:_publicDelegate];
  }

  [self willChangeValueForKey:@"delegate"];
  _publicDelegate = delegate;
  [self didChangeValueForKey:@"delegate"];

  if (_publicDelegate) {
    [_delegateSplitter addDelegate:_publicDelegate];
  }
}

#pragma mark - UIScrollViewDelegate

- (void)scrollViewWillEndDragging:(UIScrollView *)scrollView
                     withVelocity:(CGPoint)velocity
              targetContentOffset:(inout CGPoint *)targetContentOffset
{
  if (self.snapToOffsets && self.snapToOffsets.count > 0) {
    // An alternative to enablePaging and snapToInterval which allows setting custom
    // stopping points that don't have to be the same distance apart. Often seen in
    // apps which feature horizonally scrolling items. snapToInterval does not enforce
    // scrolling one interval at a time but guarantees that the scroll will stop at
    // a snap offset point.

    // Find which axis to snap
    BOOL isHorizontal = [self isHorizontal:scrollView];
    CGFloat velocityAlongAxis = isHorizontal ? velocity.x : velocity.y;
    CGFloat offsetAlongAxis = isHorizontal ? scrollView.contentOffset.x : scrollView.contentOffset.y;

    // Calculate maximum content offset
    CGSize viewportSize = self.bounds.size;
    CGFloat maximumOffset = isHorizontal ? MAX(0, scrollView.contentSize.width - viewportSize.width)
                                         : MAX(0, scrollView.contentSize.height - viewportSize.height);

    // Calculate the snap offsets adjacent to the initial offset target
    CGFloat targetOffset = isHorizontal ? targetContentOffset->x : targetContentOffset->y;
    CGFloat smallerOffset = 0.0;
    CGFloat largerOffset = maximumOffset;

    for (unsigned long i = 0; i < self.snapToOffsets.count; i++) {
      CGFloat offset = [[self.snapToOffsets objectAtIndex:i] floatValue];

      if (offset <= targetOffset) {
        if (targetOffset - offset < targetOffset - smallerOffset) {
          smallerOffset = offset;
        }
      }

      if (offset >= targetOffset) {
        if (offset - targetOffset < largerOffset - targetOffset) {
          largerOffset = offset;
        }
      }
    }

    // Calculate the nearest offset
    CGFloat nearestOffset = targetOffset - smallerOffset < largerOffset - targetOffset ? smallerOffset : largerOffset;

    CGFloat firstOffset = [[self.snapToOffsets firstObject] floatValue];
    CGFloat lastOffset = [[self.snapToOffsets lastObject] floatValue];

    // if scrolling after the last snap offset and snapping to the
    // end of the list is disabled, then we allow free scrolling
    if (!self.snapToEnd && targetOffset >= lastOffset) {
      if (offsetAlongAxis >= lastOffset) {
        // free scrolling
      } else {
        // snap to end
        targetOffset = lastOffset;
      }
    } else if (!self.snapToStart && targetOffset <= firstOffset) {
      if (offsetAlongAxis <= firstOffset) {
        // free scrolling
      } else {
        // snap to beginning
        targetOffset = firstOffset;
      }
    } else if (velocityAlongAxis > 0.0) {
      targetOffset = largerOffset;
    } else if (velocityAlongAxis < 0.0) {
      targetOffset = smallerOffset;
    } else {
      targetOffset = nearestOffset;
    }

    // Make sure the new offset isn't out of bounds
    targetOffset = MIN(MAX(0, targetOffset), maximumOffset);

    // Set new targetContentOffset
    if (isHorizontal) {
      targetContentOffset->x = targetOffset;
    } else {
      targetContentOffset->y = targetOffset;
    }
  } else if (self.snapToInterval) {
    // An alternative to enablePaging which allows setting custom stopping intervals,
    // smaller than a full page size. Often seen in apps which feature horizonally
    // scrolling items. snapToInterval does not enforce scrolling one interval at a time
    // but guarantees that the scroll will stop at an interval point.
    CGFloat snapToIntervalF = (CGFloat)self.snapToInterval;

    // Find which axis to snap
    BOOL isHorizontal = [self isHorizontal:scrollView];

    // What is the current offset?
    CGFloat velocityAlongAxis = isHorizontal ? velocity.x : velocity.y;
    CGFloat targetContentOffsetAlongAxis = targetContentOffset->y;
    if (isHorizontal) {
      // Use current scroll offset to determine the next index to snap to when momentum disabled
      targetContentOffsetAlongAxis = self.disableIntervalMomentum ? scrollView.contentOffset.x : targetContentOffset->x;
    } else {
      targetContentOffsetAlongAxis = self.disableIntervalMomentum ? scrollView.contentOffset.y : targetContentOffset->y;
    }

    // Offset based on desired alignment
    CGFloat frameLength = isHorizontal ? self.frame.size.width : self.frame.size.height;
    CGFloat alignmentOffset = 0.0f;
    if ([self.snapToAlignment isEqualToString:@"center"]) {
      alignmentOffset = (frameLength * 0.5f) + (snapToIntervalF * 0.5f);
    } else if ([self.snapToAlignment isEqualToString:@"end"]) {
      alignmentOffset = frameLength;
    }

    // Pick snap point based on direction and proximity
    CGFloat fractionalIndex = (targetContentOffsetAlongAxis + alignmentOffset) / snapToIntervalF;

    NSInteger snapIndex = velocityAlongAxis > 0.0 ? ceil(fractionalIndex)
        : velocityAlongAxis < 0.0                 ? floor(fractionalIndex)
                                                  : round(fractionalIndex);
    CGFloat newTargetContentOffset = (snapIndex * snapToIntervalF) - alignmentOffset;

    // Set new targetContentOffset
    if (isHorizontal) {
      targetContentOffset->x = newTargetContentOffset;
    } else {
      targetContentOffset->y = newTargetContentOffset;
    }
  }
}

#pragma mark -

- (BOOL)isHorizontal:(UIScrollView *)scrollView
{
  return scrollView.contentSize.width > self.frame.size.width;
}

@end