ScrollView.js

/**
 * 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.
 *
 * @format
 * @flow strict-local
 */

'use strict';

const AnimatedImplementation = require('../../Animated/src/AnimatedImplementation');
const Platform = require('../../Utilities/Platform');
const React = require('react');
const ReactNative = require('../../Renderer/shims/ReactNative');
require('../../Renderer/shims/ReactNative'); // Force side effects to prevent T55744311
const ScrollResponder = require('../ScrollResponder');
const ScrollViewStickyHeader = require('./ScrollViewStickyHeader');
const StyleSheet = require('../../StyleSheet/StyleSheet');
const View = require('../View/View');

const dismissKeyboard = require('../../Utilities/dismissKeyboard');
const flattenStyle = require('../../StyleSheet/flattenStyle');
const invariant = require('invariant');
const processDecelerationRate = require('./processDecelerationRate');
const resolveAssetSource = require('../../Image/resolveAssetSource');
const splitLayoutProps = require('../../StyleSheet/splitLayoutProps');
const setAndForwardRef = require('../../Utilities/setAndForwardRef');

import type {EdgeInsetsProp} from '../../StyleSheet/EdgeInsetsPropType';
import type {PointProp} from '../../StyleSheet/PointPropType';
import type {ViewStyleProp} from '../../StyleSheet/StyleSheet';
import type {ColorValue} from '../../StyleSheet/StyleSheetTypes';
import type {
  PressEvent,
  ScrollEvent,
  LayoutEvent,
} from '../../Types/CoreEventTypes';
import type {HostComponent} from '../../Renderer/shims/ReactNativeTypes';
import type {State as ScrollResponderState} from '../ScrollResponder';
import type {ViewProps} from '../View/ViewPropTypes';
import type {Props as ScrollViewStickyHeaderProps} from './ScrollViewStickyHeader';

import ScrollViewNativeComponent from './ScrollViewNativeComponent';
import ScrollContentViewNativeComponent from './ScrollContentViewNativeComponent';
import AndroidHorizontalScrollViewNativeComponent from './AndroidHorizontalScrollViewNativeComponent';
import AndroidHorizontalScrollContentViewNativeComponent from './AndroidHorizontalScrollContentViewNativeComponent';

let AndroidScrollView;
let AndroidHorizontalScrollContentView;
let AndroidHorizontalScrollView;
let RCTScrollView;
let RCTScrollContentView;

if (Platform.OS === 'android') {
  AndroidScrollView = ScrollViewNativeComponent;
  AndroidHorizontalScrollView = AndroidHorizontalScrollViewNativeComponent;
  AndroidHorizontalScrollContentView = AndroidHorizontalScrollContentViewNativeComponent;
} else {
  RCTScrollView = ScrollViewNativeComponent;
  RCTScrollContentView = ScrollContentViewNativeComponent;
}

// Public methods for ScrollView
export type ScrollViewImperativeMethods = $ReadOnly<{|
  getScrollResponder: $PropertyType<ScrollView, 'getScrollResponder'>,
  getScrollableNode: $PropertyType<ScrollView, 'getScrollableNode'>,
  getInnerViewNode: $PropertyType<ScrollView, 'getInnerViewNode'>,
  getInnerViewRef: $PropertyType<ScrollView, 'getInnerViewRef'>,
  getNativeScrollRef: $PropertyType<ScrollView, 'getNativeScrollRef'>,
  scrollTo: $PropertyType<ScrollView, 'scrollTo'>,
  scrollToEnd: $PropertyType<ScrollView, 'scrollToEnd'>,
  flashScrollIndicators: $PropertyType<ScrollView, 'flashScrollIndicators'>,

  // ScrollResponder.Mixin public methods
  scrollResponderZoomTo: $PropertyType<
    typeof ScrollResponder.Mixin,
    'scrollResponderZoomTo',
  >,
  scrollResponderScrollNativeHandleToKeyboard: $PropertyType<
    typeof ScrollResponder.Mixin,
    'scrollResponderScrollNativeHandleToKeyboard',
  >,
|}>;

export type ScrollResponderType = ScrollViewImperativeMethods;

type IOSProps = $ReadOnly<{|
  /**
   * Controls whether iOS should automatically adjust the content inset
   * for scroll views that are placed behind a navigation bar or
   * tab bar/ toolbar. The default value is true.
   * @platform ios
   */
  automaticallyAdjustContentInsets?: ?boolean,
  /**
   * The amount by which the scroll view content is inset from the edges
   * of the scroll view. Defaults to `{top: 0, left: 0, bottom: 0, right: 0}`.
   * @platform ios
   */
  contentInset?: ?EdgeInsetsProp,
  /**
   * Used to manually set the starting scroll offset.
   * The default value is `{x: 0, y: 0}`.
   * @platform ios
   */
  contentOffset?: ?PointProp,
  /**
   * When true, the scroll view bounces when it reaches the end of the
   * content if the content is larger then the scroll view along the axis of
   * the scroll direction. When false, it disables all bouncing even if
   * the `alwaysBounce*` props are true. The default value is true.
   * @platform ios
   */
  bounces?: ?boolean,
  /**
   * By default, ScrollView has an active pan responder that hijacks panresponders
   * deeper in the render tree in order to prevent accidental touches while scrolling.
   * However, in certain occasions (such as when using snapToInterval) in a vertical scrollview
   * You may want to disable this behavior in order to prevent the ScrollView from blocking touches
   */
  disableScrollViewPanResponder?: ?boolean,
  /**
   * When true, gestures can drive zoom past min/max and the zoom will animate
   * to the min/max value at gesture end, otherwise the zoom will not exceed
   * the limits.
   * @platform ios
   */
  bouncesZoom?: ?boolean,
  /**
   * When true, the scroll view bounces horizontally when it reaches the end
   * even if the content is smaller than the scroll view itself. The default
   * value is true when `horizontal={true}` and false otherwise.
   * @platform ios
   */
  alwaysBounceHorizontal?: ?boolean,
  /**
   * When true, the scroll view bounces vertically when it reaches the end
   * even if the content is smaller than the scroll view itself. The default
   * value is false when `horizontal={true}` and true otherwise.
   * @platform ios
   */
  alwaysBounceVertical?: ?boolean,
  /**
   * When true, the scroll view automatically centers the content when the
   * content is smaller than the scroll view bounds; when the content is
   * larger than the scroll view, this property has no effect. The default
   * value is false.
   * @platform ios
   */
  centerContent?: ?boolean,
  /**
   * The style of the scroll indicators.
   *
   *   - `'default'` (the default), same as `black`.
   *   - `'black'`, scroll indicator is black. This style is good against a light background.
   *   - `'white'`, scroll indicator is white. This style is good against a dark background.
   *
   * @platform ios
   */
  indicatorStyle?: ?('default' | 'black' | 'white'),
  /**
   * When true, the ScrollView will try to lock to only vertical or horizontal
   * scrolling while dragging.  The default value is false.
   * @platform ios
   */
  directionalLockEnabled?: ?boolean,
  /**
   * When false, once tracking starts, won't try to drag if the touch moves.
   * The default value is true.
   * @platform ios
   */
  canCancelContentTouches?: ?boolean,
  /**
   * 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.
   *
   * 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
   */
  maintainVisibleContentPosition?: ?$ReadOnly<{|
    minIndexForVisible: number,
    autoscrollToTopThreshold?: ?number,
  |}>,
  /**
   * The maximum allowed zoom scale. The default value is 1.0.
   * @platform ios
   */
  maximumZoomScale?: ?number,
  /**
   * The minimum allowed zoom scale. The default value is 1.0.
   * @platform ios
   */
  minimumZoomScale?: ?number,
  /**
   * When true, ScrollView allows use of pinch gestures to zoom in and out.
   * The default value is true.
   * @platform ios
   */
  pinchGestureEnabled?: ?boolean,
  /**
   * This controls how often the scroll event will be fired while scrolling
   * (as a time interval in ms). A lower number yields better accuracy for code
   * that is tracking the scroll position, but can lead to scroll performance
   * problems due to the volume of information being send over the bridge.
   *
   * Values between 0 and 17ms indicate 60fps updates are needed and throttling
   * will be disabled.
   *
   * If you do not need precise scroll position tracking, set this value higher
   * to limit the information being sent across the bridge.
   *
   * The default value is zero, which results in the scroll event being sent only
   * once each time the view is scrolled.
   *
   * @platform ios
   */
  scrollEventThrottle?: ?number,
  /**
   * The amount by which the scroll view indicators are inset from the edges
   * of the scroll view. This should normally be set to the same value as
   * the `contentInset`. Defaults to `{0, 0, 0, 0}`.
   * @platform ios
   */
  scrollIndicatorInsets?: ?EdgeInsetsProp,
  /**
   * When true, the scroll view can be programmatically scrolled beyond its
   * content size. The default value is false.
   * @platform ios
   */
  scrollToOverflowEnabled?: ?boolean,
  /**
   * When true, the scroll view scrolls to top when the status bar is tapped.
   * The default value is true.
   * @platform ios
   */
  scrollsToTop?: ?boolean,
  /**
   * Fires when the scroll view scrolls to top after the status bar has been tapped
   * @platform ios
   */
  onScrollToTop?: (event: ScrollEvent) => void,
  /**
   * When true, shows a horizontal scroll indicator.
   * The default value is true.
   */
  showsHorizontalScrollIndicator?: ?boolean,
  /**
   * When `snapToInterval` is set, `snapToAlignment` will define the relationship
   * of the snapping to the scroll view.
   *
   *   - `'start'` (the default) will align the snap at the left (horizontal) or top (vertical)
   *   - `'center'` will align the snap in the center
   *   - `'end'` will align the snap at the right (horizontal) or bottom (vertical)
   *
   * @platform ios
   */
  snapToAlignment?: ?('start' | 'center' | 'end'),
  /**
   * The current scale of the scroll view content. The default value is 1.0.
   * @platform ios
   */
  zoomScale?: ?number,
  /**
   * This property specifies how the safe area insets are used to modify the
   * content area of the scroll view. The default value of this property is
   * "never". Available on iOS 11 and later.
   * @platform ios
   */
  contentInsetAdjustmentBehavior?: ?(
    | 'automatic'
    | 'scrollableAxes'
    | 'never'
    | 'always'
  ),
  /**
   * When true, ScrollView will emit updateChildFrames data in scroll events,
   * otherwise will not compute or emit child frame data.  This only exists
   * to support legacy issues, `onLayout` should be used instead to retrieve
   * frame data.
   * The default value is false.
   * @platform ios
   */
  DEPRECATED_sendUpdatedChildFrames?: ?boolean,
|}>;

type AndroidProps = $ReadOnly<{|
  /**
   * Enables nested scrolling for Android API level 21+.
   * Nested scrolling is supported by default on iOS
   * @platform android
   */
  nestedScrollEnabled?: ?boolean,
  /**
   * Sometimes a scrollview takes up more space than its content fills. When this is
   * the case, this prop will fill the rest of the scrollview with a color to avoid setting
   * a background and creating unnecessary overdraw. This is an advanced optimization
   * that is not needed in the general case.
   * @platform android
   */
  endFillColor?: ?ColorValue,
  /**
   * Tag used to log scroll performance on this scroll view. Will force
   * momentum events to be turned on (see sendMomentumEvents). This doesn't do
   * anything out of the box and you need to implement a custom native
   * FpsListener for it to be useful.
   * @platform android
   */
  scrollPerfTag?: ?string,
  /**
   * Used to override default value of overScroll mode.
   *
   * Possible values:
   *
   *  - `'auto'` - Default value, allow a user to over-scroll
   *    this view only if the content is large enough to meaningfully scroll.
   *  - `'always'` - Always allow a user to over-scroll this view.
   *  - `'never'` - Never allow a user to over-scroll this view.
   *
   * @platform android
   */
  overScrollMode?: ?('auto' | 'always' | 'never'),
  /**
   * Causes the scrollbars not to turn transparent when they are not in use.
   * The default value is false.
   *
   * @platform android
   */
  persistentScrollbar?: ?boolean,
  /**
   * Fades out the edges of the the scroll content.
   *
   * If the value is greater than 0, the fading edges will be set accordingly
   * to the current scroll direction and position,
   * indicating if there is more content to show.
   *
   * The default value is 0.
   *
   * @platform android
   */
  fadingEdgeLength?: ?number,
|}>;

type VRProps = $ReadOnly<{|
  /**
   * Optionally an image can be used for the scroll bar thumb. This will
   * override the color. While the image is loading or the image fails to
   * load the color will be used instead. Use an alpha of 0 in the color
   * to avoid seeing it while the image is loading.
   *
   * - `uri` - a string representing the resource identifier for the image, which
   * should be either a local file path or the name of a static image resource
   * - `number` - Opaque type returned by something like
   * `import IMAGE from './image.jpg'`.
   * @platform vr
   */
  scrollBarThumbImage?: ?($ReadOnly<{||}> | number), // Opaque type returned by import IMAGE from './image.jpg'
|}>;

type StickyHeaderComponentType = React.AbstractComponent<
  ScrollViewStickyHeaderProps,
  $ReadOnly<{setNextHeaderY: number => void, ...}>,
>;

export type Props = $ReadOnly<{|
  ...ViewProps,
  ...IOSProps,
  ...AndroidProps,
  ...VRProps,

  /**
   * These styles will be applied to the scroll view content container which
   * wraps all of the child views. Example:
   *
   * ```
   * return (
   *   <ScrollView contentContainerStyle={styles.contentContainer}>
   *   </ScrollView>
   * );
   * ...
   * const styles = StyleSheet.create({
   *   contentContainer: {
   *     paddingVertical: 20
   *   }
   * });
   * ```
   */
  contentContainerStyle?: ?ViewStyleProp,
  /**
   * When true, the scroll view stops on the next index (in relation to scroll
   * position at release) regardless of how fast the gesture is. This can be
   * used for pagination when the page is less than the width of the
   * horizontal ScrollView or the height of the vertical ScrollView. The default value is false.
   */
  disableIntervalMomentum?: ?boolean,
  /**
   * A floating-point number that determines how quickly the scroll view
   * decelerates after the user lifts their finger. You may also use string
   * shortcuts `"normal"` and `"fast"` which match the underlying iOS settings
   * for `UIScrollViewDecelerationRateNormal` and
   * `UIScrollViewDecelerationRateFast` respectively.
   *
   *   - `'normal'`: 0.998 on iOS, 0.985 on Android (the default)
   *   - `'fast'`: 0.99 on iOS, 0.9 on Android
   */
  decelerationRate?: ?('fast' | 'normal' | number),
  /**
   * When true, the scroll view's children are arranged horizontally in a row
   * instead of vertically in a column. The default value is false.
   */
  horizontal?: ?boolean,
  /**
   * If sticky headers should stick at the bottom instead of the top of the
   * ScrollView. This is usually used with inverted ScrollViews.
   */
  invertStickyHeaders?: ?boolean,
  /**
   * Determines whether the keyboard gets dismissed in response to a drag.
   *
   * *Cross platform*
   *
   *   - `'none'` (the default), drags do not dismiss the keyboard.
   *   - `'on-drag'`, the keyboard is dismissed when a drag begins.
   *
   * *iOS Only*
   *
   *   - `'interactive'`, the keyboard is dismissed interactively with the drag and moves in
   *     synchrony with the touch; dragging upwards cancels the dismissal.
   *     On android this is not supported and it will have the same behavior as 'none'.
   */
  keyboardDismissMode?: ?// default
  (| 'none' // cross-platform
    | 'on-drag'
    | 'interactive'
  ), // ios only
  /**
   * Determines when the keyboard should stay visible after a tap.
   *
   *   - `'never'` (the default), tapping outside of the focused text input when the keyboard
   *     is up dismisses the keyboard. When this happens, children won't receive the tap.
   *   - `'always'`, the keyboard will not dismiss automatically, and the scroll view will not
   *     catch taps, but children of the scroll view can catch taps.
   *   - `'handled'`, the keyboard will not dismiss automatically when the tap was handled by
   *     a children, (or captured by an ancestor).
   *   - `false`, deprecated, use 'never' instead
   *   - `true`, deprecated, use 'always' instead
   */
  keyboardShouldPersistTaps?: ?('always' | 'never' | 'handled' | true | false),
  /**
   * Called when the momentum scroll starts (scroll which occurs as the ScrollView glides to a stop).
   */
  onMomentumScrollBegin?: ?(event: ScrollEvent) => void,
  /**
   * Called when the momentum scroll ends (scroll which occurs as the ScrollView glides to a stop).
   */
  onMomentumScrollEnd?: ?(event: ScrollEvent) => void,

  /**
   * Fires at most once per frame during scrolling. The frequency of the
   * events can be controlled using the `scrollEventThrottle` prop.
   */
  onScroll?: ?(event: ScrollEvent) => void,
  /**
   * Called when the user begins to drag the scroll view.
   */
  onScrollBeginDrag?: ?(event: ScrollEvent) => void,
  /**
   * Called when the user stops dragging the scroll view and it either stops
   * or begins to glide.
   */
  onScrollEndDrag?: ?(event: ScrollEvent) => void,
  /**
   * Called when scrollable content view of the ScrollView changes.
   *
   * Handler function is passed the content width and content height as parameters:
   * `(contentWidth, contentHeight)`
   *
   * It's implemented using onLayout handler attached to the content container
   * which this ScrollView renders.
   */
  onContentSizeChange?: (contentWidth: number, contentHeight: number) => void,
  onKeyboardDidShow?: (event: PressEvent) => void,
  /**
   * When true, the scroll view stops on multiples of the scroll view's size
   * when scrolling. This can be used for horizontal pagination. The default
   * value is false.
   *
   * Note: Vertical pagination is not supported on Android.
   */
  pagingEnabled?: ?boolean,

  /**
   * When false, the view cannot be scrolled via touch interaction.
   * The default value is true.
   *
   * Note that the view can always be scrolled by calling `scrollTo`.
   */
  scrollEnabled?: ?boolean,
  /**
   * When true, shows a vertical scroll indicator.
   * The default value is true.
   */
  showsVerticalScrollIndicator?: ?boolean,
  /**
   * An array of child indices determining which children get docked to the
   * top of the screen when scrolling. For example, passing
   * `stickyHeaderIndices={[0]}` will cause the first child to be fixed to the
   * top of the scroll view. This property is not supported in conjunction
   * with `horizontal={true}`.
   */
  stickyHeaderIndices?: ?$ReadOnlyArray<number>,
  /**
   * A React Component that will be used to render sticky headers.
   * To be used together with `stickyHeaderIndices` or with `SectionList`, defaults to `ScrollViewStickyHeader`.
   * You may need to set this if your sticky header uses custom transforms (eg. translation),
   * for example when you want your list to have an animated hidable header.
   */
  StickyHeaderComponent?: StickyHeaderComponentType,
  /**
   * When set, causes the scroll view to stop at multiples of the value of
   * `snapToInterval`. This can be used for paginating through children
   * that have lengths smaller than the scroll view. Typically used in
   * combination with `snapToAlignment` and `decelerationRate="fast"`.
   *
   * Overrides less configurable `pagingEnabled` prop.
   */
  snapToInterval?: ?number,
  /**
   * When set, causes the scroll view to stop at the defined offsets.
   * This can be used for paginating through variously sized children
   * that have lengths smaller than the scroll view. Typically used in
   * combination with `decelerationRate="fast"`.
   *
   * Overrides less configurable `pagingEnabled` and `snapToInterval` props.
   */
  snapToOffsets?: ?$ReadOnlyArray<number>,
  /**
   * Use in conjunction with `snapToOffsets`. By default, the beginning
   * of the list counts as a snap offset. Set `snapToStart` to false to disable
   * this behavior and allow the list to scroll freely between its start and
   * the first `snapToOffsets` offset.
   * The default value is true.
   */
  snapToStart?: ?boolean,
  /**
   * Use in conjunction with `snapToOffsets`. By default, the end
   * of the list counts as a snap offset. Set `snapToEnd` to false to disable
   * this behavior and allow the list to scroll freely between its end and
   * the last `snapToOffsets` offset.
   * The default value is true.
   */
  snapToEnd?: ?boolean,
  /**
   * Experimental: When true, offscreen child views (whose `overflow` value is
   * `hidden`) are removed from their native backing superview when offscreen.
   * This can improve scrolling performance on long lists. The default value is
   * true.
   */
  removeClippedSubviews?: ?boolean,
  /**
   * A RefreshControl component, used to provide pull-to-refresh
   * functionality for the ScrollView. Only works for vertical ScrollViews
   * (`horizontal` prop must be `false`).
   *
   * See [RefreshControl](docs/refreshcontrol.html).
   */
  // $FlowFixMe - how to handle generic type without existential operator?
  refreshControl?: ?React.Element<any>,
  children?: React.Node,
  /**
   * A ref to the inner View element of the ScrollView. This should be used
   * instead of calling `getInnerViewRef`.
   */
  innerViewRef?: React.Ref<typeof View>,
  /**
   * A ref to the Native ScrollView component. This ref can be used to call
   * all of ScrollView's public methods, in addition to native methods like
   * measure, measureLayout, etc.
   */
  scrollViewRef?: React.Ref<
    typeof ScrollViewNativeComponent & ScrollViewImperativeMethods,
  >,
|}>;

type State = {|
  layoutHeight: ?number,
  ...ScrollResponderState,
|};

function createScrollResponder(
  node: React.ElementRef<typeof ScrollView>,
): typeof ScrollResponder.Mixin {
  const scrollResponder = {...ScrollResponder.Mixin};

  for (const key in scrollResponder) {
    if (typeof scrollResponder[key] === 'function') {
      scrollResponder[key] = scrollResponder[key].bind(node);
    }
  }

  return scrollResponder;
}

type ContextType = {|horizontal: boolean|} | null;
const Context: React.Context<ContextType> = React.createContext(null);
const standardHorizontalContext: ContextType = Object.freeze({
  horizontal: true,
});
const standardVerticalContext: ContextType = Object.freeze({horizontal: false});
type ScrollViewComponentStatics = $ReadOnly<{|
  Context: typeof Context,
|}>;

/**
 * Component that wraps platform ScrollView while providing
 * integration with touch locking "responder" system.
 *
 * Keep in mind that ScrollViews must have a bounded height in order to work,
 * since they contain unbounded-height children into a bounded container (via
 * a scroll interaction). In order to bound the height of a ScrollView, either
 * set the height of the view directly (discouraged) or make sure all parent
 * views have bounded height. Forgetting to transfer `{flex: 1}` down the
 * view stack can lead to errors here, which the element inspector makes
 * easy to debug.
 *
 * Doesn't yet support other contained responders from blocking this scroll
 * view from becoming the responder.
 *
 *
 * `<ScrollView>` vs [`<FlatList>`](https://reactnative.dev/docs/flatlist.html) - which one to use?
 *
 * `ScrollView` simply renders all its react child components at once. That
 * makes it very easy to understand and use.
 *
 * On the other hand, this has a performance downside. Imagine you have a very
 * long list of items you want to display, maybe several screens worth of
 * content. Creating JS components and native views for everything all at once,
 * much of which may not even be shown, will contribute to slow rendering and
 * increased memory usage.
 *
 * This is where `FlatList` comes into play. `FlatList` renders items lazily,
 * just when they are about to appear, and removes items that scroll way off
 * screen to save memory and processing time.
 *
 * `FlatList` is also handy if you want to render separators between your items,
 * multiple columns, infinite scroll loading, or any number of other features it
 * supports out of the box.
 */
class ScrollView extends React.Component<Props, State> {
  static Context: React$Context<ContextType> = Context;
  /**
   * Part 1: Removing ScrollResponder.Mixin:
   *
   * 1. Mixin methods should be flow typed. That's why we create a
   *    copy of ScrollResponder.Mixin and attach it to this._scrollResponder.
   *    Otherwise, we'd have to manually declare each method on the component
   *    class and assign it a flow type.
   * 2. Mixin methods can call component methods, and access the component's
   *    props and state. So, we need to bind all mixin methods to the
   *    component instance.
   * 3. Continued...
   */
  _scrollResponder: typeof ScrollResponder.Mixin = createScrollResponder(this);

  constructor(props: Props) {
    super(props);

    /**
     * Part 2: Removing ScrollResponder.Mixin
     *
     * 3. Mixin methods access other mixin methods via dynamic dispatch using
     *    this. Since mixin methods are bound to the component instance, we need
     *    to copy all mixin methods to the component instance. This is also
     *    necessary because getScrollResponder() is a public method that returns
     *    an object that can be used to execute all scrollResponder methods.
     *    Since the object returned from that method is the ScrollView instance,
     *    we need to bind all mixin methods to the ScrollView instance.
     */
    for (const key in ScrollResponder.Mixin) {
      if (
        typeof ScrollResponder.Mixin[key] === 'function' &&
        key.startsWith('scrollResponder')
      ) {
        // $FlowFixMe - dynamically adding properties to a class
        (this: any)[key] = ScrollResponder.Mixin[key].bind(this);
      }
    }

    /**
     * Part 3: Removing ScrollResponder.Mixin
     *
     * 4. Mixins can initialize properties and use properties on the component
     *    instance.
     */
    Object.keys(ScrollResponder.Mixin)
      .filter(key => typeof ScrollResponder.Mixin[key] !== 'function')
      .forEach(key => {
        // $FlowFixMe - dynamically adding properties to a class
        (this: any)[key] = ScrollResponder.Mixin[key];
      });
  }

  _scrollAnimatedValue: AnimatedImplementation.Value = new AnimatedImplementation.Value(
    0,
  );
  _scrollAnimatedValueAttachment: ?{detach: () => void, ...} = null;
  _stickyHeaderRefs: Map<
    string,
    React.ElementRef<StickyHeaderComponentType>,
  > = new Map();
  _headerLayoutYs: Map<string, number> = new Map();

  state: State = {
    layoutHeight: null,
    ...ScrollResponder.Mixin.scrollResponderMixinGetInitialState(),
  };

  UNSAFE_componentWillMount() {
    this._scrollResponder.UNSAFE_componentWillMount();
    this._scrollAnimatedValue = new AnimatedImplementation.Value(
      this.props.contentOffset?.y ?? 0,
    );
    this._scrollAnimatedValue.setOffset(this.props.contentInset?.top ?? 0);
    this._stickyHeaderRefs = new Map();
    this._headerLayoutYs = new Map();
  }

  UNSAFE_componentWillReceiveProps(nextProps: Props) {
    const currentContentInsetTop = this.props.contentInset
      ? this.props.contentInset.top
      : 0;
    const nextContentInsetTop = nextProps.contentInset
      ? nextProps.contentInset.top
      : 0;
    if (currentContentInsetTop !== nextContentInsetTop) {
      this._scrollAnimatedValue.setOffset(nextContentInsetTop || 0);
    }
  }

  componentDidMount() {
    this._updateAnimatedNodeAttachment();
  }

  componentDidUpdate() {
    this._updateAnimatedNodeAttachment();
  }

  componentWillUnmount() {
    this._scrollResponder.componentWillUnmount();
    if (this._scrollAnimatedValueAttachment) {
      this._scrollAnimatedValueAttachment.detach();
    }
  }

  _setNativeRef = setAndForwardRef({
    getForwardedRef: () => this.props.scrollViewRef,
    setLocalRef: ref => {
      this._scrollViewRef = ref;

      /*
        This is a hack. Ideally we would forwardRef to the underlying
        host component. However, since ScrollView has it's own methods that can be
        called as well, if we used the standard forwardRef then these
        methods wouldn't be accessible and thus be a breaking change.

        Therefore we edit ref to include ScrollView's public methods so that
        they are callable from the ref.
      */
      if (ref) {
        ref.getScrollResponder = this.getScrollResponder;
        ref.getScrollableNode = this.getScrollableNode;
        ref.getInnerViewNode = this.getInnerViewNode;
        ref.getInnerViewRef = this.getInnerViewRef;
        ref.getNativeScrollRef = this.getNativeScrollRef;
        ref.scrollTo = this.scrollTo;
        ref.scrollToEnd = this.scrollToEnd;
        ref.flashScrollIndicators = this.flashScrollIndicators;

        // $FlowFixMe - This method was manually bound from ScrollResponder.mixin
        ref.scrollResponderZoomTo = this.scrollResponderZoomTo;
        // $FlowFixMe - This method was manually bound from ScrollResponder.mixin
        ref.scrollResponderScrollNativeHandleToKeyboard = this.scrollResponderScrollNativeHandleToKeyboard;
      }
    },
  });

  /**
   * Returns a reference to the underlying scroll responder, which supports
   * operations like `scrollTo`. All ScrollView-like components should
   * implement this method so that they can be composed while providing access
   * to the underlying scroll responder's methods.
   */
  getScrollResponder: () => ScrollResponderType = () => {
    // $FlowFixMe - overriding type to include ScrollResponder.Mixin
    return ((this: any): ScrollResponderType);
  };

  getScrollableNode: () => ?number = () => {
    return ReactNative.findNodeHandle(this._scrollViewRef);
  };

  getInnerViewNode(): ?number {
    return ReactNative.findNodeHandle(this._innerViewRef);
  }

  getInnerViewRef(): ?React.ElementRef<typeof View> {
    return this._innerViewRef;
  }

  getNativeScrollRef: () => ?React.ElementRef<HostComponent<mixed>> = () => {
    return this._scrollViewRef;
  };

  /**
   * Scrolls to a given x, y offset, either immediately or with a smooth animation.
   *
   * Example:
   *
   * `scrollTo({x: 0, y: 0, animated: true})`
   *
   * Note: The weird function signature is due to the fact that, for historical reasons,
   * the function also accepts separate arguments as an alternative to the options object.
   * This is deprecated due to ambiguity (y before x), and SHOULD NOT BE USED.
   */
  scrollTo: (
    options?:
      | {
          x?: number,
          y?: number,
          animated?: boolean,
          ...
        }
      | number,
    deprecatedX?: number,
    deprecatedAnimated?: boolean,
  ) => void = (
    options?:
      | {
          x?: number,
          y?: number,
          animated?: boolean,
          ...
        }
      | number,
    deprecatedX?: number,
    deprecatedAnimated?: boolean,
  ) => {
    let x, y, animated;
    if (typeof options === 'number') {
      console.warn(
        '`scrollTo(y, x, animated)` is deprecated. Use `scrollTo({x: 5, y: 5, ' +
          'animated: true})` instead.',
      );
      y = options;
      x = deprecatedX;
      animated = deprecatedAnimated;
    } else if (options) {
      y = options.y;
      x = options.x;
      animated = options.animated;
    }
    this._scrollResponder.scrollResponderScrollTo({
      x: x || 0,
      y: y || 0,
      animated: animated !== false,
    });
  };

  /**
   * If this is a vertical ScrollView scrolls to the bottom.
   * If this is a horizontal ScrollView scrolls to the right.
   *
   * Use `scrollToEnd({animated: true})` for smooth animated scrolling,
   * `scrollToEnd({animated: false})` for immediate scrolling.
   * If no options are passed, `animated` defaults to true.
   */
  scrollToEnd: (options?: ?{animated?: boolean, ...}) => void = (
    options?: ?{animated?: boolean, ...},
  ) => {
    // Default to true
    const animated = (options && options.animated) !== false;
    this._scrollResponder.scrollResponderScrollToEnd({
      animated: animated,
    });
  };

  /**
   * Displays the scroll indicators momentarily.
   *
   * @platform ios
   */
  flashScrollIndicators: () => void = () => {
    this._scrollResponder.scrollResponderFlashScrollIndicators();
  };

  _getKeyForIndex(index, childArray) {
    const child = childArray[index];
    return child && child.key;
  }

  _updateAnimatedNodeAttachment() {
    if (this._scrollAnimatedValueAttachment) {
      this._scrollAnimatedValueAttachment.detach();
    }
    if (
      this.props.stickyHeaderIndices &&
      this.props.stickyHeaderIndices.length > 0
    ) {
      this._scrollAnimatedValueAttachment = AnimatedImplementation.attachNativeEvent(
        this._scrollViewRef,
        'onScroll',
        [{nativeEvent: {contentOffset: {y: this._scrollAnimatedValue}}}],
      );
    }
  }

  _setStickyHeaderRef(
    key: string,
    ref: ?React.ElementRef<StickyHeaderComponentType>,
  ) {
    if (ref) {
      this._stickyHeaderRefs.set(key, ref);
    } else {
      this._stickyHeaderRefs.delete(key);
    }
  }

  _onStickyHeaderLayout(index, event, key) {
    const {stickyHeaderIndices} = this.props;
    if (!stickyHeaderIndices) {
      return;
    }
    const childArray = React.Children.toArray(this.props.children);
    if (key !== this._getKeyForIndex(index, childArray)) {
      // ignore stale layout update
      return;
    }

    const layoutY = event.nativeEvent.layout.y;
    this._headerLayoutYs.set(key, layoutY);

    const indexOfIndex = stickyHeaderIndices.indexOf(index);
    const previousHeaderIndex = stickyHeaderIndices[indexOfIndex - 1];
    if (previousHeaderIndex != null) {
      const previousHeader = this._stickyHeaderRefs.get(
        this._getKeyForIndex(previousHeaderIndex, childArray),
      );
      previousHeader &&
        previousHeader.setNextHeaderY &&
        previousHeader.setNextHeaderY(layoutY);
    }
  }

  _handleScroll = (e: ScrollEvent) => {
    if (__DEV__) {
      if (
        this.props.onScroll &&
        this.props.scrollEventThrottle == null &&
        Platform.OS === 'ios'
      ) {
        console.log(
          'You specified `onScroll` on a <ScrollView> but not ' +
            '`scrollEventThrottle`. You will only receive one event. ' +
            'Using `16` you get all the events but be aware that it may ' +
            "cause frame drops, use a bigger number if you don't need as " +
            'much precision.',
        );
      }
    }
    if (Platform.OS === 'android') {
      if (
        this.props.keyboardDismissMode === 'on-drag' &&
        this.state.isTouching
      ) {
        dismissKeyboard();
      }
    }
    this._scrollResponder.scrollResponderHandleScroll(e);
  };

  _handleLayout = (e: LayoutEvent) => {
    if (this.props.invertStickyHeaders === true) {
      this.setState({layoutHeight: e.nativeEvent.layout.height});
    }
    if (this.props.onLayout) {
      this.props.onLayout(e);
    }
  };

  _handleContentOnLayout = (e: LayoutEvent) => {
    const {width, height} = e.nativeEvent.layout;
    this.props.onContentSizeChange &&
      this.props.onContentSizeChange(width, height);
  };

  _scrollViewRef: ?React.ElementRef<HostComponent<mixed>> = null;

  _innerViewRef: ?React.ElementRef<typeof View> = null;
  _setInnerViewRef = setAndForwardRef({
    getForwardedRef: () => this.props.innerViewRef,
    setLocalRef: ref => {
      this._innerViewRef = ref;
    },
  });

  render(): React.Node | React.Element<string> {
    let ScrollViewClass;
    let ScrollContentContainerViewClass;
    if (Platform.OS === 'android') {
      if (this.props.horizontal === true) {
        ScrollViewClass = AndroidHorizontalScrollView;
        ScrollContentContainerViewClass = AndroidHorizontalScrollContentView;
      } else {
        ScrollViewClass = AndroidScrollView;
        ScrollContentContainerViewClass = View;
      }
    } else {
      ScrollViewClass = RCTScrollView;
      ScrollContentContainerViewClass = RCTScrollContentView;
    }

    invariant(
      ScrollViewClass !== undefined,
      'ScrollViewClass must not be undefined',
    );

    invariant(
      ScrollContentContainerViewClass !== undefined,
      'ScrollContentContainerViewClass must not be undefined',
    );

    const contentContainerStyle = [
      this.props.horizontal === true && styles.contentContainerHorizontal,
      this.props.contentContainerStyle,
    ];
    if (__DEV__ && this.props.style !== undefined) {
      const style = flattenStyle(this.props.style);
      const childLayoutProps = ['alignItems', 'justifyContent'].filter(
        prop => style && style[prop] !== undefined,
      );
      invariant(
        childLayoutProps.length === 0,
        'ScrollView child layout (' +
          JSON.stringify(childLayoutProps) +
          ') must be applied through the contentContainerStyle prop.',
      );
    }

    let contentSizeChangeProps = {};
    if (this.props.onContentSizeChange) {
      contentSizeChangeProps = {
        onLayout: this._handleContentOnLayout,
      };
    }

    const {stickyHeaderIndices} = this.props;
    let children = this.props.children;

    if (stickyHeaderIndices != null && stickyHeaderIndices.length > 0) {
      const childArray = React.Children.toArray(this.props.children);

      children = childArray.map((child, index) => {
        const indexOfIndex = child ? stickyHeaderIndices.indexOf(index) : -1;
        if (indexOfIndex > -1) {
          const key = child.key;
          const nextIndex = stickyHeaderIndices[indexOfIndex + 1];
          const StickyHeaderComponent =
            this.props.StickyHeaderComponent || ScrollViewStickyHeader;
          return (
            <StickyHeaderComponent
              key={key}
              ref={ref => this._setStickyHeaderRef(key, ref)}
              nextHeaderLayoutY={this._headerLayoutYs.get(
                this._getKeyForIndex(nextIndex, childArray),
              )}
              onLayout={event => this._onStickyHeaderLayout(index, event, key)}
              scrollAnimatedValue={this._scrollAnimatedValue}
              inverted={this.props.invertStickyHeaders}
              scrollViewHeight={this.state.layoutHeight}>
              {child}
            </StickyHeaderComponent>
          );
        } else {
          return child;
        }
      });
    }
    children = (
      <Context.Provider
        value={
          this.props.horizontal === true
            ? standardHorizontalContext
            : standardVerticalContext
        }>
        {children}
      </Context.Provider>
    );

    const hasStickyHeaders =
      Array.isArray(stickyHeaderIndices) && stickyHeaderIndices.length > 0;

    const contentContainer = (
      /* $FlowFixMe(>=0.112.0 site=react_native_fb) This comment suppresses an
       * error found when Flow v0.112 was deployed. To see the error, delete
       * this comment and run Flow. */
      <ScrollContentContainerViewClass
        {...contentSizeChangeProps}
        ref={this._setInnerViewRef}
        style={contentContainerStyle}
        removeClippedSubviews={
          // Subview clipping causes issues with sticky headers on Android and
          // would be hard to fix properly in a performant way.
          Platform.OS === 'android' && hasStickyHeaders
            ? false
            : this.props.removeClippedSubviews
        }
        collapsable={false}>
        {children}
      </ScrollContentContainerViewClass>
    );

    const alwaysBounceHorizontal =
      this.props.alwaysBounceHorizontal !== undefined
        ? this.props.alwaysBounceHorizontal
        : this.props.horizontal;

    const alwaysBounceVertical =
      this.props.alwaysBounceVertical !== undefined
        ? this.props.alwaysBounceVertical
        : !this.props.horizontal;

    const DEPRECATED_sendUpdatedChildFrames = !!this.props
      .DEPRECATED_sendUpdatedChildFrames;

    const baseStyle =
      this.props.horizontal === true
        ? styles.baseHorizontal
        : styles.baseVertical;
    const props = {
      ...this.props,
      alwaysBounceHorizontal,
      alwaysBounceVertical,
      style: [baseStyle, this.props.style],
      // Override the onContentSizeChange from props, since this event can
      // bubble up from TextInputs
      onContentSizeChange: null,
      onLayout: this._handleLayout,
      onMomentumScrollBegin: this._scrollResponder
        .scrollResponderHandleMomentumScrollBegin,
      onMomentumScrollEnd: this._scrollResponder
        .scrollResponderHandleMomentumScrollEnd,
      onResponderGrant: this._scrollResponder
        .scrollResponderHandleResponderGrant,
      onResponderReject: this._scrollResponder
        .scrollResponderHandleResponderReject,
      onResponderRelease: this._scrollResponder
        .scrollResponderHandleResponderRelease,
      onResponderTerminationRequest: this._scrollResponder
        .scrollResponderHandleTerminationRequest,
      onScrollBeginDrag: this._scrollResponder
        .scrollResponderHandleScrollBeginDrag,
      onScrollEndDrag: this._scrollResponder.scrollResponderHandleScrollEndDrag,
      onScrollShouldSetResponder: this._scrollResponder
        .scrollResponderHandleScrollShouldSetResponder,
      onStartShouldSetResponder: this._scrollResponder
        .scrollResponderHandleStartShouldSetResponder,
      onStartShouldSetResponderCapture: this._scrollResponder
        .scrollResponderHandleStartShouldSetResponderCapture,
      onTouchEnd: this._scrollResponder.scrollResponderHandleTouchEnd,
      onTouchMove: this._scrollResponder.scrollResponderHandleTouchMove,
      onTouchStart: this._scrollResponder.scrollResponderHandleTouchStart,
      onTouchCancel: this._scrollResponder.scrollResponderHandleTouchCancel,
      onScroll: this._handleScroll,
      scrollBarThumbImage: resolveAssetSource(this.props.scrollBarThumbImage),
      scrollEventThrottle: hasStickyHeaders
        ? 1
        : this.props.scrollEventThrottle,
      sendMomentumEvents:
        this.props.onMomentumScrollBegin || this.props.onMomentumScrollEnd
          ? true
          : false,
      DEPRECATED_sendUpdatedChildFrames,
      // default to true
      snapToStart: this.props.snapToStart !== false,
      // default to true
      snapToEnd: this.props.snapToEnd !== false,
      // pagingEnabled is overridden by snapToInterval / snapToOffsets
      pagingEnabled: Platform.select({
        // on iOS, pagingEnabled must be set to false to have snapToInterval / snapToOffsets work
        ios:
          this.props.pagingEnabled === true &&
          this.props.snapToInterval == null &&
          this.props.snapToOffsets == null,
        // on Android, pagingEnabled must be set to true to have snapToInterval / snapToOffsets work
        android:
          this.props.pagingEnabled === true ||
          this.props.snapToInterval != null ||
          this.props.snapToOffsets != null,
      }),
    };

    const {decelerationRate} = this.props;
    if (decelerationRate != null) {
      props.decelerationRate = processDecelerationRate(decelerationRate);
    }

    const refreshControl = this.props.refreshControl;

    if (refreshControl) {
      if (Platform.OS === 'ios') {
        // On iOS the RefreshControl is a child of the ScrollView.
        // tvOS lacks native support for RefreshControl, so don't include it in that case
        return (
          /* $FlowFixMe(>=0.117.0 site=react_native_fb) This comment suppresses
           * an error found when Flow v0.117 was deployed. To see the error,
           * delete this comment and run Flow. */
          <ScrollViewClass {...props} ref={this._setNativeRef}>
            {Platform.isTV ? null : refreshControl}
            {contentContainer}
          </ScrollViewClass>
        );
      } else if (Platform.OS === 'android') {
        // On Android wrap the ScrollView with a AndroidSwipeRefreshLayout.
        // Since the ScrollView is wrapped add the style props to the
        // AndroidSwipeRefreshLayout and use flex: 1 for the ScrollView.
        // Note: we should split props.style on the inner and outer props
        // however, the ScrollView still needs the baseStyle to be scrollable
        const {outer, inner} = splitLayoutProps(flattenStyle(props.style));
        return React.cloneElement(
          refreshControl,
          {style: [baseStyle, outer]},
          <ScrollViewClass
            {...props}
            style={[baseStyle, inner]}
            ref={this._setNativeRef}>
            {contentContainer}
          </ScrollViewClass>,
        );
      }
    }
    return (
      <ScrollViewClass {...props} ref={this._setNativeRef}>
        {contentContainer}
      </ScrollViewClass>
    );
  }
}

const styles = StyleSheet.create({
  baseVertical: {
    flexGrow: 1,
    flexShrink: 1,
    flexDirection: 'column',
    overflow: 'scroll',
  },
  baseHorizontal: {
    flexGrow: 1,
    flexShrink: 1,
    flexDirection: 'row',
    overflow: 'scroll',
  },
  contentContainerHorizontal: {
    flexDirection: 'row',
  },
});

function Wrapper(props, ref) {
  return <ScrollView {...props} scrollViewRef={ref} />;
}
Wrapper.displayName = 'ScrollView';
const ForwardedScrollView = React.forwardRef(Wrapper);

// $FlowFixMe Add static context to ForwardedScrollView
ForwardedScrollView.Context = Context;

ForwardedScrollView.displayName = 'ScrollView';

module.exports = ((ForwardedScrollView: $FlowFixMe): React.AbstractComponent<
  React.ElementConfig<typeof ScrollView>,
  $ReadOnly<{|
    ...$Exact<React.ElementRef<HostComponent<mixed>>>,
    ...ScrollViewImperativeMethods,
  |}>,
> &
  ScrollViewComponentStatics);