// @flow

const speedOffset = 5;

type ListenerFunctionType = (shouldHide: boolean) => void;

export type OnScrollType = {
  nativeEvent: {
    contentInset: {bottom: number, left: number, right: number, top: number},
    contentOffset: {x: number, y: number},
    contentSize: {height: number, width: number},
    layoutMeasurement: {height: number, width: number},
    zoomScale: number,
  },
};

/**
 * Class used to detect when to show or hide a component based on scrolling
 */
export default class AutoHideHandler {
  lastOffset: number;

  isHidden: boolean;

  listeners: Array<ListenerFunctionType>;

  constructor(startHidden: boolean) {
    this.listeners = [];
    this.isHidden = startHidden;
  }

  /**
   * Adds a listener to the hide event
   *
   * @param listener
   */
  addListener(listener: (shouldHide: boolean) => void) {
    this.listeners.push(listener);
  }

  /**
   * Notifies every listener whether they should hide or show.
   *
   * @param shouldHide
   */
  notifyListeners(shouldHide: boolean) {
    this.listeners.forEach((func: ListenerFunctionType) => {
      func(shouldHide);
    });
  }

  /**
   * Callback to be used on the onScroll animated component event.
   *
   * Detects if the current speed exceeds a threshold and notifies listeners to hide or show.
   *
   * The hide even is triggered when the user scrolls down, and the show event on scroll up.
   * This does not take into account the speed when the y coordinate is negative, to prevent hiding on over scroll.
   * (When scrolling up and hitting the top on ios for example)
   *
   * //TODO Known issue:
   * When refreshing a list with the pull down gesture on ios,
   * this can trigger the hide event as it scrolls down the list to show the refresh indicator.
   * Android shows the refresh indicator on top of the list so this is not an issue.
   *
   * @param event The scroll event generated by the animated component onScroll prop
   */
  onScroll(event: OnScrollType) {
    const {nativeEvent} = event;
    const speed =
      nativeEvent.contentOffset.y < 0
        ? 0
        : this.lastOffset - nativeEvent.contentOffset.y;
    if (speed < -speedOffset && !this.isHidden) {
      // Go down
      this.notifyListeners(true);
      this.isHidden = true;
    } else if (speed > speedOffset && this.isHidden) {
      // Go up
      this.notifyListeners(false);
      this.isHidden = false;
    }
    this.lastOffset = nativeEvent.contentOffset.y;
  }
}