application-amicale/src/utils/AutoHideHandler.js
2020-06-28 12:39:13 +02:00

70 lines
2.1 KiB
JavaScript

// @flow
import * as React from 'react';
const speedOffset = 5;
/**
* Class used to detect when to show or hide a component based on scrolling
*/
export default class AutoHideHandler {
lastOffset: number;
isHidden: boolean;
listeners: Array<Function>;
constructor(startHidden: boolean) {
this.listeners = [];
this.isHidden = startHidden;
}
/**
* Adds a listener to the hide event
*
* @param listener
*/
addListener(listener: Function) {
this.listeners.push(listener);
}
/**
* Notifies every listener whether they should hide or show.
*
* @param shouldHide
*/
notifyListeners(shouldHide: boolean) {
for (let i = 0; i < this.listeners.length; i++) {
this.listeners[i](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 nativeEvent The scroll event generated by the animated component onScroll prop
*/
onScroll({nativeEvent}: Object) {
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;
}
}