Scroll Helper

Usage

1. Include `zuix.js` library

If not already included, add the following line right before the end of the head section in the HTML document:

<script src="https://cdn.jsdelivr.net/npm/zuix-dist"></script>

2. Load the scroll helper

Add the ctrl z-load attributes to the host element implementing the scrollbar (usually the main body or a div element):

<html>
<!-- ... -->
<body ctrl z-load="@lib/controllers/scroll-helper"
      z-context="scroll-helper">
<!-- ... -->
</body>
</html>

Option attributes

ctrl z-load="@lib/controllers/scroll-helper" constructor
load the scroll-helper controller on the element.
z-context optional
identifier name to be used to access this component from JavaScript.

Scripting

Get a reference to the scroll helper object

Before component is loaded

var scrollHelper; // it will be == null until component is loaded
zuix.context('scroll-helper', (sh) => scrollHelper = sh);

or at any time after component is loaded

var scrollHelper = zuix.context('scroll-helper');

Listen to events

scrollHelper.on('scroll:change', (e, data) => {
  switch (data.event) {

    case 'hit-top':
      // reached top of page
      break;

    case 'scroll':
      if (data.info.shift.y < 0) {
        // scrolling up
      } else if (data.info.shift.y > 0) {
        // scrolling down
      }
      // for all fields of the data.info
      // object see next paragraph
      break;

    case 'hit-bottom':
      // reached end of page
      break;

  }
});

Get scroll information

// get the scroll info object
const info = scrollHelper.info();

info -> {

  // last event timestamp
  timestamp: 1524263175438,

  // movement since previous event
  shift: {
    x: 0,
    y: -3
  },

  // total scroll area size
  size: {
    width: 1280,
    height: 5281
  },

  // visible area size and position
  viewport: {
    x: 0,
    y: -3313,
    width: 1280,
    height: 720
  }

}

Watching DOM elements

The watch method takes one argument that can be either an element object or a valid DOM query selector expression.

// watch elements having the 'watchable' class
scrollHelper.watch('.watchable', (el, data) => {
  switch (data.event) {

    case 'enter':
      // element entered the visible area
      break;

    case 'exit':
      // element out of the visible area
      break;

    case 'scroll':
      // element is being scrolled in the visible area
      break;

    case 'off-scroll':
      // element is being scrolled out of the visible area
      break;

  }
});

The data object passed to the watch callback is described below

data = {

    // event: 'enter' | 'exit' | 'scroll' | 'off-scroll'
    event,

    // element position, status, rect and frame
    x,       // float
    y,       // float

    visible, // boolean

    rect: {  // DOMRect
        x,
        y,
        height,
        width,
        left,
        right,
        top,
        bottom
    },

    frame: { // relative center position
        dx,
        dy,
    }

}

The frame object represents the position of the center of the element relative to the viewport size. Specifically, dx and dy are float numbers that have values between 0 and 1 when the element is inside the viewport. So when the element is at the center of the viewport, dx and dy will have a value of 0.5.
Otherwise, it will be less than 0 if the element is above the viewport top or lastly greater than 1 when the element is below the viewport bottom.

This value can be used to create responsive and synchronized scroll animations based on current viewport offset and elements position.

Other methods

// scroll to the start of page
scrollHelper.scrollStart( [duration_ms] );

// scroll to the end of page
scrollHelper.scrollEnd( [duration_ms] );

// scroll to element or offset
scrollHelper.scrollTo( element | offset_px );

Demos

Demo page from older zKit release

Controllers

Components