VisSense.js

A utility library for observing visibility changes of DOM elements. Immediately know when an element becomes hidden, partly visible or fully visible.

VisSense.js is lightweight (<4KB minified and gzipped), tested and documented. Best of all: No dependencies.

What it does

• provides methods for detecting visibility of DOM elements
• provides a convenience class with straightforward methods like isHidden, isVisible, isFullyVisible, percentage
• provides a convenience class for detecting changes in visibility

What it does not do (by default)

• detect if an element is overlapped by others
• take elements opacity into account
• take scrollbars into account - elements "hidden" behind scrollbars are considered visible

Demos and Examples

Check out this bin for a quick demo. See more examples on the demo page.

In this simple example a video will only be started if at least 75% of its area is in the users viewport:

var video = $('#video'); 
var videoVisibility = VisSense(video[0]);

if(videoVisibility.percentage() > 0.75) { 
  video.play();
}

In the following example the video will be started if it's at least 75% visible and stopped as soon as it is not visible anymore:

var video = $('#video'); 
var visibility = VisSense(video[0], { fullyvisible: 0.75 });

var visibility_monitor = visibility.monitor({
  fullyvisible: function() { 
    video.play();
  }, 
  hidden: function() { 
    video.stop(); 
  }
}).start();

See a slightly adapted version of this example live and try it on jsbin.com.

Documentation

See vissense.github.io/vissense or generate the documentation locally.

Generate documentation

Clone the repository and run grunt docs

Download

npm

Install with npm

npm install vissense --save

Bower

Install with bower

bower install vissense/vissense --save

Github

Download from Github

cdnjs

Reference from cdnjs.com

Contribute

• Issue Tracker: https://github.com/vissense/vissense/issues
• Source Code: https://github.com/vissense/vissense

Clone Repository

git clone https://github.com/vissense/vissense.git

Install dependencies

npm install -g grunt-cli
npm install -g karma-cli
npm install -g bower
npm install
bower install

Build project

grunt

Run tests

grunt test

or

grunt serve

and it automatically opens http://localhost:3000/SpecRunner.html in your browser.

API

VisSense(element [, options])

Object constructor. Options:

• hidden (default: 0) - if percentage is equal or below this limit the element is considered hidden
• fullyvisible (default: 1) - if percentage is equal or above this limit the element is considered fully visible

Note: you can omit new keyword when calling VisSense(...)

.percentage()

gets the current visible percentage (0..1)

.isHidden()

true if element is hidden

.isVisible()

true if element is visible

.isFullyVisible()

true if element is fully visible

.state()

returns an object representing the current state

{ 
  "code": 0, 
  "state": "hidden", 
  "percentage": 0, 
  "visible": false, 
  "fullyvisible": false, 
  "hidden": true,
  "previous": {}
}

.monitor([config])

This is an alias for getting a VisSense.VisMon object observing the current element. See the options below for more details.

var element = document.getElementById('video');

var visibility_monitor = VisSense(element).monitor({
  visible: function() { 
    console.log('visible');
  }, 
  hidden: function() { 
    console.log('hidden');
  }
}).start();

VisSense.VisMon(visobj [, options])

A monitor object lets you observe an element over a period of time. It emits certain events you can subscribe to, and it can be extended with custom logic.

Object constructor. Options:

• strategy a strategy (or array of strategies) for observing the element. VisSense comes with two predefined strategies. See below.
• start function to run when monitoring the element starts
• stop function to run when monitoring the element stops
• update function to run when elements update function is called
• hidden function to run when element becomes hidden
• visible function to run when element becomes visible
• fullyvisible function to run when element becomes fully visible
• visibilitychange function to run when the visibility of the element changes
• percentagechange function to run when the percentage of the element changes
• async a boolean flag indicating whether events are synchronous or asynchronous

var visobj = VisSense(document.getElementById('video'));

var visibilityMonitor = VisSense.VisMon(visobj, { 
  strategy: [
    new VisSense.VisMon.Strategy.EventStrategy({ throttle: 42 })
  ],
  visibilitychange: function() { 
    console.log('visibilitychange');
  }, 
  visible: function() { 
    console.log('element became visible');
  }
}).start();

Strategies

Strategies are hooks which let you intercept the monitoring process. e.g. updating the monitor, sending custom events, etc.

VisSense comes with two predefined strategies:

• PollingStrategy a simple strategy which invokes update() periodically.
• EventStrategy this strategy registers event handlers for visibilitychange, scroll and resize events and calls update() accordingly.

A monitor can use any number of strategies. The default monitor uses a combination of EventStrategy and PollingStrategy. Feel free to write your own strategy to cover your specific requirements (it's super easy). You can also pass an empty array if you don't want to use any strategy.

.start()

starts observing the element returns this

.stop()

stops observing the element

.update()

manually run the update procedure. this will fire all registered hooks accordingly e.g. when a percentage change happened.

.state()

returns a state object

{ 
  "code": 1, 
  "state": "visible", 
  "percentage": 0.55, 
  "visible": true, 
  "fullyvisible": false, 
  "hidden": false 
  "previous" : { 
    "code": 2, 
    "state": "fullyvisible", 
    "percentage": 1, 
    "visible": true, 
    "fullyvisible": true, 
    "hidden": false 
  }
} 

.on(event, hook)

registers an event hook

visibility_monitor.on('percentagechange', function() { ... });

Builder

There is a builder available if you want to build more complex monitor objects.

var visobj = VisSense(document.getElementById('video'));

var visibilityMonitor = VisSense.VisMon.Builder(visobj)
  .strategy(new VisSense.VisMon.Strategy.ConfigurablePollingStrategy({
    hidden: 1000,
    visible: 2000,
    fullyvisible: 5000
  }))
  .strategy(new VisSense.VisMon.Strategy.EventStrategy({ throttle: 200 }))
  .strategy(new VisSense.VisMon.Strategy.UserActivityStrategy({ inactiveAfter: 60000 }))
  .strategy(new VisSense.VisMon.Strategy.PercentageTimeTestEventStrategy('50%/10s', { 
    percentageLimit: 0.5,
    timeLimit: 10000,
    interval: 100
  }))
  .strategy({
    start: function(monitor) {
      setTimeout(function() {
        monitor.publish('mySpecialEvent');
      }, 10000);
    }
  })
  .on('start', function (monitor) {
      console.log('[Visibility Monitor] Started!');
  })
  .on('stop', function (monitor) {
    console.log('[Visibility Monitor] Stopped!');
  })
  .on('visible', function (monitor) {
    console.log('[Visibility Monitor] Element became visible!');
  })
  .on('50%/10s', function (monitor) {
    console.log('[Visibility Monitor] Element was >50% visible for 10 seconds!');
  })
  .on('mySpecialEvent', function (monitor) {
    console.log('[Visibility Monitor] MySpecialEvent received!');
  })
  .build()
  .startAsync();

This example uses external strategies. See UserActivityStrategy, PercentageTimeTestEventStrategy and ConfigurablePollingStrategy for more information.

License

The project is licensed under the MIT license. See LICENSE for details.