Smooth movement in JavaScript

Transition effects in JavaScript can be divided into two groups: movement effects and fading effects. Movement effects look smoother when acceleration and deceleration are used to prevent the animation starting and stopping abruptly. SmoothMovement is a JavaScript object that aids the creation of such transition effects. In the example below, SmoothMovement is used in two ways: to slide between the planets and to change their size. Both transitions can occur at the same time, and the target planet can be changed while a slide is in progress.

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Download SmoothMovement

Download one of the files below and upload it to your web server.

Link to the file using a script element in the head of your page:

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<script type="text/javascript" src="SmoothMovement.js"></script>

Using SmoothMovement

When creating an instance of SmoothMovement, two parameters may be specified. The first parameter, which defaults to 0, sets the position — the value used to produce movement. The second parameter, which defaults to the same value as the position, sets the target — the value that the position will smoothly change to. For example:

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// create an instance of SmoothMovement with position 0 and target 0
var movement1 = new SmoothMovement();

// create an instance of SmoothMovement with position 100 and target 100
var movement2 = new SmoothMovement(100);

// create an instance of SmoothMovement with position 0 and target 100
var movement3 = new SmoothMovement(0, 100);

The position and target can be accessed and changed at any time using the position and target properties:

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// set the target to be 100 further than the current position
movement.target = movement.position + 100;

The update function updates the position, smoothly moving it towards the target. As a convenience, it returns the new position:

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// update the position
var newPosition = movement.update();

The hasStopped function returns whether the movement has permanently stopped (and hence whether future calls to the update function will return the same value):

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// check whether movement has permanently stopped
if (movement.hasStopped()){
  // movement has stopped
}else{
  // movement is ongoing
}

Animating with SmoothMovement

The animate function aids the creation of animations using SmoothMovement. It calls the update function and a user-supplied function at regular intervals, and can call a second user-supplied function when the animation has finished.

The animate function takes up to three parameters. The first parameter is the interval, in milliseconds, between updates. The second parameter is a function to call after each update. This function will be passed the position and the SmoothMovement instance as parameters. The optional third parameter is a function to call when the animation has finished. This function will be passed the SmoothMovement instance as a parameter.

As an example, the following code collapses an element with the ID ‘help’ by smoothly reducing its height (lines 11 to 13) and finally hiding it (lines 14 to 16).

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// find the help element
var help = document.getElementById('help');

// create an instance of SmoothMovement
var movement = new SmoothMovement(help.clientHeight, 1);

// collapse the help element
help.style.overflow = 'hidden';
movement.animate(
    20,
    function(height){
      help.style.height = height + 'px';
    },
    function(){
      help.style.display = 'none';
    });

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