逼真的HTML5树叶飘落动画图文代码详解

<p id="container">
  <!-- The container is dynamically populated using the init function in leaves.js -->
  <!-- Its dimensions and position are defined using its id selector in leaves.css -->
  <p id="leafContainer"></p>
  <!-- its appearance, dimensions, and position are defined using its id selector in leaves.css -->
  <p id="message">
   <em>这是基于webkit的落叶动画</em>
  </p>
</p>

#container {
    position: relative;
    height: 700px;
    width: 500px;
    margin: 10px auto;
    overflow: hidden;
    border: 4px solid #5C090A;
    background: #4E4226 url('images/backgroundLeaves.jpg') no-repeat top left;
}

/* Defines the position and dimensions of the leafContainer p */
#leafContainer 
{
    position: absolute;
    width: 100%;
    height: 100%;
}

/* Defines the appearance, position, and dimensions of the message p */
#message
{
    position: absolute;
    top: 160px;
    width: 100%;
    height: 300px;
    background:transparent url('images/textBackground.png') repeat-x center;
    color: #5C090A;
    font-size: 220%;
    font-family: 'Georgia';
    text-align: center;
    padding: 20px 10px;
    -webkit-box-sizing: border-box;
    -webkit-background-size: 100% 100%;
    z-index: 1;
}

p {
  margin: 15px;
}

a
{
  color: #5C090A;
  text-decoration: none;
}

/* Sets the color of the "Dino's Gardening Service" message */
em 
{
    font-weight: bold;
    font-style: normal;
}

.phone {
  font-size: 150%;
  vertical-align: middle;
}

/* This CSS rule is applied to all p elements in the leafContainer p.
   It styles and animates each leafp.
*/
#leafContainer > p 
{
    position: absolute;
    width: 100px;
    height: 100px;

    /* We use the following properties to apply the fade and drop animations to each leaf.
       Each of these properties takes two values. These values respectively match a setting
       for fade and drop.
    */
    -webkit-animation-iteration-count: infinite, infinite;
    -webkit-animation-direction: normal, normal;
    -webkit-animation-timing-function: linear, ease-in;
}

/* This CSS rule is applied to all img elements directly inside p elements which are
   directly inside the leafContainer p. In other words, it matches the 'img' elements
   inside the leafps which are created in the createALeaf() function.
*/
#leafContainer > p > img {
     position: absolute;
     width: 100px;
     height: 100px;

    /* We use the following properties to adjust the clockwiseSpin or counterclockwiseSpinAndFlip
       animations on each leaf.
       The createALeaf function in the Leaves.js file determines whether a leaf has the 
       clockwiseSpin or counterclockwiseSpinAndFlip animation.
    */
     -webkit-animation-iteration-count: infinite;
     -webkit-animation-direction: alternate;
     -webkit-animation-timing-function: ease-in-out;
     -webkit-transform-origin: 50% -100%;
}

/* Hides a leaf towards the very end of the animation */
@-webkit-keyframes fade
{
    /* Show a leaf while into or below 95 percent of the animation and hide it, otherwise */
    0%   { opacity: 1; }
    95%  { opacity: 1; }
    100% { opacity: 0; }
}

/* Makes a leaf fall from -300 to 600 pixels in the y-axis */
@-webkit-keyframes drop
{
    /* Move a leaf to -300 pixels in the y-axis at the start of the animation */
    0%   { -webkit-transform: translate(0px, -50px); }
    /* Move a leaf to 600 pixels in the y-axis at the end of the animation */
    100% { -webkit-transform: translate(0px, 650px); }
}

/* Rotates a leaf from -50 to 50 degrees in 2D space */
@-webkit-keyframes clockwiseSpin
{
    /* Rotate a leaf by -50 degrees in 2D space at the start of the animation */
    0%   { -webkit-transform: rotate(-50deg); }
    /*  Rotate a leaf by 50 degrees in 2D space at the end of the animation */
    100% { -webkit-transform: rotate(50deg); }
}

/* Flips a leaf and rotates it from 50 to -50 degrees in 2D space */
@-webkit-keyframes counterclockwiseSpinAndFlip 
{
    /* Flip a leaf and rotate it by 50 degrees in 2D space at the start of the animation */
    0%   { -webkit-transform: scale(-1, 1) rotate(50deg); }
    /* Flip a leaf and rotate it by -50 degrees in 2D space at the end of the animation */
    100% { -webkit-transform: scale(-1, 1) rotate(-50deg); }
}

/* Define the number of leaves to be used in the animation */
const NUMBER_OF_LEAVES = 30;

/* 
    Called when the "Falling Leaves" page is completely loaded.
*/
function init()
{
    /* Get a reference to the element that will contain the leaves */
    var container = document.getElementById('leafContainer');
    /* Fill the empty container with new leaves */
    for (var i = 0; i < NUMBER_OF_LEAVES; i++) 
    {
        container.appendChild(createALeaf());
    }
}

/*
    Receives the lowest and highest values of a range and
    returns a random integer that falls within that range.
*/
function randomInteger(low, high)
{
    return low + Math.floor(Math.random() * (high - low));
}

/*
   Receives the lowest and highest values of a range and
   returns a random float that falls within that range.
*/
function randomFloat(low, high)
{
    return low + Math.random() * (high - low);
}

/*
    Receives a number and returns its CSS pixel value.
*/
function pixelValue(value)
{
    return value + &#39;px&#39;;
}

/*
    Returns a duration value for the falling animation.
*/

function durationValue(value)
{
    return value + &#39;s&#39;;
}

/*
    Uses an img element to create each leaf. "Leaves.css" implements two spin 
    animations for the leaves: clockwiseSpin and counterclockwiseSpinAndFlip. This
    function determines which of these spin animations should be applied to each leaf.

*/
function createALeaf()
{
    /* Start by creating a wrapper p, and an empty img element */
    var leafp = document.createElement(&#39;p&#39;);
    var image = document.createElement(&#39;img&#39;);

    /* Randomly choose a leaf image and assign it to the newly created element */
    image.src = &#39;images/realLeaf&#39; + randomInteger(1, 5) + &#39;.png&#39;;

    leafp.style.top = "-100px";

    /* Position the leaf at a random location along the screen */
    leafp.style.left = pixelValue(randomInteger(0, 500));

    /* Randomly choose a spin animation */
    var spinAnimationName = (Math.random() < 0.5) ? &#39;clockwiseSpin&#39; : &#39;counterclockwiseSpinAndFlip&#39;;

    /* Set the -webkit-animation-name property with these values */
    leafp.style.webkitAnimationName = &#39;fade, drop&#39;;
    image.style.webkitAnimationName = spinAnimationName;

    /* Figure out a random duration for the fade and drop animations */
    var fadeAndDropDuration = durationValue(randomFloat(5, 11));

    /* Figure out another random duration for the spin animation */
    var spinDuration = durationValue(randomFloat(4, 8));
    /* Set the -webkit-animation-duration property with these values */
    leafp.style.webkitAnimationDuration = fadeAndDropDuration + &#39;, &#39; + fadeAndDropDuration;

    var leafDelay = durationValue(randomFloat(0, 5));
    leafp.style.webkitAnimationDelay = leafDelay + &#39;, &#39; + leafDelay;

    image.style.webkitAnimationDuration = spinDuration;

    // add the <img> to the <p>
    leafp.appendChild(image);

    /* Return this img element so it can be added to the document */
    return leafp;
}

/* Calls the init function when the "Falling Leaves" page is full loaded */
window.addEventListener(&#39;load&#39;, init, false);