JavaScript图像动画的小demo_javascript技巧

 代码如下:






/**
* ImageLoop.js: An ImageLoop class for performing image animations
*
* Constructor Arguments:
* imageId: the id of the tag which will be animated
* fps: the number of frames to display per second
* frameURLs: an array of URLs, one for each frame of the animation
*
* Public Methods:
* start(): start the animation (but wait for all frames to load first)
* stop(): stop the animation
*
* Public Properties:
* loaded: true if all frames of the animation have loaded,
* false otherwise
*/
function ImageLoop(imageId, fps, frameURLs) {
// Remember the image id. Don't look it up yet since this constructor
// may be called before the document is loaded.
this.imageId = imageId;
// Compute the time to wait between frames of the animation
this.frameInterval = 1000 / fps;
// An array for holding Image objects for each frame
this.frames = new Array(frameURLs.length);

this.image = null; // The element, looked up by id
this.loaded = false; // Whether all frames have loaded
this.loadedFrames = 0; // How many frames have loaded
this.startOnLoad = false; // Start animating when done loading?
this.frameNumber = -1; // What frame is currently displayed
this.timer = null; // The return value of setInterval()

// Initialize the frames[] array and preload the images
for (var i = 0; i < frameURLs.length; i++) {
this.frames[i] = new Image(); // Create Image object
// Register an event handler so we know when the frame is loaded
this.frames[i].onload = countLoadedFrames; // defined later
this.frames[i].src = frameURLs[i]; // Preload the frame's image
}

// This nested function is an event handler that counts how many
// frames have finished loading. When all are loaded, it sets a flag,
// and starts the animation if it has been requested to do so.
var loop = this;
function countLoadedFrames() {
loop.loadedFrames++;
if (loop.loadedFrames == loop.frames.length) {
loop.loaded = true;
if (loop.startOnLoad) loop.start();
}
}

// Here we define a function that displays the next frame of the
// animation. This function can't be an ordinary instance method because
// setInterval() can only invoke functions, not methods. So we make
// it a closure that includes a reference to the ImageLoop object
this._displayNextFrame = function () {
// First, increment the frame number. The modulo operator (%) means
// that we loop from the last to the first frame
loop.frameNumber = (loop.frameNumber + 1) % loop.frames.length;
// Update the src property of the image to the URL of the new frame
loop.image.src = loop.frames[loop.frameNumber].src;
};
}

/**
* This method starts an ImageLoop animation. If the frame images have not
* finished loading, it instead sets a flag so that the animation will
* automatically be started when loading completes
*/
ImageLoop.prototype.start = function () {
if (this.timer != null) return; // Already started
// If loading is not complete, set a flag to start when it is
if (!this.loaded) this.startOnLoad = true;
else {
// If we haven't looked up the image by id yet, do so now
if (!this.image) this.image = document.getElementById(this.imageId);
// Display the first frame immediately
this._displayNextFrame();
// And set a timer to display subsequent frames
this.timer = setInterval(this._displayNextFrame, this.frameInterval);
}
};

/** Stop an ImageLoop animation */
ImageLoop.prototype.stop = function () {
if (this.timer) clearInterval(this.timer);
this.timer = null;
};

function de() {
var animation = new ImageLoop("loop", 1, ["/Uploads/img/img_01.jpg", "img/img_02.jpg",]);
var sta = document.getElementById("sta");
var stp = document.getElementById("stp");
sta.onclick = function () {
animation.start();
}
stp.onclick = function () {
animation.stop();
}
}
window.onload = function () {
de();
}




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