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<html><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Keywords" name="Keywords"content="JavaScript, examples, JavaScript: The Definitive Guide"><title>Example 22-4 from JavaScript: The Definitive Guide, Fifth Edition</title><style>body { font-family: arial, sans-serif}.lefttitle { font: bold x-small arial, sans-serif; margin: 5px; letter-spacing: .15em; text-transform:uppercase;}.leftcaption { font: italic x-small sans-serif; margin-bottom: 20px;}#banner { font-family:sans-serif; color:#333; font-size:x-large; font-weight:normal; padding:15px; padding-top: 3px; border-bottom:4px double #666;}#banner h1 { font-size: x-large; margin: 0px; padding: 0px;}#banner .description { font-size:large; margin-left: 2px;}pre { border: solid black 1px; background-color: #ddd; padding: 10px; margin: 0px;} </style></script></head><body><div id="banner"><h1>davidflanagan.com/javascript5</h1><span class="description">Example 22-4 from<i>JavaScript: The Definitive Guide, Fifth Edition</i></span></div><div style="float: left; 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text-align: right"><a href="index.html" tppabs="http://www.davidflanagan.com/javascript5/index.html">Table of Examples</a></div><pre>/** * ImageLoop.js: An ImageLoop class for performing image animations * * Constructor Arguments: * imageId: the id of the <img> 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 <img> 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;};</pre><div class="lefttitle" style="padding:0px; text-align: right"><a href="index.html" tppabs="http://www.davidflanagan.com/javascript5/index.html">Table of Examples</a></div><p><script type="text/javascript">amazon_ad_tag="davidflanagancom"; amazon_ad_width="300"; amazon_ad_height="250"; amazon_ad_logo="hide";</script><script type="text/javascript" src="../../www.assoc-amazon.com/s/ads.js" tppabs="http://www.assoc-amazon.com/s/ads.js"></script></div></body></html>⌨️ 快捷键说明
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