一、定義
預加載圖片是提升用戶體驗的一個好辦法，提前加載用戶所需的圖片，保證圖片快速、無縫發布，使用戶在瀏覽器網站時獲得更好用戶體驗。常用于圖片畫廊等應用中。
[注意]若使用即時加載，加載的圖片與頁面的其他內容一起加載會增加頁面的整體加載時間，所以使用window.onload比較合適。
二、兩種思路
1、使用背景圖像
使用頁面無用元素的背景圖片預加載
<style>body{ margin: 0;}img{ width: 100px; height: 100px;}ul{ margin: 0; padding: 0; list-style: none;}.list li{ height: 0; width: 0;}</style></head><body><button>載入圖片</button><img src="img/test.png" alt="測試"><ul class="list"> <li id="preload1"></li> <li id="preload2"></li> <li id="preload3"></li> <li id="preload4"></li></ul><script>var oBtn = document.getElementsByTagName('button')[0];var oImg0 = document.images[0];var array = ["img/img1.gif","img/img2.gif","img/img3.gif","img/img4.gif"]var iNow = -1;oBtn.onclick = function(){ iNow++; iNow = iNow%4; oImg0.src = array[iNow];}function preLoadImg(){ preload1.style.background = "url('img/img1.gif')"; preload2.style.background = "url('img/img2.gif')"; preload3.style.background = "url('img/img3.gif')"; preload4.style.background = "url('img/img4.gif')";}window.onload = function(){ preLoadImg(); }</script></body>2、使用Image()
通過new Image()或document.createElement('img')創建<img>標簽，然后通過<img>src賦值語句來加載圖片
<style>body{ margin: 0;}img{ width: 100px; height: 100px;}</style></head><body><button>載入圖片</button><img src="img/test.png" alt="測試"><script>var oBtn = document.getElementsByTagName('button')[0];var oImg0 = document.images[0];var array = ["img/img1.gif","img/img2.gif","img/img3.gif","img/img4.gif"]var iNow = -1;oBtn.onclick = function(){ iNow++; iNow = iNow%4; oImg0.src = array[iNow];}var aImages = [];function preLoadImg(array){ for(var i = 0, len = preLoadImg.arguments[0].length; i < len; i++){ aImages[i] = new Image(); aImages[i].src = preLoadImg.arguments[0][i]; }}window.onload = function(){ preLoadImg(array); }</script></body>三、onload事件
利用圖像的onload事件可以確切地知道圖片是否被真正加載，并可能在后續執行一系列對圖片的操作功能，如獲取當前圖片的實際寬高及索引等
[注意1]圖片的src賦值語句必須放在圖片的onload事件后面。否則可能出現圖片已經加載完畢、但事件綁定尚未完成的情況
<button>載入圖片</button><script>var oBtn = document.getElementsByTagName('button')[0];oBtn.onclick = function(){ preLoadImg('img/test.png');}function preLoadImg(url){ var oImg = document.createElement('img'); //在本機環境下，IE8-瀏覽器下oImg的onload事件放在src后面將無法載入圖片 oImg.src = url; oImg.onload = function(){ document.body.appendChild(oImg); oImg.onload = null; oImg = null; } }</script> [注意2]Image對象的onload屬性引用了一個匿名函數對象，而匿名函數通過其作用域引用Image對象，這種循環引用會有IE6中導致內存泄漏，因此，應該解除循環引用。
【遞歸寫法】
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>Document</title><style>body{ margin: 0;}img{ width: 100px; height: 100px;}</style></head><body><button>載入圖片</button><img src="img/test.png" alt="測試"><script>var oBtn = document.getElementsByTagName('button')[0];var oImg0 = document.images[0];var array = ["img/img1.gif","img/img2.gif","img/img3.gif","img/img4.gif"]var iNow = -1;oBtn.onclick = function(){ iNow++; iNow = iNow%4; oImg0.src = array[iNow];}var oImg = document.createElement('img');var iDown = 0; preLoadImg();function preLoadImg(){ oImg.onload = function(){ iDown++; alert('第' + iDown + '張圖片的寬:' + this.width + ' 高:' + this.height); if(iDown < array.length){ preLoadImg(); }else{ oImg.onload = null; oImg = null; } } oImg.src = array[iDown]; }</script></body></html>【考慮onerror的更完善寫法】
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>Document</title><style>body{ margin: 0;}img{ width: 100px; height: 100px;}</style></head><body><button>載入圖片</button><img src="img/test.png" alt="測試"><script>var oBtn = document.getElementsByTagName('button')[0];var oImg0 = document.images[0];var array = ["img/img1.gif","img/img2.gif","img/img3.gif","img/img4.gif"]var iNow = -1;oBtn.onclick = function(){ iNow++; iNow = iNow%4; oImg0.src = array[iNow];}var iDown = 0;var oImage = new Image();function preLoadImg(arr){ function loadImgTest(arr){ iDown++; if(iDown < arr.length){ preLoadImg(arr); }else{ alert('ok'); oImg.onload = null; oImg = null; } } oImage.onload = function(){ loadImgTest(arr); }; oImage.onerror = function(){ loadImgTest(arr); }; oImage.src = arr[iDown];}preLoadImg(array);</script></body></html>【循環寫法】
<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>Document</title><style>body{ margin: 0;}img{ width: 100px; height: 100px;}</style></head><body><button>載入圖片</button><img src="img/test.png" alt="測試"><script>var oBtn = document.getElementsByTagName('button')[0];var oImg0 = document.images[0];var array = ["img/img1.gif","img/img2.gif","img/img3.gif","img/img4.gif"]var iNow = -1;oBtn.onclick = function(){ iNow++; iNow = iNow%4; oImg0.src = array[iNow];}function preLoadImg(arr,callback){ var aImages = []; var iDown = 0; for(var i = 0; i < arr.length; i++){ aImages[i] = new Image(); aImages[i].onload = function(){ loadImgTest(arr,callback); }; aImages[i].onerror = function(){ loadImgTest(arr,callback); }; aImages[i].src = arr[iDown]; } function loadImgTest(arr,callback){ iDown++; if(iDown == arr.length){ alert('ok'); callback && callback.call(aImages); } } }preLoadImg(array,function(){ console.log(this[0].width);});</script></body></html>應用：預加載模糊變清晰<!DOCTYPE html><html lang="en"><head><meta charset="UTF-8"><title>Document</title><style>body{ margin: 0;}img{ width: 500px; height: 500px;}</style></head><body><button>載入圖片</button><img src="#" alt="測試"><script>var oBtn = document.getElementsByTagName('button')[0];var oImg0 = document.images[0];var arrayB = ["img/img1.gif","img/img2.gif","img/img3.gif","img/img4.gif"];var arrayL = ["img/img1.jpg","img/img2.jpg","img/img3.jpg","img/img4.jpg"];var iNow = -1;oBtn.onclick = function(){ iNow++; iNow = iNow%4; oImg0.src = arrayL[iNow]; aftLoadImg(arrayB,oImg0);}var aImages = [];window.onload = function(){ preLoadImg(arrayL); }function preLoadImg(arr){ for(var i = 0, len = arr.length; i < len; i++){ aImages[i] = new Image(); aImages[i].src = arr[i]; }}function aftLoadImg(arr,obj){ var oImg = new Image(); oImg.onload = function(){ obj.src = arr[iNow]; } oImg.src = arr[iNow];}</script></body></html>希望本文所述對大家的javascript程序設計有所幫助。
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