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有時需要擷取圖片的尺寸，這需要在圖片載入完成以後才可以。有三種方式實現，下面一一介紹。
<!DOCTYPE HTML> <html> <head> <meta charset="utf-8"> <title>img - load event</title> </head> <body> <img id="img1" src="http://pic1.win4000.com/wallpaper/f/51c3bb99a21ea.jpg"> <p id="p1">loading...</p> <script type="text/javascript"> img1.onload = function() { p1.innerHTML = 'loaded' } </script> </body> </html>
測試，所有瀏覽器都顯示出了“loaded”，說明所有瀏覽器都支援img的load事件。
二、readystatechange事件
<!DOCTYPE HTML> <html> <head> <meta charset="utf-8"> <title>img - readystatechange event</title> </head> <body> <img id="img1" src="http://pic1.win4000.com/wallpaper/f/51c3bb99a21ea.jpg"> <p id="p1">loading...</p> <script type="text/javascript"> img1.onreadystatechange = function() { if(img1.readyState=="complete"||img1.readyState=="loaded"){ p1.innerHTML = 'readystatechange:loaded' } } </script> </body> </html>
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readyState為complete和loaded則表明圖片已經載入完畢。測試IE6-IE10支援該事件，其它瀏覽器不支援。
三、img的complete屬性
<!DOCTYPE HTML> <html> <head> <meta charset="utf-8"> <title>img - complete attribute</title> </head> <body> <img id="img1" src="http://pic1.win4000.com/wallpaper/f/51c3bb99a21ea.jpg"> <p id="p1">loading...</p> <script type="text/javascript"> function imgLoad(img, callback) { var timer = setInterval(function() { if (img.complete) { callback(img) clearInterval(timer) } }, 50) } imgLoad(img1, function() { p1.innerHTML('載入完畢') }) </script> </body> </html>
輪詢不斷監測img的complete屬性，如果為true則表明圖片已經載入完畢，停止輪詢。該屬性所有瀏覽器都支援。
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