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WebWorker Image preloader proof of concept (Tested in Mobile Safari 6.0/IOS 6.1.3 and Chrome 33)
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| <!DOCTYPE html> | |
| <html> | |
| <head> | |
| <title>WebWorker image preloading</title> | |
| <meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0, user-scalable=no" /> | |
| </head> | |
| <body> | |
| <div id="output"></div> | |
| <script id="imgloader" type="javascript/worker"> | |
| // Not race proof or robust. Proof of concept. | |
| self.onmessage = function (e) { | |
| var urls = e.data, | |
| done = urls.length, | |
| onload = function () { | |
| if (--done === 0) { | |
| self.postMessage('Done!'); | |
| self.close(); | |
| } | |
| }; | |
| urls.forEach(function (url) { | |
| var xhr = new XMLHttpRequest(); | |
| xhr.responseType = 'blob'; | |
| xhr.onload = xhr.onerror = onload; | |
| xhr.open('GET', url, true); | |
| xhr.send(); | |
| }); | |
| }; | |
| </script> | |
| <script> | |
| // Server must expose CORS headers for XHR preload. | |
| var imgs = [ | |
| 'http://i.imgur.com/JmvCQXd.jpg', | |
| 'http://i.imgur.com/L4ipvCE.jpg', | |
| 'http://i.imgur.com/fKDIYIP.jpg', | |
| 'http://i.imgur.com/4ad4bo5.jpg', | |
| 'http://i.imgur.com/VukIBgD.jpg' | |
| ], | |
| URL = window.URL || window.webkitURL, | |
| url = URL.createObjectURL( | |
| new Blob( | |
| [document.getElementById('imgloader').textContent], | |
| { type: "text/javascript" } | |
| ) | |
| ), | |
| worker = new Worker(url); | |
| worker.onmessage = function (e) { | |
| var frag = document.createDocumentFragment(), | |
| count = imgs.length, | |
| onload = function () { | |
| delete this.onload; | |
| frag.appendChild(this); | |
| if (--count === 0) { | |
| console.log('done') | |
| document.getElementById('output').appendChild(frag); | |
| } | |
| }; | |
| // These will come immediately from cache. | |
| // Ensure cache is not disabled by dev tools. | |
| imgs.forEach(function (img) { | |
| var el = document.createElement('img'); | |
| // Avoid early reflows as images load without sizes. Wait for onload. | |
| el.onload = onload; | |
| el.src = img; | |
| }); | |
| URL.revokeObjectURL(url); | |
| }; | |
| // Mix in a cache-bust on every run. | |
| imgs = imgs.map(function (img) { | |
| return img + '?ts=' + Date.now(); | |
| }); | |
| worker.postMessage(imgs); | |
| </script> | |
| </body> | |
| </html> |
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