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コード

var loader={
	xhr: function(){
		return new XMLHttpRequest();
	},
	download: function(url, key, tag){
		var req=this.xhr();
		req.open('GET', url, false);
		req.overrideMimeType('text/plain; charset=x-user-defined');
		req.send(null);
		this.save(key, tag+base64encode(req.responseText));
	},
	save: function(key, value){
		localStorage[key] = value;
	},
	get: function(key){
		return localStorage[key];
	},
	has: function(key){
		return Boolean( localStorage[key] );
	},
	clear: function(key){
		if( key ){
			delete localStorage[key];
		}else{
			localStorage.clear();
		}
	}
};

function saveAudio(key, elAudio){
	var url, mime;
	var elSources=elAudio.getElementsByTagName('source');
	for( var i=0, elSource, t, s; elSource=elSources[i]; i++ ){
		t=elSource.type;
		s=elSource.src;
		if( elAudio.canPlayType(t) ){
			url=s;
			mime=t;
			break;
		}
	}
	if( url ){
		loader.download(url, key, 'data:'+mime+';base64,');
	}
	return url;
}
function playAudio(key){
//console.log( localStorage[key] );
	
	if( !loader.has(key) ){
		alert('can not play');
		return false;
	}
	
	var audio=new Audio(loader.get(key));
	audio.play();
}

// XHRでダウンロード、エンコード
function saveImageFromNetwork(key, img, mime){
	
	loader.download(img.src, key, 'data:'+mime+';base64,');
	
	return true;
}

// canvas経由でエンコード
function saveImage(key, img, mime){
	var canvas=document.createElement('canvas');
	canvas.width=img.width; canvas.height=img.height;
	canvas.getContext('2d').drawImage(img,0,0);
	
	loader.save(key, canvas.toDataURL(mime));
	
	return true;
}
function showImage(key){
//console.log( localStorage[key] );
	var img=new Image();
	img.src=loader.get(key);
	document.getElementById('output').appendChild(img);
}

/**/
var key1='audio1';
var key2='image2';
var key3='image3';

onload= function(){
	if( !loader.has(key1) ){
		saveAudio(key1, document.getElementsByTagName('audio')[0]);
	}
	
	if( !loader.has(key2) ){
		saveImage(key2, document.images[0], 'image/png');
	}
	
	if( !loader.has(key3) ){
		saveImageFromNetwork(key3, document.getElementsByTagName('img')[0], 'image/png');
	}
//playAudio(key1);
//showImage(key2);
//showImage(key3);
	
};/*onload*/