javascript客户端解决方案 缓存提供程序

相信每一个开发者都知道缓存的重要性。从头至尾有缓存的后台(memcached,xcache等。) 来减轻db的压力。对内容分发网络（CDN）缓存中希望你的浏览器缓存那些不止一次的加载资源。当然, 有客户端缓存,所以你不要重复昂贵的操作(即使是算法或大量的运算)。

这是介绍的是一个不错的javascript的方面的客户端解决方案,可选配支持HTML5本地存储器.

Starting Simple
复制代码 代码如下:
function CacheProvider() {
// values will be stored here
this._cache = {};
}Feature detect on local storage
try {
CacheProvider.hasLocalStorage = ('localStorage' in window) && window['localStorage'] !== null;
} catch (ex) {
CacheProvider.hasLocalStorage = false;
}

这里使用try catch的主要原因是 尽管firefox支持该属性，但是需要在about:config中设置并开启，否则将会报错。所以一个简单的if else不能满足需求。

下面我们将增加对象本地存储机制的支持。这个技术是借鉴了Christopher Blizzard的一篇不错的文章 Saving data with local storage – for which those who didn't know, you can only store string's into local storage. Thus we have this…

in / out JSON parsing
复制代码 代码如下:
if (CacheProvider.hasLocalStorage) {
Storage.prototype.setObject = function(key, value) {
this.setItem(key, JSON.stringify(value));
};

Storage.prototype.getObject = function(key) {
return JSON.parse(this.getItem(key));
};
}

现在就到了我们的三个核心方法了，分别是 get, set, 和clear.

Core class functionality
复制代码 代码如下:
CacheProvider.prototype = {

/**
* {String} k - the key
* {Boolean} local - get this from local storage?
* {Boolean} o - is the value you put in local storage an object?
*/
get: function(k, local, o) {
if (local && CacheProvider.hasLocalStorage) {
var action = o ? 'getObject' : 'getItem';
return localStorage[action](k) || undefined;
} else {
return this._cache[k] || undefined;
}
},

/**
* {String} k - the key
* {Object} v - any kind of value you want to store
* however only objects and strings are allowed in local storage
* {Boolean} local - put this in local storage
*/
set: function(k, v, local) {
if (local && CacheProvider.hasLocalStorage) {
if (typeof v !== 'string')) {
// make assumption if it's not a string, then we're storing an object
localStorage.setObject(k, v);
} else {
try {
localStorage.setItem(k, v);
} catch (ex) {
if (ex.name == 'QUOTA_EXCEEDED_ERR') {
// developer needs to figure out what to start invalidating
throw new Exception(v);
return;
}
}
}
} else {
// put in our local object
this._cache[k] = v;
}
// return our newly cached item
return v;
},

/**
* {String} k - the key
* {Boolean} local - put this in local storage
* {Boolean} o - is this an object you want to put in local storage?
*/
clear: function(k, local, o) {
if (local && CacheProvider.hasLocalStorage) {
localStorage.removeItem(k);
}
// delete in both caches - doesn't hurt.
delete this._cache[k];
}

};

如何运用？
注意在这篇文章的开始，就说了Cache Provider 是可选支配的本地存储，首先然让我们看一个没有本地存储的例子：

getElementsByClassName
复制代码 代码如下:
var cache = new CacheProvider;

window.getElementsByClassName = getElementsByClassName || function(c) {
var reg = cache.get(c) || cache.set(c, new RegExp("(?:^|s+)" + c + "(?:s+|$)"));
var elements = document.getElementsByTagName('*');
var results = [];
for (var i = 0; i < elements.length; i++) {
if (elements[i].className.match(reg)) {
results.push(elements[i]);
}
}
return results;
};

备注：下次你调用类函数的时候， 将会用预先编译好的正则表达式替代够建造一个表达式。


再举一个例子：比如 对于大的应用程序需要i18n，你可以缓存一个编译好的html字符串进入本地存储中。
复制代码 代码如下:
var i18nCache = new CacheProvider;

if (i18nCache.get('topnav')) {
$('#nav').html(i18nCache.get('topnav'));
} else {
ajax('top-nav.tmpl', function(html) {
i18nCache.set('topnav', html);
$('#nav').html(i18nCache.get('topnav'));
});
}

除此之外，你开可以做很多外部资源缓存到本地的事情，加油:)