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CHAPTER 4 CACHE CONTROLLER PATTERN
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parameters. The first parameter is the URL that is downloaded, and the second URL is the complete function that receives the results. To illustrate how the HTML content is cached, it is not helpful to show the pages after they have been downloaded. Showing images after the fact illustrates that content is available and presented but does not illustrate where the content came from whether it was from the cache or from an HTTP request. A better way to show that there is a cache with content is to set a breakpoint in the code and illustrate the contents of the cache with a debugger. Figure 4-4 shows the variable CacheController and the cache that it contains.
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Figure 4-4. Mozilla debugger illustrating that there are two items in the cache
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In Figure 4-4, the this variable in the middle-left window is the CacheController instance. The property _cache is an Array instance that contains the cached objects. Notice that stored in the cache are the objects chunkhtml01.html and chunkimage02.html, which happen to be the HTTP content retrieved by the buttons.
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Implementing CacheController and CacheProxy
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Implementing CacheController requires implementing a script-defined passive cache. It would seem that writing a passive cache is a bad idea because the browser already does this. The scriptdefined passive cache is necessary because of the browser-defined passive cache inconsistencies. The script-defined passive cache does not implement any sophisticated cache algorithm. However, if you wanted to extend the functionality of the cache, you could. The variable CacheController implements the client side of the HTTP Validation model. Implementing the HTTP Validation model on the client side requires the client to receive, store, and send entity tags when sending requests and receiving responses. Then based on the HTTP return codes, the cache controller receives, stores, and returns new content, or returns old content to the consuming script of the passive cache. The following is the implementation of CacheController (the details of the getURL function are missing because that function is fairly complicated and will be explained in pieces later in the chapter): var CacheController = { cache : new Array(), EBVN prefetch : function( url) { }, didNotFindETagError : function(url) { } getCachedURL : function(url) { var func = function(status, statusText, responseText, responseXML) { } CacheController.getURL(url, func, true); }, getURL : function(url, inpCP, calledFromCache) { } } At first glance, the cache seems to expose one property and three functions. The reality is that CacheController is making extensive use of JavaScript anonymous functions, making the implementation contain more functions than illustrated. The effectiveness of CacheController depends completely on the HTTP server; if the server does not use entity tags, no caching will occur and CacheController will pass all requests directly to the user s complete function implementation. The property _cache is an Array instance that contains a series of objects representing the cache. Each entry in the cache is associated with and found by using a URL. HTTP content is added to the array when the CacheController internally-defined complete method s parameter status has a value of 200, indicating a successful downloading of HTTP content. The CacheController internally-defined complete method is an anonymous function assigned to asynchronous.complete. The property prefetch is a function that is assigned by the HTML code to preload HTML content pieces into the cache. If the default prefetch function implementation is used, the predictive cache becomes passive because the prefetch function does nothing.
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CHAPTER 4 CACHE CONTROLLER PATTERN
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The function getCachedURL retrieves HTTP content from a server, and is called by the prefetch function defined by HTML code. The implementation of getCachedURL passes three parameters to getURL. The first parameter is the URL that is being retrieved. The second parameter is the complete function implementation, which for the prefetch implementation is neither required nor desired and hence is an empty function declaration. The third parameter, and the only one required to be passed by the getCachedURL function, stops the prefetch function from being called again. Otherwise, a recursive loop of calling getURL that calls prefetch that calls getURL and so on could be initiated. The method getURL retrieves HTTP content that is added to the cache. The method is called by the still undefined CacheProxy. Following is an implementation of getURL without the implementation of the anonymous functions: getURL : function(url, inpCP, calledFromCache) { var asynchronous = new Asynchronous(); var cacheProxy = inpCP; asynchronous.openCallback = function(xmlhttp) { } asynchronous.complete = function(status, statusText, responseText, responseXML) { } asynchronous.get(url); if( calledFromCache != true) { CacheController.prefetch(url); EBVN } } Within the implementation of getURL, the anonymous function has been used several times. The anonymous function solves the problem of associating variables with object instances, as explained in 2. In the abbreviated implementation, each time getURL is called, an instance of Asynchronous is created. This was done on purpose so that multiple HTTP requests could retrieve data concurrently. Remember that CacheController is a single instance. The function openCallback is new and is used to perform a callback operation after the XMLHttpRequest.open method has been called. The implementation of openCallback is called by Asynchronous after the XMLHttpRequest.open method has been called. The function openCallback is required because it is not possible to call the method XMLHttpRequest.setRequestHeader until the XMLHttpRequest.open method has been called. The anonymous function assignment of asynchronous.complete is needed so that when a URL has been retrieved, the data can be processed. In the implementation of asynchronous. complete are the details of the Cache Controller pattern implementation. The method asynchronous.get calls XMLHttpRquest and the server. After the call has been made and the getURL method is not called from a prefetch implementation (calledFromCache != true), the prefetch implementation is called. In this implementation of CacheManager, the prefetch function is called before the request has a chance to return with the data. The prefetch function is called before a response can be generated because it is assumed that the prefetch logic can deduce from a URL what an associated URL is. However, in some situations a URL cannot be deduced because the associated URLs are defined in the response of the request. This happens when the Decoupled Navigation pattern is implemented. In that case, the CacheManager prefetch-calling functionality has to be
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