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CHAPTER 8 PERSISTENT COMMUNICATIONS PATTERN
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3. In the background, the HTTP server has put the asynchronous request that is waiting for a message on the reading stream on hold. 4. When the asynchronous request returns with a response, the response is processed and then a timer is started to call the function that executes another asynchronous request. What is important about the sequence of events is to call the timer only when the asynchronous request has returned. Calling the timer earlier would result in an infinite number of requests being made immediately, causing the web browser to lock and the server to suffer a denial of service attack. Using the timer, we are confronted with another problem: the window.setTimeout method requires a reference to a text-based script. The reference cannot be an object reference, because JavaScript when converting an object reference will reference either an undefined reference or a value that does not exist. The problem is clearly illustrated in the following source code: function runIt(value) { window.setTimeout("Loop(value)", 1000); } The function runIt has a parameter, value, which is an object reference and is used in the script expression of the method setTimeout. The problem is that the script expression is a piece of text, and the timer can execute only a piece of text, not a reference to the variable value. A solution is not to reference the variable, but to copy the value of the variable to the text script, E B V N as the following source code illustrates: function runIt(value) { window.setTimeout("Loop(" + value + ")", 1000); } In the modified source code, the function Loop will be called properly with the value of the variable value. Knowing that a variable has to be converted to a value is a step closer to the solution, but is not the entire solution. The main problem is that the variable value will reference an object instance, and serializing an object instance is a bad practice because it results in multiple object instances with similar states. Having multiple versions of the same object results in object state consistency problems. The solution is to pass a value that represents an index of an array. The resulting implementation uses the array property this.instances and associated properties (callDelay, index, instances, and instanceCount) to store and manage references of the reading streams shown in abbreviated detail as follows: this.index = this.instanceCount.counter; this.instances[ this.index] = this; this.instanceCount.counter ++; ClientCommunicator.prototype.instances = new Array(); ClientCommunicator.prototype.instanceCount = new CounterHack();
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CHAPTER 8 PERSISTENT COMMUNICATIONS PATTERN
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The property instances is assigned an Array instance, but notice that the property instances is associated with the prototype property. This means that whenever ClientCommunicator is instantiated, all instances will share the same Array instances. The result is that whatever the ClientCommunicator instance references, the property prototype. instances will manipulate the same array. Of course, the property instances could have been a global variable, but using the prototype property is object oriented. The other property, instanceCount, is an example of where the global variable concept does not work. I am going to backtrack a bit on my assertion that the property prototype is object oriented. Let me restate the assertion and say that the effect is object oriented. When defining a property associated with prototype, the values of the properties are copied from the property prototype to the property of the ClientCommunicator instance. When the property is a value type such as an integer or a double value, each ClientCommunicator instance will have its own value. If the property is a reference, the reference value is copied. Hence the property instanceCount must refer to a JavaScript reference type. The property index is the index reference for each ClientCommunicator instance. Following is the source code for the ClientCommunicator.start implementation, which is used to start the polling of the reading stream: function ClientCommunicator_start() { if(this.baseURL != null) { this.doLoop = true; window.setTimeout("PrivateLoop(" + this.index + ")", this.callDelay); } EBVN else { throw new Error("Must specify baseURL before starting communications"); } } The method ClientCommunicator_start will start the polling only if the property baseURL is assigned. If the property is assigned, a polling operation is started by calling the method setTimeout with the index (this.index) of the ClientCommunicator instance. If the property is not assigned, an Error exception is generated. When the setTimeout method expires, the function PrivateLoop is called and used to perform physical reading from the reading stream. The implementation of PrivateLoop is as follows: function PrivateLoop(index) { var tempReference = ClientCommunicator.prototype.instances[ index]; tempReference.server2Client.openCallback = function(xmlhttp) { xmlhttp.setRequestHeader("Accept", tempReference.preferredTypes); } tempReference.server2Client.complete = function(status, statusText, responseText, responseXML) { if(status == 200) { if(tempReference.listen != null) { tempReference.listen(status, statusText, responseText, responseXML); } }
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