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HTML forms. The World Wide Web Consortium (W3C) spec includes a few other common methods, including HEAD, which fetches the headers only for a file; PUT, for uploading documents to the server; and DELETE, for removing documents.
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Subsequent headers do a lot of negotiation, with the client telling the server what content types, character sets, and so on it can understand. Because I ve visited Google before, it also sends a cookie, a short message telling Google who I am. The response headers, shown here, also contain quite a lot of information:
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HTTP/1.x 302 Found Location: http://www.google.co.uk/cxfer c=PREF%3D: TM%3D1116601572:S%3DzFxPsBpXhZzknVMF&prev=/ Set-Cookie: PREF=ID=cabd38877dc0b6a1:CR=1:TM=1116601572: LM=1116943140:S=fRfhD-u49xp9UE18; expires=Sun, 17-Jan-2038 19:14:07 GMT; path=/; domain=.google.com Content-Type: text/html Server: GWS/2.1 Transfer-Encoding: chunked Content-Encoding: gzip Date: Tue, 24 May 2005 17:59:00 GMT Cache-Control: private, x-gzip-ok=""
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The first line indicates the status of the response. A 302 response indicates a redirection to a different page. In addition, another cookie is passed back for this session. The content type of the response (aka MIME type) is also declared. A further request is made on the strength of the redirect instruction, resulting in a second response with the following headers:
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HTTP/1.x 200 OK Cache-Control: private Content-Type: text/html Content-Encoding: gzip Server: GWS/2.1 Content-Length: 1196 Date: Tue, 24 May 2005 17:59:00 GMT
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Status code 200 indicates success, and the Google home page will be attached to the body of this response for display. The content-type header tells the browser that it is html. Our sendRequest() method is constructed so that the second and third parameters, which we probably won t need most of the time, are optional, defaulting to using POST to retrieve the resource with no parameters passed in the request body. The code in this listing sets the request in motion and will return control to us immediately, while the network and the server take their own sweet time.
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Loading data asynchronously using XML technologies
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This is good for responsiveness, but how do we find out when the request has completed
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2.5.4 Using callback functions to monitor the request
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The second part of the equation for handling asynchronous communications is setting up a reentry point in your code for picking up the results of the call once it has finished. This is generally implemented by assigning a callback function, that is, a piece of code that will be invoked when the results are ready, at some unspecified point in the future. The window.onload function that we saw in listing 2.9 is a callback function. Callback functions fit the event-driven programming approach used in most modern UI toolkits keyboard presses, mouse clicks, and so on will occur at unpredictable points in the future, too, and the programmer anticipates them by writing a function to handle them when they do occur. When coding UI events in JavaScript, we assign functions to the onkeypress, onmouseover, and similarly named properties of an object. When coding server request callbacks, we encounter similar properties called onload and onreadystatechange. Both Internet Explorer and Mozilla support the onreadystatechange callback, so we ll use that. (Mozilla also supports onload, which is a bit more straightforward, but it doesn t give us any information that onreadystatechange doesn t.) A simple callback handler is demonstrated in listing 2.10.
Listing 2.10 Using a callback handler
var READY_STATE_UNINITIALIZED=0; var READY_STATE_LOADING=1; var READY_STATE_LOADED=2; var READY_STATE_INTERACTIVE=3; var READY_STATE_COMPLETE=4; var req; function sendRequest(url,params,HttpMethod){ if (!HttpMethod){ HttpMethod="GET"; } req=getXMLHTTPRequest(); if (req){ req.onreadystatechange=onReadyStateChange; req.open(HttpMethod,url,true); req.setRequestHeader ("Content-Type", "application/x-www-form-urlencoded"); req.send(params); } }
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function onReadyStateChange(){ var ready=req.readyState; var data=null; if (ready==READY_STATE_COMPLETE){ data=req.responseText; }else{ data="loading...["+ready+"]"; } //... do something with the data... }
First, we alter our sendRequest() function to tell the request object what its callback handler is, before we send it off. Second, we define the handler function, which we have rather unimaginatively called onReadyStateChange(). readyState can take a range of numerical values. We ve assigned descriptively named variables to each here, to make our code easier to read. At the moment, the code is only interested in checking for the value 4, corresponding to completion of the request. Note that we declare the request object as a global variable. Right now, this keeps things simple while we address the mechanics of the XMLHttpRequest object, but it could get us into trouble if we were trying to fire off several requests simultaneously. We ll show you how to get around this issue in section 3.1. Let s put the pieces together now, to see how to handle a request end to end.
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