For the first time about AJAX I have heard in the spring of this year. At once has become interested in this technology, and, as well as it is necessary in such cases, has gone to search behind clauses which could answer the questions which have arisen at me: What is such? How it works? And in what of advantage? What it is necessary to establish in addition on server/klient for work with AJAX? How it can be used on the site?. Prochtya about ten clauses, I have received the answer only on first two questions, but on the others the answer so anywhere have not found. And only after perusal of several publications in English I have finally understood, that to what. It and podviglo me to a writing of the given material.
First of all clause is addressed to the prepared people writing the programs for the Internet and familiar with such terms as object, a method, properties. However it can notebook dress thrusting kissed partially appear useful and that who simply is interested in the given question. In the list of the literature there is a necessary list of references, having taken advantage with which quite really to master technology from zero.
On a course of a statement a browser we shall understand browsers as the term: Internet Explorer 5. +, Safari 1. and 2. +, Netscape +, Opera 8. +, Mozilla Firefox (plus means the given version and newer). If speech begins to come about other versions, it will be separately mentioned.
What is such? How it works and in what of advantage? To understand, what advantages gives AJAX, it is necessary to know, how work veb-prilojeniya now. And they work on client-server technology (fig. 1). Work veb-programmy' on client-server technology the User in a browser opens any page page. On page there are hyperlinks which conduct on other pages. By pressing any of them the browser sends inquiry URL about a server with which this reference is connected. If in the nature there is no the server connected with this reference (for example when, typing URL in an address line, you were mistaken at a writing of a name of a resource), notebook dress thrusting kissed or there are problems of communication with the Internet the browser will generate the page similar shown on picture (so it looks in Opera-e): In case of existence of a server, but absence on it of the document specified in inquiry a server will create HTML page with the description of a mistake. For example, it can be all known 404-th mistake (the document is not found). Or, if all is true, the server will give new page to the answer. In any case, in a browser the new page new_page even if in comparison with old on it the pair words has changed only will be loaded. Essential enough minus of the given technology. Besides work is conducted in a synchronous mode. That is after the browser has sent on a server inquiry it the answer and while the answer is not received anything expects from it will not undertake. And at times the answer, and loading of new page can last too long. So long, that the user can not wait loadings of page and is notebook dress thrusting kissed simple close it. Therefore veb-programmmisty' resort to some dodges.
For example, using , it is possible to change dynamically background figure of the block without perezagruzki pages. It provides necessary interactivity. At prompting the cursor of the mouse the initial background picture pic_1.gif is replaced with a cell of the table pic_2.gif. At leaving the cursor from a cell there is a return process.! DOCTYPE HTML PUBLIC - // W3C // DTD HTML 4. Transitional // EN
Or such example. In a file with the image in format JPEG it is possible to write down (and to read through) metadata EXIF. Photoresources much the Internet allow to show for a displayed photo and these data. But they appear only after pressing the corresponding button the screen. Thus updating of page does not occur. It is done very simply. EXIF data are on page, but are located in the latent block by means of CSS (display: none). By pressing the button the block notebook dress thrusting kissed becomes visible (display: block). Minuses are obvious: together with page data who, probably, will not be used are transferred; to display it is possible only those data who have been sent together with page.
Once the client-server technology promoted development of a network. But as it quite often happens that moved progress to the certain period of history, hardly starts this progress to brake later. In this case became obvious, that the client-server technology in the pure state cannot meet all requirements which are shown to the some veb-prilojeniyam. Any new decision was required. And it has appeared in the form of AJAX the approach to a writing veb-prilojeniy.
The wide circulation of this concept has begun with the publication in an English-speaking part of the Internet of clause of Dzhessi James Garretta the New approach to veb-prilojeniyam in February, 20 . Clause was written by the designer and for designers. But with clause has familiarized many person, including programmers. Which have hastened to throw the author electronic messages with questions on technical details of realization of the given approach. Letters, probably, was so much, that in December of same year Garretu in the foreword of one of books on AJAX it notebook dress thrusting kissed was necessary to give the explanatory, that it is not the programmer, and questions on concrete realization AJAX-prilojeniy not to it (The truth is, I've never built an Ajax application. Sure, I've worked on Ajax projects....).
So in what key difference AJAX of technology from classical client-server? This presence AJAX-dvijka, two-piece: klient-prilojenie (written in language JavaScript) and server-prilojenie (written in any server language). Also a little other logic of dialogue prilojeniya-klienta with a server (fig. 2). Work veb-programmy' on technology AJAX Now at activation of any element of management of the interface a browser does not do inquiry of new page from a server, and starts a client part. And already prilojenie-klient, in turn, addresses to a server through inquiry requst and requests only those data who should change on page. After data notebook dress thrusting kissed acquisition data from prilojeniya-servera klient-prilojenie makes updating a part of page through DOM methods without an overload of all page as a whole. Thus work in an asynchronous mode is possible. That is when the user does not wait receptions of the answer from a server and overloads of page, and proceeds to work with page page, indifferently.
Big plus of the given approach that it does not exclude work under the client-server scheme. That is on the same page the part of elements of management can realize client-server technology, and a part - technology AJAX.
Here I would like to make small lyrical digression. The idea is a lot of clauses in which it is spoken that AJAX it not the technology, is the approach to a writing of the appendix. Basically, truly, everything, that surrounds us and there are ideas, but personally I insist on the term technology AJAX for underlining that fact that is one more technology of interaction of client and server appendices (alongside notebook dress thrusting kissed with klient-and fayl-serverny'mi technologies).
What it is necessary to establish in addition on server/klient for work with AJAX? All above-stated was cleanly theoretical reasonings of the developer which has written Garrett. Time has come to pass to practical consideration of on what is based AJAX and that it is particularly necessary to use for writing AJAX-prilojeniy.
AJAX it is deciphered as Asynchronous JavaScript + XML (asynchronous JavaScript+XML) and already it specifies, on what the technology leans. And leans practically on all same, as others veb-prilojeniya:
DOM for dynamic change of page in reply to actions of the user;
XML [,] for data exchange between client and server parts;
JavaScript actually for writing AJAX of a cursor, for maintenance of interactivity
When I have decided to work with AJAX-prilojeniem has thought, that for support it is necessary for something to put on a server. Any module, can, in the notebook dress thrusting kissed form of dll. But as has then understood, anything anywhere to establish it is not necessary. It is required to write only correctly a cursor and all. There is a question, and what for AJAX-dvijki extend in a network? The answer is simple: these are available appendices which are intended for any problems and which in a ready kind can be used on the site. For example, there is a library for PHP which refers to xAjax. It includes as the client script written on JavaScript, and a server script with which JavaScript cooperates. Remember it! I had a case when the person thought, that AJAX-prilojenie it only JavaScript, using XMLHttpRequest object, and all. But it is far not all. There is still a script on the party of a server which logic notebook dress thrusting kissed of work also should be projected, and then to write a code.
I shall not consider ready libraries. In the documentation going with them, all is painted in detail enough. Remains only, using them API to connect them to the site. Our purpose will familiarize with a stuffing of such appendices. This knowledge will help, as most to write Ajax-prilojenie, and to understand the available decision.
The basis of any AJAX-prilojeniya is XMLHttpRequest object, with its consideration and we shall begin.
At the given object it is a lot of properties and methods , but not all from them are supported by leaders to browsers. Complete support is only in FireFox-e. Therefore I result only that will work in modern browsers .
PROPERTIES: readonly onreadystatechange function Specifies function of a return call (callback function) which will be caused each time when will change readyState property. In spite of the fact that function is caused, parameters to transfer in it to not turn out. But about it notebook dress thrusting kissed hardly later in an example. readonly readyState integer a condition of inquiry. Can accept values:
- not not initialized (uninitialized), the method open has not been caused yet
- it is loaded (loaded), the method send has been caused also reciprocal zagolovki/status (property status) are received;
- interactive (interactive), there is a reception of data which are accessible through property responseText;
- completed (completed), in reply to inquiry headings and the status are received not only all, but also data from a server are accepted all, the answer is completed.
readonly responseText string the Answer of a server in the form of the ordinary text. Only reading. readonly responseXML object the answer of a server in the form of object DOM Document. It is used, if the answer of a server is correct XML the document. If the document not correct, data are not received or yet opravleny' property is equal notebook dress thrusting kissed NULL. Only reading. readonly status string the status of the answer. For example: 2 (OK), 4 (the document is not found), 5 (a time overload of a server).
METHODS: void abort Interrupts HTTP inquiry or data acquisition. Clears all properties of object by which initial values are appropriated. The method is useful together with the timer, when after certain time from the moment of inquiry (a start in a time-out) the answer from a server and has not been received. string getAllResponseHeaders returns all headings of the answer of a server in the form of the formatted line. Each new heading begins with a new line. string getResponseHeader (string header)
To return heading with a name header. void open (string method, string uri, [boolean asynch])
Prepares inquiry to the address of uri a method method (POST or GET) with the notebook dress thrusting kissed instruction of a mode asynch, an asynchronous mode or not. As a result of a call property readyState to become equal 1. void send (string data)
Initiates inquiry to a server. In inquiry data data. void setHeader (string header, string value) are transferred
Appropriates to heading with a name header, value value. Before the beginning of use of this method do not forget to cause open !
How it can be used on the site? Now, when at us is not only necessary theoretical knowledge, but also representation of that, on what practically leans AJAX, it is possible to break to a writing of the appendix. Further I result one example from the moment of a formulation of a problem before realization full to it, in the form of the appendix, explaining some subtleties on a course of a statement.
So, at notebook dress thrusting kissed us a problem: it is necessary to realize a database (DB) of drivers for various devices. Thus a DB so greater, that there is no sense to send it prilojeniyu-klientu and to do sample of it by means of JavaScript. Because of change of one value on page to overload it too it is undesirable. Personally I for realization of the given problem apply to server scripts PHP, and realizations of a DB I apply XML a file
Listing of a DB a file data.xml:? xml version = 1. encoding = windows-1251?
How in this a DB the person conducts search? Most likely, it from a root element would go on a tree of the document up to those while in the necessary branch has not found the reference or was convinced, that the driver for the given device is not present in base. Also we also shall act, using for a finding of the necessary unit or a set of notebook dress thrusting kissed units of expression of language XPath .
Listing of the form for sending data index.htm:! DOCTYPE HTML PUBLIC - // W3C // DTD HTML 4. Transitional // EN body
In the form of there are two variables: path and flag. The required way which sends on a server is stored In the first. As one element in the form of already is, this variable already has an initial value. The second variable serves to specify to a server script, that from the document it is necessary to take certain element Device. Besides the format of returned data from a server will change.
Now we shall consider JS-dvijok. All functions of a client part are collected in a script ajax.js: y = new Object function sendRequest (url, data) function responseServer function sendData (obj)
var path = Elpath.value + '/Device [' + obj.value + '] ' function addSelect (obj) function reset
}; As I already spoke, in function of property onreadystatechange it is impossible to transfer parameters. It is impossible to transfer parameters which are objects more precisely. Therefore right at the beginning we create a variable in which we shall store the reference to the object which has caused function. As the given variable notebook dress thrusting kissed is in a global visibility range of variables to address to it it will be possible from any part of the program. At present it is the most reasonable way to transfer parameters call-back functions of property onreadystatechange object
At approach of event onblur (the element select has lost focus) function sendData which prepares POST data for opravki inquiry is caused. Besides it forms XPath expression depending on value of a variable flag= (for example, // Devices/VideoCards) or flag= (for example, // Devices/VideoCards/AGP/Sapphire/Device [ATI 9600XT 2 DDR (128bit)]).
Further we cause function sendRequest in which we transfer URL a server script, and also a variable of type a line in which contain ready POST-danny'e. And first of all we create XMLHttpRequest object, the reference on which we store in a variable requestObj. Function httpRequest is kross-brauzernoy, and will work in all browsers.
Once httpRequest I did the reference to function at once at loading page through body onload = notebook dress thrusting kissed httpRequest and any more did not create XMLHttpRequest object. But as it has appeared, it works for all browsers except for IE, which each time demands to create new object. Therefore the call of the given function is done each time before sending of data.
After sending data the browser looks forward to hearing from a server. At each change of property readyState function responseServer will be caused. If the status of the answer has come with a code 2 (all normally), function addSelect will be caused which will add obtained data in DOM the current document. All browsers will look forward to hearing from a server. However after some time (time-out) will compulsorily appoint XMLHttpRequest.readyState = and will cease to expect the answer from a server. For example, for Opera value of a time-out makes seconds. Using other statuses, it is possible to add in a cursor obrabotchik mistakes in answers of a server.
Function addSelect adds in DOM the current document of notebook dress thrusting kissed one more unit DIV in which places the answer from a server. There can be a question why property responseText is used, instead of responseXML? Someone necessarily will have a desire, using this property to import the answer of a server (and the server script in the answer sends XML the document) directly in DOM the document. There was such desire and at me. I wished to import a root element sent XML a file and all its descendants a method importNode. But the browser imported an element without descendants, even in spite of the fact that the second parameter of the given method has been established in true: importNode (Object importedNode, true). Therefore not exact realization of this method while excludes its use.
On it work of a client part comes to an end. Remained not notebook dress thrusting kissed considered function reset is called to return DOM the document to an initial kind. To reach the same it is possible, having updated page on F5, but AJAX-dvijok just and is written to avoid perezagruzki page. Therefore all the elements added in the document by a script, should be removed from it also by a script.
In reply to inquiry the server script forms XML data of a kind: select name = ElementValue onblur = sendData (this)
If the required unit has name Device the usual formatted text comes back. The server script is written on PHP V and will not work on earlier versions of this interpreter as expansion for work with DOM has been entered into this language only from the fifth version, and has replaced with itself expansion DOM XML which interface mismatched the specification. And now we shall look at a code of a server script // we create object XPat // we create object XML-dokumen / //
Otherwise we create attribute with JS a code which and prisvoi / notebook dress thrusting kissed $optgroup-appendChild ($option) // the element / $root - appendChild ($DOMText) $NamedNodeMapAttr = $DOMNode-attributes / $DOMElement = $replyXML-createElement (' a ') $root - appendChild ($DOMText) is inserted in XML-dokument turned out
The list of the useful literature: W3C.. Jesse James Garrett. W3C.. W3C.. W3C.. W3C.. D.Lebedev.. Mozilla Developer Center.. W3C.. XULPlanet.. W3C.. (
It is published on September, 18th, 20 Other discussions in conference: [the day before yesterday, ] [8/31/2007, ] [8/31/2007, ] Comments? Amendments? Additions?
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