ajax英文文献Word文档下载推荐.doc

ajax英文文献Word文档下载推荐.doc

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XMLHttpRequest;

WebApplications

I.INTRODUCTION

AjaxisshorthandforAsynchronousJavaScriptandXML（andDHTML,andsoon）.ThephrasewascoinedbyJesseJamesGarrettofAdaptivePathandis,accordingtoJesse,notmeanttobeanacronym.However,Ajaxisfarmorethanjustafad;

it'

sapowerfulapproachtobuildingWebsitesandit'

snotnearlyashardtolearnasanentirenewlanguage.

Figure1.Thetraditionalmodelforwebapplications（left）comparedtotheAjaxmodel（right）

Ajaxisn’tatechnology.It’sreallyseveraltechnologies,eachflourishinginitsownright,comingtogetherinpowerfulnewways.Ajaxincorporates:

.standards-basedpresentationusingXHTMLandCSS;

.dynamicdisplayandinteractionusingtheDocumentObjectModel;

.datainterchangeandmanipulationusingXMLandXSLT;

.asynchronousdataretrievalusingXMLHttpRequest;

.JavaScriptbindingeverythingtogether.

Theclassicwebapplicationmodelworkslikethis:

MostuseractionsintheinterfacetriggeranHTTPrequestbacktoawebserver.Theserverdoessomeprocessing—retrievingdata,crunchingnumbers,talkingtovariouslegacysystems—andthenreturnsanHTMLpagetotheclient.It’samodeladaptedfromtheWeb’soriginaluseas

ahypertextmedium,butasfansoftheElementsofUserExperienceknow,whatmakestheWebgoodforhypertextdoesn’tnecessarilymakeitgoodforsoftwareapplications.

Thisapproachmakesalotoftechnicalsense,butitdoesn’tmakeforagreatuserexperience.Whiletheserverisdoingitsthing,what’stheuserdoing?

That’sright,waiting.Andateverystepinatask,theuserwaitssomemore.Obviously,wewouldn’tmakeuserswaitaroundindesigningtheWebfromscratchforapplications.Onceaninterfaceisloaded,whyshouldtheuserinteractioncometoahalteverytimetheapplicationneedssomethingfromtheserver?

Infact,whyshouldtheuserseetheapplicationgototheserveratall?

II.MAKEASYNCHRONOUSREQUESTSWITH

JAVASCRIPTANDAJAX

MostWebapplicationsusearequest/responsemodelthatgetsanentireHTMLpagefromtheserver.Theresultisaback-and-forththatusuallyinvolvesclickingabutton,waitingfortheserver,clickinganotherbutton,andthenwaitingsomemore.WithAjaxandtheXMLHttpRequestobject,wecanusearequest/responsemodelthatneverleavesuserswaitingforaservertorespond.

A.IntroducingXMLHttpRequestXMLHttpRequesthasactuallybeenaroundinseveralbrowsersforquiteawhile,whichisthekeytoWeb2.0,Ajax,andprettymucheverything.Togiveareallyquickoverview,thesearejustafewofthemethodsandpropertieswillbeusedonthisobject:

.open（）:

Setsupanewrequesttoaserver.

.send（）:

Sendsarequesttoaserver.

.abort（）:

Bailsoutofthecurrentrequest.

.readyState:

ProvidesthecurrentHTMLreadystate.

.responseText:

Thetextthattheserversendsbacktorespondtoarequest.

Eachofthesemethodsandpropertiesrelatetosendingarequestanddealingwitharesponse.Infact,ifwesaweverymethodandpropertyofXMLHttpRequest,theywouldallrelatetothatverysimplerequest/responsemodel.Soclearly,wewon'

tlearnaboutanamazingnewGUIobjectorsomesortofsuper-secretapproachtocreatinguserinteraction;

programmerswillworkwithsimplerequestsandsimpleresponses.Itmightnotsoundexciting,butcarefuluseofthisoneobjectcantotallychangeourapplications.

First,weneedtocreateanewvariableandassignittoaninstanceoftheXMLHttpRequestobject.That'

sprettysimpleinJavaScript,justusethenewkeywordwiththeobjectname,likeinFig.2.

Figure2.CreateanewXMLHttpRequestobject

Inreallife,thingscangowrongandthiscodedoesn'

tprovideanyerror-handling.Aslightlybetterapproachistocreatethisobjectandhaveitgracefullyfailifsomethinggoeswrong.Forexample,manyolderbrowsers（believeitornot,peoplearestillusingoldversionsofNetscapeNavigator）don'

tsupportXMLHttpRequestandWebapplicationsneedtoletthoseusersknowthatsomethinghasgonewrong.Fig.3showshowtocreatethisobjectsoifsomethingfails,itthrowsoutaJavaScriptalert.

Figure3.CreateXMLHttpRequestwithsomeerror-handlingabilities

ItturnsoutthatMicrosoftsupportsAjax,butcallsitsversionofXMLHttpRequestsomethingdi