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1、UsingtheXMLHttpRequestObjectNowthatwe’vediscussedthehistoryofdynamicWebapplicationsandintroducedAjax,it’stimetocovertheheartofthematter:howtousetheXMLHttpRequestobject.WhileAjaxismoreofatechniquethanatechnology,withoutwidespreadsupportforXMLHttpRequest,GoogleSuggestandTa-daListwouldn’texistasw
2、ecurrentlyknowthem.Andyouwouldn’tbereadingthisbook!XMLHttpRequestwasoriginallyimplementedinInternetExplorer5asanActiveXcomponent.ThatitworkedonlyinInternetExplorerkeptmostdevelopersfromusingXMLHttpRequestuntilitsrecentadoptionasadefactostandardinMozilla1.0andSafari1.2.It’simportanttonotethatXM
3、LHttpRequestisnotaW3Cstandard,thoughmuchofthefunctionalityiscoveredinanewproposal:theDOMLevel3LoadandSaveSpecification.Becauseitisnotastandard,itsbehaviormaydifferslightlyfrombrowsertobrowser,thoughmostmethodsandpropertiesarewidelysupported.Currently,Firefox,Safari,Opera,Konqueror,andInternetE
4、xplorerallimplementthebehavioroftheXMLHttpRequestobjectsimilarly.Thatsaid,ifasignificantnumberofyourusersstillaccessyoursiteorapplicationwitholderbrowsers,youwillneedtoconsideryouroptions.AswediscussedinChapter1,ifyouaregoingtouseAjaxtechniques,youneedtoeitherdevelopanalternativesiteorallowyou
5、rapplicationtodegradegracefully.WithmostusagestatisticsindicatingthatonlyasmallfractionofbrowsersinusetodaylackXMLHttpRequestsupport,thechancesofthisbeingaproblemareslim.However,youneedtocheckyourWeblogsanddeterminewhatclientsyourcustomersareusingtoaccessyoursites.OverviewoftheXMLHttpRequestOb
6、jectYoumustfirstcreateanXMLHttpRequestobjectusingJavaScriptbeforeyoucanusetheobjecttosendrequestsandprocessresponses.SinceXMLHttpRequestisnotaW3Cstandard,youcanuseJavaScriptinacoupleofwaystocreateaninstanceofXMLHttpRequest.InternetExplorerimplementsXMLHttpRequestasanActiveXobject,andotherbrows
7、erssuchasFirefox,Safari,andOperaimplementitasanativeJavaScriptobject.Becauseofthesedifferences,theJavaScriptcodemustcontainlogictocreateaninstanceofXMLHttpRequestusingtheActiveXtechniqueorusingthenativeJavaScriptobjecttechnique.Theprevi
