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1、iTePub.Net-CollectPerformanceThischaptercovers■ProfilingAjaxapplications■Managingmemoryfootprints■Usingdesignpatternsforconsistentperformance■Handlingbrowser-specificperformanceissues279LicensedtojonathanzhengiTePub.Net-Collect280CHAPTER8PerformanceInthepreviou
2、sthreechapters,wehavebuiltupourunderstandingofhowAjaxapplicationscanbemaderobustandreliable—abletowithstandreal-lifeusagepatternsandchangesinrequirements.Designpatternshelpustokeepourcodeorganized,andtheprincipleofseparationofconcernskeepsthecouplinginourcodelowenoughtoallowustore
3、spondquicklytochangeswithoutbreakingthings.Ofcourse,tomakeourapplicationreallyuseful,italsohastobeabletofunc-tionatareasonablespeedandwithoutbringingtherestofouruser’scomputertoagrindinghalt.Sofar,we’vebeenoperatinginahigh-techShangri-lainwhichouruser’sworkstationshaveinfinitereso
4、urcesandwebbrowsersknowhowtomakeuseofthemeffectively.Inthischapter,we’lldescendtothegrubbysidestreetsoftherealworldandlookattheissueofperformance.We’llbetakingouridealisticrefactoringanddesignpatternswithus.Evendownhere,theycanpro-videavocabulary—andvaluableinsights—intoperformanc
5、eissuesthatwemightencounter.8.1Whatisperformance?Theperformanceofacomputerprogramhingesontwofactors:howfastitcanrunandhowmuchofthesystemresources(mostcrucially,memoryandCPUload)ittakesup.Aprogramthatistooslowisfrustratingtoworkwithformosttasks.Inamodernmultitaskingoperatingsystem,
6、aprogramthatmakestherestofauser’sactivitiesgrindtoahaltisdoublyfrustrating.Thesearebothrelativeissues.ThereisnofixedpointatwhichexecutionspeedorCPUusagebecomesaccept-able,andperceptionisimportanthere,too.Asprogrammers,weliketofocusonthelogicofourapplications.Performanceisanecessar
7、yevilthatweneedtokeepaneyeon.Ifwedon’t,ouruserswillcertainlyremindus.Likechess,computerlanguagesofferself-containedworldsthatoperatebyawell-specifiedsetofrules.Withinthatsetofrules,everythingisproperlydefinedandfullyexplicable.Thereisacertainalluretothiscomfortableclockworkworld,a
8、ndasprogrammers,wecanbetemptedtob
