Windows10环境下的Http20编程接口.docx

Windows10环境下的Http20编程接口.docx

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Windows10环境下的Http20编程接口

NOVEMBER23,20159:

27AM

DemystifyingHttpClientAPIsintheUniversalWindowsPlatform

By WindowsAppsTeam

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AsaUniversalWindowsPlatform（UWP）appdeveloper,ifyouaretryingtocommunicateoverHTTPwithawebserviceoranyserverendpoint,youhavemultipleAPIchoices.TwoofthemostusedandrecommendedAPIsforimplementingtheHTTPclientroleinamanagedUWPappare System.Net.Http.HttpClient and Windows.Web.Http.HttpClient.TheseAPIsshouldbepreferredoverolder,discouragedAPIssuchasWebClientandHttpWebRequest（althoughasmallsubsetofHttpWebRequestisavailableinUWPforbackwardcompatibility）.

WehavereceivedseveralquestionsaboutthedifferencesbetweentheseAPIs,equivalentfunctionalitiesbetweenthetwo,whichonetousewhen,andsoon.Inthispost,wewilltrytoaddressthesequestionsandhelpclarifythepurposeofthesetwoAPIs.

BriefOverview

TheSystem.Net.Http.HttpClientAPIwasfirstintroducedin.NET4.5,andavariantwasmadeavailableviaaNuGetpackagedown-levelfor.NET4.0andWindowsPhone8Silverlightapps.ThegoalofthisAPIwastoprovideasimpler,cleanerabstractionlayerandflexibilityforimplementingtheHTTPclientrole,ascomparedtotheolderHttpWebRequestAPI.Forexample,itallowschainingcustomhandlers,bywhichdeveloperscouldintercepteachrequestorresponse,andimplementcustomlogic.UpuntilWindows8.1,thisAPIhadafullymanaged.NETimplementationunderneath.InWindows10,theimplementationofthisAPIforUWPhasbeenchangedtolayeritontopofWindows.Web.HttpandtheWinINetHTTPstackofWindows.

Ontheotherhand,theWindows.Web.Http.HttpClientAPI,wasfirstintroducedinWindows8.1andwasavailableonWindowsPhone8.1aswell.TheprimarymotivationbehindcreatingthisAPIwastoconsolidatethedisparateHTTPAPIsavailablefordifferentWindowsapplanguages（C#,VB,C++,JavaScript）intoasingleonethatsupportsallthefeaturesfromeachofthoseAPIs.MostofthebasicAPIdesignwasderivedfromthatofSystem.Net.HttpandtheimplementationisbasedontheWinINetHTTPstackofWindows.

WhenusingtheseAPIsinaWindowsStoreapp,thesupportedOSversionsandprogramminglanguagesareasfollows:

API

OSVersions

SupportedLanguages

System.Net.Http.HttpClient

Windows,WindowsPhone8onwards

.NETlanguagesonly

Windows.Web.Http.HttpClient

Windows,WindowsPhone8.1onwards

AllWindowsStoreapplanguages

WhichoneshouldIuse?

SincebothoftheseAPIsareavailableinUWP,thebiggestquestionforHTTPdevelopersiswhichonetouseintheirapp.Theansweristhatitdependsonacoupleoffactors:

1.DoyouneedtointegratewithnativeUIforcollectingusercredentials,controlHTTPcachereadandwritebehavior;orpassinaspecificSSLclientcertificateforauthentication?

Ifyes–thenuseWindows.Web.Http.HttpClient.Atthetimeofthiswriting,theWindows.Web.HttpAPIprovidesgreatercontroloverHTTPsettingsinUWPthantheSystem.Net.HttpAPI.Infutureversions,theSystem.Net.HttpAPImayalsobeenhancedtosupportthesefeaturesonUWP.

2.Doyouintendtowritecross-platform.NETcode（acrossUWP/ASP.NET5/iOSandAndroid）?

Ifyes–thenuseSystem.Net.HttpAPI.Thisallowsyoutowritecodethatyoucanre-useonother.NETplatformssuchasASP.NET5and.NETFrameworkdesktopapplications.Thanksto Xamarin,thisAPIisalsosupportedoniOSandAndroid,soyoucanreuseyourcodeontheseplatformsaswell.

ObjectModel

NowthatweunderstandthemotivationbehindcreatingthesetwosimilarAPIsandtherationaleforchoosingbetweenthetwo,let’slookcloserattheobjectmodelforeachofthese.

System.Net.Http

Thetopmostabstractionlayeristhe HttpClient object,whichrepresentsthecliententityintheclient-servermodeloftheHTTPprotocol.Thisclientcanissuemultiplerequests（representedby HttpRequestMessage）totheserverandreceivethecorrespondingresponses（representedby HttpResponseMessage）.TheentitybodyandcontentheadersofeachHTTPrequestorresponseisrepresentedbythe HttpContent baseclass,andderivedclassessuchasStreamContent,MultipartContentandStringContent.TheyprovidedifferentrepresentationsoftheHTTPentitybody.EachoftheseclassesprovideasetofReadAs*methodstoreadouttheentitybodyofarequestorresponseasastring,bytearrayorastream.

EachHttpClientobjecthasahandlerobjectunderneaththatrepresentsalltheHTTP-relatedsettingsofthatclient.Conceptually,youcanthinkofthehandlerasrepresentingtheHTTPstackunderneaththeclient.Itisresponsibleforsendingtheclient’sHTTPrequeststotheserverandconveyingtheresponsebacktotheclient.

ThedefaulthandlerclassusedintheSystem.Net.HttpAPIis HttpClientHandler.WhenyoucreateanewinstanceofanHttpClientobject—forexample,callnewHttpClient（）—anHttpClientHandlerobjectisautomaticallycreatedforyouwiththedefaultHTTPstacksettings.Ifyouwanttomodifyanyofthedefaultsettingssuchascachingbehavior,automaticcompression,credentialsorproxy,youcancreateyourowninstanceofanHttpClientHandlerdirectly,modifyitspropertiesandthenpassitintotheconstructorofHttpClient,asfollows:

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HttpClientHandlermyHandler=newHttpClientHandler（）;

myHandler.AllowAutoRedirect=false;

HttpClientmyClient=newHttpClient（myHandler）;

ChainingofHandlers

OneofthekeyadvantagesoftheSystem.Net.Http.HttpClientAPIdesignistheabilitytoinsertcustomhandlersandcreateachainofhandlerobjectsunderneathanHttpClientobject.Forexample,let’ssayyouarebuildinganappthatqueriesawebserviceforsomedata.YouhavecustomlogictohandleHTTP4xx（clienterror）and5xx（servererror）responsesfromtheserverandwanttotakespecificretrysteps,suchastryingadifferentendpointoraddingusercredentials.YouwouldideallywanttoseparatethisHTTP-relatedworkfromtherestofyourbusinesslogicwhichjustcaresaboutthedatareturnedfromthewebservice.

Thiscanbeachievedbycreatinganewhandlerclassthatderivesfromthe DelegatingHandler class（e.g.CustomHandler1）,thencreateanewinstanceofitandpassthatintotheHttpClientconstructor.TheInnerHandlerpropertyoftheDelegatingHandlerclassisusedtospecifythenexthandlerinthechain–forexample,youcouldaddanothercustomhandler（e.g.CustomHandler2）tothechain.Forthelasthandler,youcansettheinnerhandlertoanHttpClientHandlerinstance–thiswillpasstherequestontotheHTTPstackoftheOS.Here’showthislooksconceptually:

Andhereisthesamplecodetoachievethis:

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publicclassCustomHandler1:

DelegatingHandler

{

    //Constructorsandothercodehere.

    protectedasyncoverrideTaskSendAsync（

        HttpRequestMessagerequest,CancellationTokencancellationToken）

    {

        //ProcesstheHttpRequestMessageobjecthere.

        Debug.WriteLine（"ProcessingrequestinCustomHandler1"）;

        //Onceprocessingisdone,callDelegatingHandler.SendAsynctopassitonthe

        //innerhandler.

        HttpResponseMessageresponse=awaitbase.SendAsync（request,cancellationToken）;

        //ProcesstheincomingHttpResponseMessageobjecthere.

        Debug.WriteLine（"ProcessingresponseinCustomHandler1"）;

        returnresponse;

    }

}

publicclassCustomHandler2:

DelegatingHandler

{

    //SimilarcodeasCustomHandler1.

}

publicclassFoo

{

    publicvoidCreateHttpClientWithChain（）

    {

        HttpClientHandlersystemHandler=newHttpClientHandler（）;

        CustomHandler1myHandler1=newCustomHandler1（）;

        CustomHandler2myHandler2=newCustomHandler2（）;

        //Chainthehandlerstogether.

        myHandler1.InnerHandler=myHandler2;

        myHandler2.InnerHandler=systemHandler;

        //Createtheclientobjectwiththetopmosthandlerinthechain.

        HttpClientmyClient=newHttpClient（myHandler1）;

    }

}

Notes:

1.Ifyouintendtosendtherequesttoaremoteserverendpoint,thelastHandlerinachainistypicallyHttpClientHandler,whichactuallysendstherequestoutonthewireandreceivetheresponsefromtheHTTPstackoftheOS.Alternatively,youcoulduseamockhandlerthatpretendstobetheserverandreturnsfabricatedresponses.

2.Addingprocessinglogicinthehandlerbeforepassingtherequesttotheinnerhandlerortheresponsetotheupperhandlercanleadtoperformanceloss.Itisbesttoavoidexpensivesynchronousoperationsinthisscenario.

FormoredetailsontheconceptofchainingHandlers,see thispost byHenrikNielsen（NotethatitreferstotheASP.NETWebAPIversionoftheAPI,whichisslightlydifferentfromthe.NETframeworkonediscussedhere.Theconceptofchaininghandlersiscommonthough.）

Windows.Web.Http

TheobjectmodelfortheWindows.Web.HttpAPIisverysimilartothatoftheSystem.Net.Httpversiondescribedabove–italsohastheconceptofacliententity,ahandler（called“filter”inthisnamespace）andtheoptionofinsertingcustomlogicbetweentheclientandthesystem-defaultfilter.

MostofthetypesaredirectanalogsofthetypesintheSystem.Net.Httpobjectmodel,asfollows:

HTTPclientroleaspect

System.Net.Httptype

CorrespondingWindows.Web.Httptype

Cliententity

HttpClient

HttpClient

HTTPrequest

HttpRequestMessage

HttpRequestMessage

HTTPresponse

HttpResponseMessage

HttpResponseMessage

EntitybodyofanHTTPrequestorresponse

HttpContent

IHttpContent

Represent