畢業(yè)設計外文文獻-創(chuàng)建控制器

附錄A外文翻譯—原文部分文章來源：NateBarbettiniTheLittleASP.NETCoreBook[Z]ADDINCNKISM.UserStyleCreateacontrollerTherearealreadyafewcontrollersintheproject'sControllersdirectory,includingtheHomeControllerthatrendersthedefaultwelcomescreenyouseewhenyouvisithttp://localhost:5000.Youcanignorethesecontrollersfornow.Createanewcontrollerfortheto-dolistfunctionality,calledTodoController,andaddthefollowingcode:Controllers/TodoController.csusingSystem;usingSystem.Collections.Generic;usingSystem.Linq;usingSystem.Threading.Tasks;usingMicrosoft.AspNetCore.Mvc;namespaceAspNetCoreTodo.Controllers{publicclassTodoController:Controller{//Actionsgohere}}Routesthatarehandledbycontrollersarecalledactions,andarerepresentedbymethodsinthecontrollerclass.Forexample,theHomeControllerincludesthreeactionmethods(Index,About,andContact)whicharemappedbyASP.NETCoretotheserouteURLs:localhost:5000/Home->Index()localhost:5000/Home/About->About()localhost:5000/Home/Contact->Contact()Thereareanumberofconventions(commonpatterns)usedbyASP.NETCore,suchasthepatternthatFooControllerbecomes/Foo,andtheIndexactionnamecanbeleftoutoftheURL.Youcancustomizethisbehaviorifyou'dlike,butfornow,we'llsticktothedefaultconventions.AddanewactioncalledIndextotheTodoController,replacingthe//Actionsgoherecomment:publicclassTodoController:Controller{publicIActionResultIndex(){//Getto-doitemsfromdatabase//Putitemsintoamodel//Renderviewusingthemodel}}Actionmethodscanreturnviews,JSONdata,orHTTPstatuscodeslike200OKand404NotFound.TheIActionResultreturntypegivesyoutheflexibilitytoreturnanyofthesefromtheaction.It'sabestpracticetokeepcontrollersaslightweightaspossible.Inthiscase,thecontrollerwillberesponsibleforgettingtheto-doitemsfromthedatabase,puttingthoseitemsintoamodeltheviewcanunderstand,andsendingtheviewbacktotheuser'sbrowser.Beforeyoucanwritetherestofthecontrollercode,youneedtocreateamodelandaview.CreatemodelsTherearetwoseparatemodelclassesthatneedtobecreated:amodelthatrepresentsato-doitemstoredinthedatabase(sometimescalledanentity),andthemodelthatwillbecombinedwithaview(theMVinMVC)andsentbacktotheuser'sbrowser.Becausebothofthemcanbereferredtoas"models",I'llrefertothelatterasaviewmodel.First,createaclasscalledTodoItemintheModelsdirectory:Models/TodoItem.csusingSystem;usingSystem.ComponentModel.DataAnnotations;namespaceAspNetCoreTodo.Models{publicclassTodoItem{publicGuidId{get;set;}publicboolIsDone{get;set;}[Required]publicstringTitle{get;set;}publicDateTimeOffset?DueAt{get;set;}}}Thisclassdefineswhatthedatabasewillneedtostoreforeachto-doitem:anID,atitleorname,whethertheitemiscomplete,andwhattheduedateis.Eachlinedefinesapropertyoftheclass:TheIdpropertyisaguid,oragloballyuniqueidentifier.Guids(orGUIDs)arelongstringsoflettersandnumbers,like43ec09f2-7f70-4f4b-9559-65011d5781bb.Becauseguidsarerandomandareextremelyunlikelytobeaccidentallyduplicated,theyarecommonlyusedasuniqueIDs.Youcouldalsouseanumber(integer)asadatabaseentityID,butyou'dneedtoconfigureyourdatabasetoalwaysincrementthenumberwhennewrowsareaddedtothedatabase.Guidsaregeneratedrandomly,soyoudon'thavetoworryaboutauto-incrementing.TheIsDonepropertyisaboolean(true/falsevalue).Bydefault,itwillbefalseforallnewitems.Lateryou'llusewritecodetoswitchthispropertytotruewhentheuserclicksanitem'scheckboxintheview.TheTitlepropertyisastring(textvalue).Thiswillholdthenameordescriptionoftheto-doitem.The[Required]attributetellsASP.NETCorethatthisstringcan'tbenullorempty.TheDueAtpropertyisaDateTimeOffset,whichisaC#typethatstoresadate/timestampalongwithatimezoneoffsetfromUTC.Storingthedate,time,andtimezoneoffsettogethermakesiteasytorenderdatesaccuratelyonsystemsindifferenttimezones.Noticethe?questionmarkaftertheDateTimeOffsettype?ThatmarkstheDueAtpropertyasnullable,oroptional.Ifthe?wasn'tincluded,everyto-doitemwouldneedtohaveaduedate.TheIdandIsDonepropertiesaren'tmarkedasnullable,sotheyarerequiredandwillalwayshaveavalue(oradefaultvalue).StringsinC#arealwaysnullable,sothere'snoneedtomarktheTitlepropertyasnullable.C#stringscanbenull,empty,orcontaintext.Eachpropertyisfollowedbyget;set;,whichisashorthandwayofsayingthepropertyisread/write(or,moretechnically,ithasagetterandsettermethods).Atthispoint,itdoesn'tmatterwhattheunderlyingdatabasetechnologyis.ItcouldbeSQLServer,MySQL,MongoDB,Redis,orsomethingmoreexotic.ThismodeldefineswhatthedatabaseroworentrywilllooklikeinC#soyoudon'thavetoworryaboutthelow-leveldatabasestuffinyourcode.Thissimplestyleofmodelissometimescalleda"plainoldC#object"orPOCO.TheviewmodelOften,themodel(entity)youstoreinthedatabaseissimilarbutnotexactlythesameasthemodelyouwanttouseinMVC(theviewmodel).Inthiscase,theTodoItemmodelrepresentsasingleiteminthedatabase,buttheviewmightneedtodisplaytwo,ten,orahundredto-doitems(dependingonhowbadlytheuserisprocrastinating).Becauseofthis,theviewmodelshouldbeaseparateclassthatholdsanarrayofTodoItems:Models/TodoViewModel.csnamespaceAspNetCoreTodo.Models{publicclassTodoViewModel{publicTodoItem[]Items{get;set;}}}Nowthatyouhavesomemodels,it'stimetocreateaviewthatwilltakeaTodoViewModelandrendertherightHTMLtoshowtheusertheirto-dolist.CreateaviewViewsinASP.NETCorearebuiltusingtheRazortemplatinglanguage,whichcombinesHTMLandC#code.(Ifyou'vewrittenpagesusingHandlebarsmoustaches,ERBinRubyonRails,orThymeleafinJava,you'vealreadygotthebasicidea.)MostviewcodeisjustHTML,withtheoccasionalC#statementaddedintopulldataoutoftheviewmodelandturnitintotextorHTML.TheC#statementsareprefixedwiththe@symbol.TheviewrenderedbytheIndexactionoftheTodoControllerneedstotakethedataintheviewmodel(asequenceofto-doitems)anddisplayitinanicetablefortheuser.Byconvention,viewsareplacedintheViewsdirectory,inasubdirectorycorrespondingtothecontrollername.Thefilenameoftheviewisthenameoftheactionwitha.cshtmlextension.CreateaTododirectoryinsidetheViewsdirectory,andaddthisfile:Views/Todo/Index.cshtml@modelTodoViewModel@{ViewData["Title"]="Manageyourtodolist";}<divclass="panelpanel-defaulttodo-panel"><divclass="panel-heading">@ViewData["Title"]</div><tableclass="tabletable-hover"><thead><tr><td>✔</td><td>Item</td><td>Due</td></tr></thead>@foreach(variteminModel.Items){<tr><td><inputtype="checkbox"class="done-checkbox"></td><td>@item.Title</td><td>@item.DueAt</td></tr>}</table><divclass="panel-footeradd-item-form"><!--TODO:Additemform--></div></div>Attheverytopofthefile,the@modeldirectivetellsRazorwhichmodeltoexpectthisviewtobeboundto.ThemodelisaccessedthroughtheModelproperty.Assumingthereareanyto-doitemsinModel.Items,theforeachstatementwillloopovereachto-doitemandrenderatablerow(<tr>element)containingtheitem'snameandduedate.Acheckboxisalsorenderedthatwilllettheusermarktheitemascomplete.ThelayoutfileYoumightbewonderingwheretherestoftheHTMLis:whataboutthe<body>tag,ortheheaderandfooterofthepage?ASP.NETCoreusesalayoutviewthatdefinesthebasestructurethateveryotherviewisrenderedinsideof.It'sstoredinViews/Shared/_Layout.cshtml.ThedefaultASP.NETCoretemplateincludesBootstrapandjQueryinthislayoutfile,soyoucanquicklycreateawebapplication.Ofcourse,youcanuseyourownCSSandJavaScriptlibrariesifyou'dlike.CustomizingthestylesheetThedefaulttemplatealsoincludesastylesheetwithsomebasicCSSrules.Thestylesheetisstoredinthewwwroot/cssdirectory.AddafewnewCSSstylerulestothebottomofthesite.cssfile:wwwroot/css/site.cssdiv.todo-panel{margin-top:15px;}tabletr.done{text-decoration:line-through;color:#888;}YoucanuseCSSruleslikethesetocompletelycustomizehowyourpageslookandfeel.ASP.NETCoreandRazorcandomuchmore,suchaspartialviewsandserver-renderedviewcomponents,butasimplelayoutandviewisallyouneedfornow.TheofficialASP.NETCoredocumentation(at)containsanumberofexamplesifyou'dliketolearnmore.AddaserviceclassYou'vecreatedamodel,aview,andacontroller.Beforeyouusethemodelandviewinthecontroller,youalsoneedtowritecodethatwillgettheuser'sto-doitemsfromadatabase.Youcouldwritethisdatabasecodedirectlyinthecontroller,butit'sabetterpracticetokeepyourcodeseparate.Why?Inabig,real-worldapplication,you'llhavetojugglemanyconcerns:Renderingviewsandhandlingincomingdata:thisiswhatyourcontrolleralreadydoes.Performingbusinesslogic,orcodeandlogicthat'srelatedtothepurposeand"business"ofyourapplication.Inato-dolistapplication,businesslogicmeansdecisionslikesettingadefaultduedateonnewtasks,oronlydisplayingtasksthatareincomplete.Otherexamplesofbusinesslogicincludecalculatingatotalcostbasedonproductpricesandtaxrates,orcheckingwhetheraplayerhasenoughpointstolevelupinagame.Savingandretrievingitemsfromadatabase.Again,it'spossibletodoallofthesethingsinasingle,massivecontroller,butthatquicklybecomestoohardtomanageandtest.Instead,it'scommontoseeapplicationssplitupintotwo,three,ormore"layers"ortiersthateachhandleone(andonlyone)concern.Thishelpskeepthecontrollersassimpleaspossible,andmakesiteasiertotestandchangethebusinesslogicanddatabasecodelater.Separatingyourapplicationthiswayissometimescalledamulti-tierorn-tierarchitecture.Insomecases,thetiers(layers)areisolatedincompletelyseparateprojects,butothertimesitjustreferstohowtheclassesareorganizedandused.Theimportantthingisthinkingabouthowtosplityourapplicationintomanageablepieces,andavoidhavingcontrollersorbloatedclassesthattrytodoeverything.Forthisproject,you'llusetwoapplicationlayers:apresentationlayermadeupofthecontrollersandviewsthatinteractwiththeuser,andaservicelayerthatcontainsbusinesslogicanddatabasecode.Thepresentationlayeralreadyexists,sothenextstepistobuildaservicethathandlesto-dobusinesslogicandsavesto-doitemstoadatabase.Mostlargerprojectsusea3-tierarchitecture:apresentationlayer,aservicelogiclayer,andadatarepositorylayer.Arepositoryisaclassthat'sonlyfocusedondatabasecode(nobusinesslogic).Inthisapplication,you'llcombinetheseintoasingleservicelayerforsimplicity,butfeelfreetoexperimentwithdifferentwaysofarchitectingthecode.CreateaninterfaceTheC#languageincludestheconceptofinterfaces,wherethedefinitionofanobject'smethodsandpropertiesisseparatefromtheclassthatactuallycontainsthecodeforthosemethodsandproperties.Interfacesmakeiteasytokeepyourclassesdecoupledandeasytotest,asyou'llseehere(andlaterintheAutomatedtestingchapter).You'lluseaninterfacetorepresenttheservicethatcaninteractwithto-doitemsinthedatabase.Byconvention,interfacesareprefixedwith"I".CreateanewfileintheServicesdirectory:Services/ITodoItemService.csusingSystem;usingSystem.Collections.Generic;usingSystem.Threading.Tasks;usingAspNetCoreTodo.Models;namespaceAspNetCoreTodo.Services{publicinterfaceITodoItemService{Task<TodoItem[]>GetIncompleteItemsAsync();}}NotethatthenamespaceofthisfileisAspNetCoreTodo.Services.Namespacesareawaytoorganize.NETcodefiles,andit'scustomaryforthenamespacetofollowthedirectorythefileisstoredin(AspNetCoreTodo.ServicesforfilesintheServicesdirectory,andsoon).Becausethisfile(intheAspNetCoreTodo.Servicesnamespace)referencestheTodoItemclass(intheAspNetCoreTodo.Modelsnamespace),itneedstoincludeausingstatementatthetopofthefiletoimportthatnamespace.Withouttheusingstatement,you'llseeanerrorlike:Thetypeornamespacename'TodoItem'couldnotbefound(areyoumissingausingdirectiveoranassemblyreference?)Sincethisisaninterface,thereisn'tanyactualcodehere,justthedefinition(ormethodsignature)oftheGetIncompleteItemsAsyncmethod.ThismethodrequiresnoparametersandreturnsaTask<TodoItem[]>.Ifthissyntaxlooksconfusing,think:"aTaskthatcontainsanarrayofTodoItems".TheTasktypeissimilartoafutureorapromise,andit'susedherebecausethismethodwillbeasynchronous.Inotherwords,themethodmaynotbeabletoreturnthelistofto-doitemsrightawaybecauseitneedstogotalktothedatabasefirst.(Moreonthislater.)CreatetheserviceclassNowthattheinterfaceisdefined,you'rereadytocreatetheactualserviceclass.I'llcoverdatabasecodeindepthintheUseadatabasechapter,sofornowyou'lljustfakeitandalwaysreturntwohard-codeditems:Services/FakeTodoItemService.csusingSystem;usingSystem.Collections.Generic;usingSystem.Threading.Tasks;usingAspNetCoreTodo.Models;namespaceAspNetCoreTodo.Services{publicclassFakeTodoItemService:ITodoItemService{publicTask<TodoItem[]>GetIncompleteItemsAsync(){varitem1=newTodoItem{Title="LearnASP.NETCore",DueAt=DateTimeOffset.Now.AddDays(1)};varitem2=newTodoItem{Title="Buildawesomeapps",DueAt=DateTimeOffset.Now.AddDays(2)};returnTask.FromResult(new[]{item1,item2});}}}ThisFakeTodoItemServiceimplementstheITodoItemServiceinterfacebutalwaysreturnsthesamearrayoftwoTodoItems.You'llusethistotestthecontrollerandview,andthenaddrealdatabasecodeinUseadatabase.UsedependencyinjectionBackintheTodoController,addsomecodetoworkwiththeITodoItemService:publicclassTodoController:Controller{privatereadonlyITodoItemService_todoItemService;publicTodoController(ITodoItemServicetodoItemService){_todoItemService=todoItemService;}publicIActionResultIndex(){//Getto-doitemsfromdatabase//Putitemsintoamodel//Passtheviewtoamodelandrender}}SinceITodoItemServiceisintheServicesnamespace,you'llalsoneedtoaddausingstatementatthetop:usingAspNetCoreTodo.Services;ThefirstlineoftheclassdeclaresaprivatevariabletoholdareferencetotheITodoItemService.ThisvariableletsyouusetheservicefromtheIndexactionmethodlater(you'llseehowinaminute).ThepublicTodoController(ITodoItemServicetodoItemService)linedefinesaconstructorfortheclass.Theconstructorisaspecialmethodthatiscalledwhenyouwanttocreateanewinstanceofaclass(theTodoControllerclass,inthiscase).ByaddinganITodoItemServiceparametertotheconstructor,you'vedeclaredthatinordertocreatetheTodoController,you'llneedtoprovideanobjectthatmatchestheITodoItemServiceinterface.Interfacesareawesomebecausetheyhelpdecouple(separate)thelogicofyourapplication.SincethecontrollerdependsontheITodoItemServiceinterface,andnotonanyspecificclass,itdoesn'tknoworcarewhichclassit'sactuallygiven.ItcouldbetheFakeTodoItemService,adifferentonethattalkstoalivedatabase,orsomethingelse!Aslongasitmatchestheinterface,thecontrollercanuseit.Thismakesitreallyeasytotestpartsofyourapplicationseparately.I'llcovertestingindetailintheAutomatedtestingchapter.NowyoucanfinallyusetheITodoItemService(viatheprivatevariableyoudeclared)inyouractionmethodtogetto-doitemsfromtheservicelayer:publicIActionResultIndex(){varitems=await_todoItemService.GetIncompleteItemsAsync();//...}RememberthattheGetIncompleteItemsAsyncmethodreturnedaTask<TodoItem[]>?ReturningaTaskmeansthatthemethodwon'tnecessarilyhavearesultrightaway,butyoucanusetheawaitkeywordtomakesureyourcodewaitsuntiltheresultisreadybeforecontinuingon.TheTaskpatterniscommonwhenyourcodecallsouttoadatabaseoranAPIservice,becauseitwon'tbeabletoreturnarealresultuntilthedatabase(ornetwork)responds.Ifyou'veusedpromisesorcallbacksinJavaScriptorotherlanguages,Taskisthesameidea:thepromisethattherewillbearesult-sometimeinthefuture.Ifyou'vehadtodealwith"callbackhell"inolderJavaScriptcode,you'reinluck.Dealingwithasynchronouscodein.NETismucheasierthankstothemagicoftheawaitkeyword!awaitletsyourcodepauseonanasyncoperation,andthenpickupwhereitleftoffwhentheunderlyingdatabaseornetworkrequestfinishes.Inthemeantime,yourapplicationisn'tblocked,becauseitcanprocessotherrequestsasneeded.Thispatternissimplebuttakesalittlegettingusedto,sodon'tworryifthisdoesn'tmakesenserightaway.Justkeepfollowingalong!TheonlycatchisthatyouneedtoupdatetheIndexmethodsignaturetoreturnaTask<IActionResult>insteadofjustIActionResult,andmarkitasasync:publicasyncTask<IActionResult>Index(){varitems=await_todoItemService.GetIncompleteItemsAsync();//Putitemsintoamodel//Passtheviewtoamodelandrender}You'realmostthere!You'vemadetheTodoControllerdependontheITodoItemServiceinterface,butyouhaven'tyettoldASP.NETCorethatyouwanttheFakeTodoItemServicetobetheactualservicethat'susedunderthehood.ItmightseemobviousrightnowsinceyouonlyhaveoneclassthatimplementsITodoItemService,butlateryou'llhavemultipleclassesthatimplementthesameinterface,sobeingexplicitisnecessary.Declaring(or"wiringup")whichconcreteclasstouseforeachinterfaceisdoneintheConfigureServicesmethodoftheStartupclass.Rightnow,itlookssomethinglikethis:Startup.cspublicvoidConfigureServices(IServiceCollectionservices){//(...somecode)services.AddMvc();}ThejoboftheConfigureServicesmethodisaddingthingstotheservicecontainer,orthecollectionofservicesthatASP.NETCoreknowsabout.Theservices.AddMvclineaddstheservicesthattheinternalASP.NETCoresystemsneed(asanexperiment,trycommentingoutthisline).AnyotherservicesyouwanttouseinyourapplicationmustbeaddedtotheservicecontainerhereinConfigureServices.AddthefollowinglineanywhereinsidetheConfigureServicesmethod:services.AddSingleton<ITodoItemService,FakeTodoItemService>();ThislinetellsASP.NETCoretousetheFakeTodoItemServicewhenevertheITodoItemServiceinterfaceisrequestedinaconstructor(oranywhereelse).AddSingletonaddsyourservicetotheservicecontainerasasingleton.ThismeansthatonlyonecopyoftheFakeTodoItemServiceiscreated,andit'sreusedwhenevertheserviceisrequested.Later,whenyouwriteadifferentserviceclassthattalkstoadatabase,you'lluseadifferentapproach(calledscoped)instead.I'llexplainwhyintheUseadatabasechapter.That'sit!WhenarequestcomesinandisroutedtotheTodoController,ASP.NETCorewilllookattheavailableservicesandautomaticallysupplytheFakeTodoItemServicewhenthecontrollerasksforanITodoItemService.Becausetheservicesare"injected"fromtheservicecontainer,thispatterniscalleddependencyinjection.FinishthecontrollerThelaststepistofinishthecontrollercode.Thecontrollernowhasalistofto-doitemsfromtheservicelayer,anditneedstoputthoseitemsintoaTodoViewModelandbindthatmodeltotheviewyoucreatedearlier:Controllers/TodoController.cspublicasyncTask<IActionResult>Index(){varitems=await_todoItemService.GetIncompleteItemsAsync();varmodel=newTodoViewModel(){Items=items};returnView(model);}Ifyouhaven'talready,makesuretheseusingstatementsareatthetopofthefile:usingAspNetCoreTodo.Services;usingAspNetCoreTodo.Models;Ifyou'reusingVisualStudioorVisualStudioCode,theeditorwillsuggesttheseusingstatementswhenyouputyourcursoronaredsquigglyline.TestitoutTostarttheapplication,pressF5(ifyou'reusingVisualStudioorVisualStudioCode),orjusttypedotnetrunintheterminal.Ifthecodecompileswithouterrors,theserverwillstartuponport5000bydefault.Ifyourwebbrowserdidn'topenautomatically,openitandnavigatetohttp://localhost:5000/todo.You'llseetheviewyoucreated,withthedatapulledfromyourfakedatabase(fornow).Althoughit'spossibletogodirectlytohttp://localhost:5000/todo,itwouldbenicertoaddanitemcalledMyto-dostothenavbar.Todothis,youcaneditthesharedlayoutfile.UpdatethelayoutThelayoutfileatViews/Shared/_Layout.cshtmlcontainsthe"base"HTMLforeachview.Thisincludesthenavbar,whichisrenderedatthetopofeachpage.Toaddanewitemtothenavbar,findtheHTMLcodefortheexistingnavbaritems:Views/Shared/_Layout.cshtml<ulclass="navnavbar-nav"><li><aasp-area=""asp-controller="Home"asp-action="Index">Home</a></li><li><aasp-area=""asp-controller="Home"asp-action="About">About</a></li><li><aasp-area=""asp-controller="Home"asp-action="Contact">Contact</a></li></ul>AddyourownitemthatpointstotheTodocontrollerinsteadofHome:<li><aasp-controller="Todo"asp-action="Index">Myto-dos</a></li>Theasp-controllerandasp-actionattributesonthe<a>elementarecalledtaghelpers.Beforetheviewisrendered,ASP.NETCorereplacesthesetaghelperswithrealHTMLattributes.Inthiscase,aURLtothe/Todo/Indexrouteisgeneratedandaddedtothe<a>elementasanhrefattribute.Thismeansyoudon'thavetohard-codetheroutetotheTodoController.Instead,ASP.NETCoregeneratesitforyouautomatically.Ifyou'veusedRazorinASP.NET4.x,you'llnoticesomesyntaxchanges.Insteadofusing@Html.ActionLink()togeneratealinktoanaction,taghelpersarenowtherecommendedwaytocreatelinksinyourviews.Taghelpersareusefulforforms,too(you'llseewhyinalaterchapter).Youcanlearnaboutothertaghelpersinthedocumentationat.附錄B外文翻譯—譯文部分創(chuàng)建控制器在項目的Controllers目錄里，已經預置了幾個控制器，其中有渲染默認歡迎頁的HomeController，就是你訪問http://localhost:5000看到的那個頁面。暫時不用管這些控制器。給待辦清單功能創(chuàng)建一個新的控制器，取名叫TodoController，并添加如下代碼：Controllers/TodoController.csusingSystem;usingSystem.Collections.Generic;usingSystem.Linq;usingSystem.Threading.Tasks;usingMicrosoft.AspNetCore.Mvc;namespaceAspNetCoreTodo.Controllers{publicclassTodoController:Controller{//在這里添加Actions}}由控制器本身處理的路由叫action，在控制器類里用方法表示。比如，HomeController包含三個action方法（Index，About，和Contact），由ASP.NETCore分別映射到如下的URL：localhost:5000/Home->Index()localhost:5000/Home/About->About()localhost:5000/Home/Contact->Contact()ASP.NETCore中有幾個慣例（常見的模式），比如這個FooController映射到/Foo的模式，還有Index的action名可以在URL里省略。如果你有需要，可以自定義這些行為，不過就目前的情況，讓我們暫且遵循這些慣例。在TodoController里，添加一個名為Index的action，把那句//Actionsgohere注釋替換掉：publicclassTodoController:Controller{publicIActionResultIndex(){//從數(shù)據(jù)庫獲取to-do條目//把條目置于model中//使用model渲染視圖}}一個action方法可以返回視圖、JSON數(shù)據(jù)，或者200OK和404NotFound之類的狀態(tài)碼。返回類型IActionResult給了你足夠的靈活性，以返回上面提到的任意一個。使控制器盡可能保持輕量化，是一個良好的習慣。在當前的情形里，這個控制器應該僅僅完成這些事情：從數(shù)據(jù)庫取出待辦事項的記錄，把這些事項包裝在一個可用于視圖的模型中，并把這個視圖發(fā)送到用戶的瀏覽器。繼續(xù)編碼這個控制器之前，你需要創(chuàng)建模型和視圖。創(chuàng)建模型我們需要創(chuàng)建兩個獨立的模型類：一個模型表示保存在數(shù)據(jù)庫里的條目（有時候也稱為一個記錄(entity)），另一個模型將與視圖結合（MVC里的MV）發(fā)送到用戶的瀏覽器。因為他們都可以被稱為模型，我將稱后者為視圖模型（viewmodel）。首先，在Models目錄下，創(chuàng)建一個名為TodoItem的類：Models/TodoItem.csusingSystem;namespaceAspNetCoreTodo.Models{publicclassTodoItem{publicGuidId{get;set;}publicboolIsDone{get;set;}publicstringTitle{get;set;}publicDateTimeOffset?DueAt{get;set;}}}這個類定義了每個待辦事項都要保存的內容：一個ID、一個標題或者名稱、該事項是否已經完成，以及截至日期是什么時候。每行定義了這個類的一個屬性：Id屬性是一個guid，或者說是全局(globally)唯一(unique)標識符(identifier).Guid（或者GUID）是一個由字母和數(shù)字組成的長長的字符串，看起來是這樣的43ec09f2-7f70-4f4b-9559-65011d5781bb。因為guid是隨機的，并極少會有重復值，所以常被用作唯一標識。你也可以用數(shù)字（整形integer）作為數(shù)據(jù)庫記錄的標識，但你需要在數(shù)據(jù)庫里配置，以便這個數(shù)字在添加新條目的時候始終增長。因為Guid是隨機生產的，所以就你不必再擔心這個自增的問題了。IsDone屬性是一個布爾值（值為true/false）。默認情況下，所有新建條目的該值為false。你后面會編寫代碼，在用戶在視圖里點擊某個條目的復選框時，修改這個屬性為true。Title屬性是一個字符串，用于保存待辦事項的名稱或者簡述。DueAt屬性是一個DateTimeOffset，C#用于這種類型保存一個日期/時間的戳記和一個與UTC偏移量表示的時區(qū)。把時期、時間和時區(qū)一起保存，有助于在不同時區(qū)的系統(tǒng)上準確地顯示時間。看到DateTimeOffset類型后面那個問號?了嗎？它表示DueAt屬性可空(nullable)，或者說是可選的。如果不加這個?，每個待辦事項都必須帶有一個截止日期。Id和IsDone屬性沒有標記為可空，所以是必須的，并可以確保始終有值（或者是一個缺省值）。C#里的字符串總是可空的，所以沒必要給Title屬性添加可空標記。C#字符串可以沒有值，也可以是空白字符串或者包含任意文本。每個屬性后面都跟著get;set;，這是個簡寫，表示該屬性可讀/可寫(read/write)（或者，更確切地說，它有getter和setter方法各一個）?，F(xiàn)在，暫且不必關心底層數(shù)據(jù)庫采用的是哪種實現(xiàn)。它可以是SQLServer，MySQL，MongoDB，Redis，或者什么其它稀奇古怪的玩意兒。這個模型定義了數(shù)據(jù)庫里的行或者記錄在C#里看起來是什么樣的，所以你無須在代碼層面擔心數(shù)據(jù)庫層面的東西。這種模型簡單的風格被稱為“樸實可愛的C#對象(plainoldC#object)”或者POCO。視圖模型通常，你保存在數(shù)據(jù)庫里的模型（實體），跟你在MVC里用的模型（視圖模型）非常相似，但又不盡相同。在現(xiàn)在的情形下，TodoItem模型代表單一的一個數(shù)據(jù)庫里的條目，而視圖則需要展示兩個、十個，甚至是一百個待辦事項（取決于用戶拖延癥的病情輕重）。因此，視圖模型應該是一個獨立的類，里面包含著一個TodoItem的數(shù)組：Models/TodoViewModel.csnamespaceAspNetCoreTodo.Models{publicclassTodoViewModel{publicTodoItem[]Items{get;set;}}}好了，現(xiàn)在模型也有了，是時候創(chuàng)建一個接收TodoViewModel并以HTML向用戶展示待辦事項列表的視圖了。創(chuàng)建視圖ASP.NETCore里的視圖使用Razor模板語言編寫，這種模板語言混合了HTML和C#的代碼。（如果你在JavaScript下用Jade、Pug或者Handlebarsmoustaches，在RubyonRails下用ERB，在Java下用Thymeleaf寫過頁面，那你就已經了解其基本概念了.）絕大多數(shù)視圖代碼就是HTML，偶爾摻雜一點C#語句，用以從視圖模型里抽取數(shù)據(jù)并轉換為文本或者HTML。這些C#語句以符號@作為前綴。由TodoController中的actionIndex生成的視圖，需要從視圖模型（一個待辦事項的數(shù)組）獲取數(shù)據(jù)，并用一個適當?shù)谋砀裾故窘o用戶。按規(guī)定，視圖要置于Views目錄里，在一個與所屬控制器同名的子目錄下。視圖文件的文件名就是action的名字加上一個.cshtml擴展名。Views/Todo/Index.cshtml@modelTodoViewModel@{ViewDa