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Presenting View Controllers from Other View Controllers

2014-11-02 14:29 253 查看

转自：https://developer.apple.com/library/ios/featuredarticles/ViewControllerPGforiPhoneOS/ModalViewControllers/ModalViewControllers.html

PresentingViewControllersfromOtherViewControllers

Theabilitytopresentviewcontrollersisatoolthatyouhaveatyourdisposalforinterruptingthecurrentworkflowanddisplayinganewsetofviews.Mostcommonly,anapppresentsaviewcontrollerasatemporaryinterruptiontoobtainimportantinformation
fromtheuser.However,youcanalsousepresentedviewcontrollerstoimplementalternateinterfacesforyourappatspecifictimes.

HowViewControllersPresentOtherViewControllers

Apresentedviewcontrollerisnotaspecificsubclassof

UIViewController

(as

UITabBarController

UINavigationController

is).
Instead,anyviewcontrollercanbepresentedbyyourapp.However,liketabbarandnavigationcontrollers,youpresentviewcontrollerswhenyouwanttoconveyaspecificmeaningabouttherelationshipbetweenthepreviousviewhierarchyandthenewlypresented
viewhierarchy.
Whenyoupresentamodalviewcontroller,thesystemcreatesarelationshipbetweentheviewcontrollerthatdidthepresentingandtheviewcontrollerthatwaspresented.Specifically,theviewcontrollerthat
didthepresentingupdatesits

presentedViewController

property
topointtoitspresentedviewcontroller.Similarly,thepresentedviewcontrollerupdatesits

presentingViewController

property
topointbacktotheviewcontrollerthatpresentedit.Figure10-1showstherelationshipbetweentheviewcontrollermanagingthemainscreenintheCalendar
appandthepresentedviewcontrollerusedtocreatenewevents.
Figure10-1PresentedviewsintheCalendar
app.

Anyviewcontrollerobjectcanpresentasingleviewcontrolleratatime.Thisistrueevenforviewcontrollersthatwerethemselvespresentedbyanotherviewcontroller.Inotherwords,youcanchainpresented
viewcontrollerstogether,presentingnewviewcontrollersontopofotherviewcontrollersasneeded.Figure10-2showsavisualrepresentationofthe
chainingprocessandtheactionsthatinitiateit.Inthiscase,whentheusertapstheiconinthecameraview,theapppresentsaviewcontrollerwiththeuser’sphotos.Tappingtheactionbuttoninthephotolibrary’stoolbarpromptstheuserforanappropriate
actionandthenpresentsanotherviewcontroller(thepeoplepicker)inresponsetothataction.Selectingacontact(orcancelingthepeoplepicker)dismissesthatinterfaceandtakestheuserbacktothephotolibrary.TappingtheDonebuttonthendismisses
thephotolibraryandtakestheuserbacktothecamerainterface.
Figure10-2Creatingachainofmodal
viewcontrollers

Eachviewcontrollerinachainofpresentedviewcontrollershaspointerstotheotherobjectssurroundingitinthechain.Inotherwords,apresentedviewcontrollerthatpresentsanotherviewcontrollerhas
validobjectsinbothits

presentingViewController

and

presentedViewController

properties.
Youcanusetheserelationshipstotracethroughthechainofviewcontrollersasneeded.Forexample,iftheusercancelsthecurrentoperation,youcanremoveallobjectsinthechainbydismissingthefirstpresentedviewcontroller.Dismissingaviewcontroller
dismissesnotonlythatviewcontrollerbutalsoanyviewcontrollersitpresented.
InFigure
10-2,apointworthnotingisthatthepresentedviewcontrollersarebothnavigationcontrollers.Youcanpresent

UINavigationController

objects
inthesamewaythatyouwouldpresentacontentviewcontroller.
Whenpresentinganavigationcontroller,youalwayspresentthe

UINavigationController

objectitself,ratherthanpresenting
anyoftheviewcontrollersonitsnavigationstack.However,individualviewcontrollersonthenavigationstackmaypresentotherviewcontrollers,includingothernavigationcontrollers.Figure
10-3showsmoredetailoftheobjectsthatareinvolvedintheprecedingexample.Asyoucansee,thepeoplepickerisnotpresentedbythephotolibrarynavigationcontrollerbutbyoneofthecontentviewcontrollersonitsnavigationstack.
Figure10-3Presentingnavigation
controllersmodally

PresentationStylesforModalViews

ForiPadapps,youcanpresentcontentusingseveraldifferentstyles.IniPhoneapps,presentedviewsalwayscoverthevisibleportionofthewindow,butwhenrunningonaniPad,viewcontrollersusethevalue
intheir

modalPresentationStyle

propertyto
determinetheirappearancewhenpresented.Differentoptionsforthispropertyallowyoutopresenttheviewcontrollersothatitfillsalloronlypartofthescreen.
Figure10-4showsthecorepresentationstylesthatareavailable.(The

UIModalPresentationCurrentContext

style
letsaviewcontrolleradoptthepresentationstyleofitsparent.)Ineachpresentationstyle,thedimmedareasshowtheunderlyingcontentbutdonotallowtapsinthatcontent.Therefore,unlikeapopover,yourpresentedviewsmuststillhavecontrolsthat
allowtheusertodismisstheview.
Figure10-4iPadpresentationstyles

Forguidanceonwhentousethedifferentpresentationstyles,seeModal
ViewiniOSHumanInterfaceGuidelines.

PresentingaViewControllerandChoosingaTransitionStyle

Whenaviewcontrollerispresentedusingastoryboardsegue,itisautomaticallyinstantiatedandpresented.Thepresentingviewcontrollercanconfigurethedestinationviewcontrollerbeforeitispresented.
Formoreinformation,seeConfiguring
theDestinationControllerWhenaSegueisTriggered.
Ifyouneedtopresentaviewcontrollerprogrammatically,youmustdothefollowing:

Createtheviewcontrolleryouwanttopresent.

Setthe

modalTransitionStyle

propertyof
theviewcontrollertothedesiredvalue.

Assignadelegateobjecttotheviewcontroller.Typically,thedelegateisthepresentingviewcontroller.The
delegateisusedbythepresentedviewcontrollerstonotifythepresentingviewcontrollerwhenitisreadytobedismissed.Itmayalsocommunicateotherinformationbacktothedelegate.

Callthe

presentViewController:animated:completion:

method
ofthecurrentviewcontroller,passingintheviewcontrolleryouwanttopresent.

The

presentViewController:animated:completion:

methodpresentstheviewforthespecifiedviewcontrollerobjectandconfigures
thepresenting-presentedrelationshipsbetweenthenewviewcontrollerandthecurrentviewcontroller.Unlessyouarerestoringyourapptosomepreviousstate,youusuallywanttoanimatetheappearanceofthenewviewcontroller.Thetransitionstyleyou
shouldusedependsonhowyouplantousethepresentedviewcontroller.Table10-1liststhetransitionstylesyoucanassigntothe

modalTransitionStyle

property
ofthepresentedviewcontrolleranddescribeshowyoumightuseeachone.

Table10-1Transitionstylesformodalviewcontrollers

Transitionstyle	Usage
UIModalTransitionStyleCoverVertical	Usethisstylewhenyouwanttointerruptthecurrentworkflowtogatherinformationfromtheuser.Youcanalsouseittopresentcontentthattheusermightormightnotmodify. Forthisstyleoftransition,contentviewcontrollersshouldprovidebuttonstodismisstheviewcontrollerexplicitly.Typically,theseareaDonebuttonandanoptionalCancelbutton. Ifyoudonotexplicitlysetatransitionstyle,thisstyleisusedbydefault.
UIModalTransitionStyleFlipHorizontal	Usethisstyletochangetheworkmodeofyourapptemporarily.Themostcommonusageforthisstyleistodisplaysettingsthatmightchangefrequently,suchasintheStocksandWeatherapps.Thesesettingscanbemeantfortheentireapportheycanbespecific tothecurrentscreen. Forthisstyleoftransition,youusuallyprovidesomesortofbuttontoreturntheusertothenormalrunningmodeofyourapp.
UIModalTransitionStyleCrossDissolve	Usethisstyletopresentanalternateinterfacewhenthedevicechangesorientations.Insuchacase,yourappisresponsibleforpresentinganddismissingthealternateinterfaceinresponsetoorientationchangenotifications. Media-basedappscanalsousethisstyletofadeinscreensdisplayingmediacontent. Foranexampleofhowtoimplementanalternateinterfaceinresponsetodeviceorientationchanges,seeCreating anAlternateLandscapeInterface.

Listing10-1showshowtopresentaviewcontrollerprogrammatically.Whentheuseraddsanewrecipe,the
apppromptstheuserforbasicinformationabouttherecipebypresentinganavigationcontroller.AnavigationcontrollerwaschosensothattherewouldbeastandardplacetoputaCancelandDonebutton.Usinganavigationcontrolleralsomakesiteasier
toexpandthenewrecipeinterfaceinthefuture.Allyouwouldhavetodoispushnewviewcontrollersonthenavigationstack.

Listing10-1Presentingaviewcontrollerprogrammatically

-(void)add:(id)sender{

//Createtherootviewcontrollerforthenavigationcontroller

//ThenewviewcontrollerconfiguresaCancelandDonebuttonforthe

//navigationbar.

RecipeAddViewController*addController=[[RecipeAddViewControlleralloc]

init];

//ConfiguretheRecipeAddViewController.Inthiscase,itreportsany

//changestoacustomdelegateobject.

addController.delegate=self;

//Createthenavigationcontrollerandpresentit.

UINavigationController*navigationController=[[UINavigationControlleralloc]

initWithRootViewController:addController];

[selfpresentViewController:navigationControlleranimated:YEScompletion:nil];

WhentheusertapseithertheDoneortheCancelbuttonfromthenewrecipeentryinterface,theappdismissestheviewcontrollerandreturnstheusertothemainview.SeeDismissing
aPresentedViewController.

PresentationContextsProvidetheAreaCoveredbythePresentedViewController

Theareaofthescreenusedtodefinethepresentationareaisdeterminedbythepresentationcontext.Bydefault,thepresentationcontextisprovidedbytherootviewcontroller,whoseframeisusedtodefine
theframeofthepresentationcontext.However,thepresentingviewcontroller,oranyotherancestorintheviewcontrollerhierarchy,canchoosetoprovidethepresentationcontextinstead.Inthatcase,whenanotherviewcontrollerprovidesthepresentation
context,itsframeisusedinsteadtodeterminetheframeofthepresentedview.Thisflexibilityallowsyoutolimitthemodalpresentationtoasmallerportionofthescreen,leavingothercontentvisible.
Whenaviewcontrollerispresented,iOSsearchesforapresentationcontext.Itstartsatthepresentingviewcontrollerbyreadingits

definesPresentationContext

property.
Ifthevalueofthispropertyis

YES

,
thenthepresentingviewcontrollerdefinesthepresentationcontext.Otherwise,itcontinuesupthroughtheviewcontrollerhierarchyuntilaviewcontrollerreturns

YES

or
untilitreachesthewindow’srootviewcontroller.
Whenaviewcontrollerdefinesapresentationcontext,itcanalsochoosetodefinethepresentationstyle.Normally,thepresentedviewcontrollerdetermineshowitpresentedusingits

modalTransitionStyle

property.
Aviewcontrollerthatsets

definesPresentationContext

YES

can
alsoset

providesPresentationContextTransitionStyle

YES

.
If

providesPresentationContextTransitionStyle

is
setto

YES

,
iOSusesthepresentationcontext’s

modalPresentationStyle

to
determinehowthenewviewcontrollerispresented.

DismissingaPresentedViewController

Whenitcomestimetodismissapresentedviewcontroller,thepreferredapproachistoletthepresentingviewcontrollerdismissit.Inotherwords,wheneverpossible,thesameviewcontrollerthatpresented
theviewcontrollershouldalsotakeresponsibilityfordismissingit.Althoughthereareseveraltechniquesfornotifyingthepresentingviewcontrollerthatitspresentedviewcontrollershouldbedismissed,thepreferredtechniqueisdelegation.
Formoreinformation,seeUsing
DelegationtoCommunicatewithOtherControllers.

PresentingStandardSystemViewControllers

Anumberofstandardsystemviewcontrollersaredesignedtobepresentedbyyourapp.Thebasicrulesforpresentingtheseviewcontrollersarethesameastherulesforpresentingyourcustomcontentviewcontrollers.
However,becauseyourappdoesnothaveaccesstotheviewhierarchymanagedbythesystemviewcontrollers,youcannotsimplyimplementactionsforthecontrolsintheviews.Interactionswiththesystemviewcontrollerstypicallytakeplacethroughadelegate
object.
Eachsystemviewcontrollerdefinesacorrespondingprotocol,whosemethodsyouimplementinyourdelegateobject.Eachdelegateusually
implementsamethodtoeitheracceptwhateveritemwasselectedorcanceltheoperation.Yourdelegateobjectshouldalwaysbereadytohandlebothcases.Oneofthemostimportantthingsthedelegatemustdoisdismissthepresentedviewcontrollerbycalling
the

dismissModalViewControllerAnimated:

method
oftheviewcontrollerthatdidthepresenting(inotherwords,theparentofthepresentedviewcontroller.)
Table10-2listsseveralofthestandardsystemviewcontrollersfoundiniOS.Formoreinformationabout
eachoftheseclasses,includingthefeaturesitprovides,seethecorrespondingclassreferencedocumentation.

Table10-2Standardsystemviewcontrollers

Framework	Viewcontrollers
AddressBookUI	ABNewPersonViewController ABPeoplePickerNavigationController ABPersonViewController ABUnknownPersonViewController
EventKitUI	EKEventEditViewController EKEventViewController
GameKit	GKAchievementViewController GKLeaderboardViewController GKMatchmakerViewController GKPeerPickerController GKTurnBasedMatchmakerViewController
MessageUI	MFMailComposeViewController MFMessageComposeViewController
MediaPlayer	MPMediaPickerController MPMoviePlayerViewController
UIKit	UIImagePickerController UIVideoEditorController

Note:Althoughthe

MPMoviePlayerController

class
intheMediaPlayerframeworkmighttechnicallybethoughtofasamodalcontroller,thesemanticsforusingitareslightlydifferent.Insteadofpresentingtheviewcontrolleryourself,youinitializeitandtellittoplayitsmediafile.Theviewcontroller
thenhandlesallaspectsofpresentinganddismissingitsview.(However,the

MPMoviePlayerViewController

class
canbeusedinsteadof

MPMoviePlayerController

asastandardviewcontrollerforplayingmovies.)

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