Creating Menus.docx

Creating Menus.docx

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CreatingMenus

CreatingMenus

Menusareanimportantpartofanapplicationthatprovideafamiliarinterfacefortheusertoaccessapplicationfunctionsandsettings.Androidoffersaneasyprogramminginterfaceforyoutoprovideapplicationmenusinyourapplication.

Androidprovidesthreetypesofapplicationmenus:

OptionsMenu

TheprimarymenuforanActivity,whichappearswhentheuserpressesthedeviceMENUkey.WithintheOptionsMenuaretwogroups:

IconMenu

ThemenuitemsvisibleatthebottomofthescreenatthepressoftheMENUkey.Itsupportsamaximumofsixmenuitems.Thesearetheonlymenuitemsthatsupporticonsandtheonlymenuitemsthatdonotsupportcheckboxesorradiobuttons.

ExpandedMenu

Theverticallistofmenuitemsexposedbythe"More"menuitemintheIconMenu.WhentheIconMenuisfull,theexpandedmenuiscomprisedofthesixthmenuitemandtherest.

ContextMenu

Afloatinglistofmenuitemsthatappearswhentheuserperformsalong-pressonaView.

Submenu

AfloatinglistofmenuitemsthattheuseropensbypressingamenuitemintheOptionsMenuoracontextmenu.Asubmenuitemcannotsupportanestedsubmenu.

DefiningMenus

InsteadofinstantiatingMenuobjectsinyourapplicationcode,youshoulddefineamenuandallitsitemsinanXMLmenuresource,theninflatethemenuresource（loaditasaprogrammableobject）inyourapplicationcode.DefiningyourmenusinXMLisagoodpracticebecauseitseparatesyourinterfacedesignfromyourapplicationcode（thesameaswhenyoudefineyourActivitylayout）.

Todefineamenu,createanXMLfileinsideyourproject'sres/menu/directoryandbuildthemenuwiththefollowingelements:

Thisexampledefinesamenuwithtwomenuitems.Eachitemincludestheattributes:

android:

id

AresourceIDthat'suniquetotheitemsothattheapplicationcanrecognizetheitemwhentheuserselectsit.

android:

icon

Adrawableresourcethatistheiconvisibletotheuser.

android:

title

Astringresourcethatisthetitlevisibletotheuser.

FormoreabouttheXMLsyntaxandattributesforamenuresource,seetheMenuResourcereference.

InflatingaMenuResource

Youcaninflateyourmenuresource（converttheXMLresourceintoaprogrammableobject）usingMenuInflater.inflate（）.Forexample,thefollowingcodeinflatesthegame_menu.xmlfiledefinedaboveduringtheonCreateOptionsMenu（）callbackmethod,tobeusedfortheOptionsMenu:

@Override

publicbooleanonCreateOptionsMenu（Menumenu）{

  MenuInflaterinflater=getMenuInflater（）;

  inflater.inflate（R.menu.game_menu,menu）;

  returntrue;

}

ThegetMenuInflater（）methodreturnsaMenuInflaterfortheActivity.Withthisobject,youcancallinflate（）,whichinflatesamenuresourceintoaMenuobject.Inthisexample,themenuresourcedefinedbygame_menu.xmlisinflatedintotheMenuthatwaspassedintoonCreateOptionsMenu（）.（Thiscallbackmethodforcreatinganoptionmenuisdiscussedmoreinthenextsection.）

CreatinganOptionsMenu

Figure1.ScreenshotofanOptionsMenu.

TheOptionsMenuiswhereyoushouldincludebasicapplicationfunctionsandnecessarynavigationitems（forexample,abuttontoopenapplicationsettings）.TheusercanopentheOptionsMenuwiththedeviceMENUkey.Figure1showsascreenshotofanOptionsMenu.

Whenopened,thefirstvisibleportionoftheOptionsMenuiscalledtheIconMenu.Itholdsthefirstsixmenuitems.IfyouaddmorethansixitemstotheOptionsMenu,AndroidplacesthesixthitemandthoseafteritintotheExpandedMenu,whichtheusercanopenwiththe"More"menuitem.

WhentheuseropenstheOptionsMenuforthefirsttime,AndroidcallsyourActivity'sonCreateOptionsMenu（）method.OverridethismethodinyourActivityandpopulatetheMenuthatispassedintothemethod.PopulatetheMenubyinflatingamenuresourceasdescribedinInflatingaMenuResource.（Youcanalsopopulatethemenuincode,usingadd（）toaddmenuitems.）

WhentheuserselectsamenuitemfromtheOptionsMenu,thesystemcallsyourActivity'sonOptionsItemSelected（）method.ThismethodpassestheMenuItemthattheuserselected.YoucanidentifythemenuitembycallinggetItemId（）,whichreturnstheuniqueIDforthemenuitem（definedbytheandroid:

idattributeinthemenuresourceorwithanintegerpassedtotheadd（）method）.YoucanmatchthisIDagainstknownmenuitemsandperformtheappropriateaction.

Forexample:

@Override

publicbooleanonOptionsItemSelected（MenuItemitem）{

  //Handleitemselection

  switch（item.getItemId（））{

  caseR.id.new_game:

    newGame（）;

    returntrue;

  caseR.id.quit:

    quit（）;

    returntrue;

  default:

    returnsuper.onOptionsItemSelected（item）;

  }

}

Inthisexample,getItemId（）queriestheIDfortheselectedmenuitemandtheswitchstatementcomparestheIDagainsttheresourceIDsthatwereassignedtomenuitemsintheXMLresource.Whenaswitchcasesuccessfullyhandlestheitem,itreturns"true"toindicatethattheitemselectionwashandled.Otherwise,thedefaultstatementpassesthemenuitemtothesuperclassincaseitcanhandletheitemselected.（Ifyou'vedirectlyextendedtheActivityclass,thenthesuperclassreturns"false",butit'sagoodpracticetopassunhandledmenuitemstothesuperclassinsteadofdirectlyreturning"false".）

Tip:

IfyourapplicationcontainsmultipleactivitiesandsomeofthemprovidethesameOptionsMenu,considercreatinganActivitythatimplementsnothingexcepttheonCreateOptionsMenu（）andonOptionsItemSelected（）methods.ThenextendthisclassforeachActivitythatshouldsharethesameOptionsMenu.Thisway,youhavetomanageonlyonesetofcodeforhandlingmenuactionsandeachdecendentclassinheritsthemenubehaviors.

Ifyouwanttoaddmenuitemstooneofyourdecendentactivities,overrideonCreateOptionsMenu（）inthatActivity.Callsuper.onCreateOptionsMenu（menu）sotheoriginalmenuitemsarecreated,thenaddnewmenuitemswithmenu.add（）.Youcanalsooverridethesuperclass'sbehaviorforindividualmenuitems.

Changingthemenuwhenitopens

TheonCreateOptionsMenu（）methodiscalledonlythefirsttimetheOptionsMenuisopened.Thesystemkeepsandre-usestheMenuyoudefineinthismethoduntilyourActivityisdestroyed.IfyouwanttochangetheOptionsMenueachtimeitopens,youmustoverridetheonPrepareOptionsMenu（）method.ThispassesyoutheMenuobjectasitcurrentlyexists.Thisisusefulifyou'dliketoremove,add,disable,orenablemenuitemsdependingonthecurrentstateofyourapplication.

Note:

YoushouldneverchangeitemsintheOptionsMenubasedontheViewcurrentlyinfocus.Whenintouchmode（whentheuserisnotusingatrackballord-pad）,Viewscannottakefocus,soyoushouldneverusefocusasthebasisformodifyingitemsintheOptionsMenu.Ifyouwanttoprovidemenuitemsthatarecontext-sensitivetoaView,useaContextMenu.

CreatingaContextMenu

Acontextmenuisconceptuallysimilartothemenudisplayedwhentheuserperformsa"right-click"onaPC.Youshoulduseacontextmenutoprovidetheuseraccesstoactionsthatpertaintoaspecificitemintheuserinterface.OnAndroid,acontextmenuisdisplayedwhentheuserperformsa"longpress"（pressandhold）onanitem.

YoucancreateacontextmenuforanyView,thoughcontextmenusaremostoftenusedforitemsinaListView.Whentheuserperformsalong-pressonaniteminaListViewandthelistisregisteredtoprovideacontextmenu,thelistitemsignalstotheuserthatacontextmenuisavailablebyanimatingitsbackgroundcolor—ittransitionsfromorangetowhitebeforeopeningthecontextmenu.（TheContactsapplicationdemonstratesthisfeature.）

RegisteraListView

IfyourActivityusesaListViewandyouwantalllistitemstoprovideacontextmenu,registerallitemsforacontextmenubypassingtheListViewtoregisterForContextMenu（）.Forexample,ifyou'reusingaListActivity,registeralllistitemslikethis:

registerForContextMenu（getListView（））;

InorderforaViewtoprovideacontextmenu,youmust"register"theviewforacontextmenu.CallregisterForContextMenu（）andpassittheViewyouwanttogiveacontextmenu.WhenthisViewthenreceivesalong-press,itdisplaysacontextmenu.

Todefinethecontextmenu'sappearanceandbehavior,overrideyourActivity'scontextmenucallbackmethods,onCreateContextMenu（）andonContextItemSelected（）.

Forexample,here'sanonCreateContextMenu（）thatusesthecontext_menu.xmlmenuresource:

@Override

publicvoidonCreateContextMenu（ContextMenumenu,Viewv,

                ContextMenuInfomenuInfo）{

 super.onCreateContextMenu（menu,v,menuInfo）;

 MenuInflaterinflater=getMenuInflater（）;

 inflater.inflate（R.menu.context_menu,menu）;

}

MenuInflaterisusedtoinflatethecontextmenuf