Download An Android Project

AndroidProjectsandApps
ClassCheck
o PowerupAndroidStudioandloadHelloWorldonyourphones.I’llcomearoundandverifythatyou’reabletopushan
APKfiletoyourphone.
AndroidDocumentation
1. https://developer.android.comistheprimarysourcefordocumentationabouttheAndroidplatformandAPIs.
2. Atthetopofthemainpageare3links:Design,DevelopandDistribute.
•
DesigntellsyouabouthowthedifferentfeaturesavailableinAndroidcanbeusedtomakeanesthetically
interestingapplication.
•
We’llusetheDeveloplinkalot.
o Herewe’llfindAPIGuidesthatdiscusshowthevariouscomponentsaredesignedtoworktogether.Weneed
toknowhowtheframeworkandcomponentsworktogetherbeforewecancode.
o We’llalsofindundertheReferencelink,theactualclassspecifications.We’llneedtoconsulttheseregularly
whenwe’recoding.
o TheToolslinkleadsyoutoresourcesforthevariousbuild,debugandreleasetoolsandservicesthatAndroid
StudioandotherIDEsusetodevelopAndroidapps.
CSCI-300MobileApplicationDevelopment
AndroidProjectFiles
Let’slookinsideanAndroidStudioproject.Atthetopofthetopleftpanelwecanchoose3differenttabslabeledProject,
Structure,andCaptures.ClickonProject.Now,atthetopoftheleftmostpaneisadropdownmenu,chooseAndroid.
Thefilesthatcompriseaprojectarestoredinthreedirectories,app,buildandgradle.Youshouldseethecontentsoftheapp
andgradledirectories.
Theappdirectorycontainsthreesubdirectories:
1. manifests–Amanifestfileisanxmlfilethattellsthedeviceinformationabouttheapp,forexamplewhichactivityto
loadfirstandwhichprivilegestheapprequests.
2. java–ThisdirectorycontainsyourappsJavasourcecode.
3. res–Theresourcedirectorycontainsfoursubdirectories:
o drawable–foriconsandotherimages
o layout–xmlfilesthatdefinetheposition(andotherproperties)ofthevariouscomponentsonascreenlike
editboxes,spinners,etc.
o mipmap–forapplaunchicons
o values–xmlfilesthatprovideaplacetodefinestringandnumericconstants.
Note:AllresourcesarecompiledintoasingleRobjectthatcanbereferencedinyourjavacode.
TheGradleScriptsdirectorycontainsconfigurationfilesforcompilingandbuildingyourapp.Inparticularthebuild.gradle
(Module:app)filecontainsinformationaboutwhichversionsoftheSDKtheapplicationwillrunonandtheapplicationId.The
applicationIdmustbethesameasthenameofthedirectoryunderthejavadirectory.
AndroidStudioShortcuts
AtthebottomoftheAndroidStudioTipsandTrickspageyou’llfindshortcutsformanyfrequentlyusedcommands.
CSCI-300MobileApplicationDevelopment
ApplicationFundamentals
Let’swalkthroughthefundamentsoftheAndroidplatform.
o AnappiswritteninJava,andalongwithappresources,iscompiledintoanapkfile.
o Androidisamulti-userLinuxoperatingsystem.
o WhenanappisinstalledonadeviceitisgivenauniqueuserID.
o TheOSsettheownerforalltheapp’sfilestotheuserIDassignedtotheappsothatonlyappcanaccessthem.
o Eachapprunsinitsownprocessthatinturnrunsitsownvirtualmachinesothatnoappcan,bydefault,accessthe
resourcesofanother.
o AppscanhoweverbedesignedtoshareauserID,process,virtualmachine,orresource.
o Appsmayalsosharecommunityresourceslikehardware,SMSmessages,contacts,etc.Permissiontousethese
resourcesmustbegrantedbytheuseratinstalltime.
ApplicationComponents
Thereare4distinctcomponentsthatstructureanapplication:Activities,Services,ContentprovidersandBroadcastreceivers.
1. AnActivitydefinesasinglescreenthatcontainsauserinterface.MostappshavemorethanoneActivity,thatis,more
thanonescreen.EachActivityisimplementedusingasubclassoftheActivityclass.TheusermovesfromoneActivity
toanotheroftenbyusingcontrols(menuoptions,buttons,etc).
2. AServiceisacomponentthatdoesnothaveauserinterfaceandrunsinthebackground.Servicescanremainrunning
whentheuserswitchestoanotherapplication.AserviceisimplementedusingasubclassoftheServiceclass.
• synchingdatawithdatabase
• playingmusic
3. AContentProviderisacomponentthatmanagesshareddata(infiles,db,etc).Itmaybroadcasttootherappsthatithas
dataandallowotherappstoreadandwritethedata,solongastheapplicationhasobtainedtheuser’spermission.It
CSCI-300MobileApplicationDevelopment
mayalsokeepthedataprivatetotheapp.AcontentproviderisimplementedusingasubclassoftheContentProvider
class.
o e.g.contacts
4. ABroadcastReceiverlistensforspecificsystem-widemessagesandrespondsbyrunningeventhandlers.These
messagescanemanatefromthesystem,suchasalowbatterymessage,orcanemanatefromapplications.Abroadcast
receiverisimplementedusingasubclassoftheBroadcastReceiverclass.
RegisteringComponentsintheManifest
Allactivities,servicesandcontentprovidersmustberegisteredinthemanifestfileorelsetheyarenotvisibletotheapp.
BroadcastReceiverscanbelistedinthemanifestfileortheycanberegistereddynamicallyusingtheregisterReceiver()method.
TheHelloWorldapp,forexample,hasoneActivityimplementedinapp/java/your_namespace/MainActivity.Whenthe
AndroidStudiowizardcreatedtheclassfile,itaddedanentryinthemanifestfile.Thisentryregistersourclassnamed
MainActivityasthestartingActivityfortheapp.
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
CSCI-300MobileApplicationDevelopment
Activities
Activitiesarecomponentsthattakeuptheentirescreen.Theuseroftenmovesfromoneactivitybypressingmenuoptions
andbuttons.Activitiesarestoredinastackinmemory.Eachapplicationhasitsownstack.ThesetofActivitieswithinastack
arereferredtoasatask.
Androidallowsmultipletaskstobeinmemoryatanypointintime,butonlyonetaskandoneActivityinthattaskhasfocusat
anytime.
Whenauserlaunchesanapp,thecurrentlyrunningtask’sstateispreserved,theActivitythatisinfocusisstoppedandthe
taskisputinthebackground.AnewtaskiscreatedforthenewappanditsmainActivity(asdefinedinthemanifest)isplaced
onanewstack.
Whentheuserpressesamenuoptionorbuttontostartanewactivitywithintheapp,thecurrentActivityisstoppedandthe
newActivityisplacedonthetopofthestackandgainsfocus.ThisprocesscancontinuecreatingmultipleActivitiesinthe
task’sstack.
WhentheuserpressesthebackbuttonthetopActivityispoppedfromthestackandtheActivitynowontopisrestarted.
Whentheuserpressesthehomebutton,thecurrentActivityisstopped,andthetask(stackincluded)isputinthebackground.
Note:Ifanapphasmultipleentrypoints(buttons,menuoptions,etc)toanActivity,thenmultipleinstancesofthesame
Activitycanexistinthestack–sobecareful.Thisisusuallynotwhatyouintended.
Note:Thesystemmightdestroybackgroundtasksifitneedsadditionalmemory.Topreservedatawhenthishappenswecan
implementacallbackmethodthatisinvokedbeforethetaskisdestroyed.SeeSavingActivityState.
CSCI-300MobileApplicationDevelopment
CreatingNewActivities
TocreateanewActivity,selectthedirectorywhereyouwantthenewActivitytoresideintheAndroidfilehierarchy.Then
selectFile>New>Activity>EmptyActivity.
Note:DONOTCHOOSENew>FileorNew>JavaClass.BychoosingNew>Activity,AndroidStudioautomaticallyaddsthe
Activitytothemanifestfile.Theseothermethodsdonot.
AndroidStudiowilldisplayaformwhereyouenterinformationabouttheactivity.ActivityNamespecifiesthenameofthe
Javaclassthatwillbeadded.ByselectingGenerateLayoutFile,askeletonxmlfilewillbecreated.Thelayoutfileiswhereyou
specifytheGUI.ByselectingtheLauncherActivitycheckboxtheappwillbemodifiedtolaunchthecurrentactivitywhenthe
appisfirststarted.ByselectingBackwardsCompatibilitythenewactivitywillextendtheAppCompatActivityclass.Thisis
preferred(moreonthislater).Last,thePackagenamefieldspecifiestheJavapackagethattheclasswillbestoredin.
CSCI-300MobileApplicationDevelopment
ApplicationLifeCycle
WedescribedabovewhathappenswhendifferentappschangefocusinAndroid.Inthissectionwe’lldiscussanapplication’s
lifecycle.
Activitieshave3states:Running(resumed),Paused(,andStopped.
1. Running-TheActivityhastheuserfocus.
2. Paused-TheActivityisbehindanotheractivitybutcanbepartiallyseen.Itsstateismaintained.
3. Stopped–Theactivityisobscuredbyanotheractivity.Itsstateismaintained.
Whenanapphasmultipleactivitiesthatarepoppedonandoffthestack,weoftenwanttodothingsbeforeanActivitylooses
focusordothingsbeforeanActivityonthestackregainsfocus.Wecanperformthesetasksimplementingcallbackmethods
whicharecalledbeforethestateofanActivityischanged.
ThemostcommonlyusedcallbacksareonCreate()andononPause().
public class ExampleActivity extends Activity
{
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// A new activity is created.
// Display layout here
}
@Override
CSCI-300MobileApplicationDevelopment
protected void onRestart() {
super.onStart();
// Either the apps task was restarted or the user hit the
// back button which destroyed the stacks top Activity,
// leaving this Activity active.
}
@Override
protected void onStart() {
super.onStart();
// The activity is about to become visible.
}
@Override
protected void onResume() {
super.onResume();
// The activity is about to gain focus and the activity is put
// on the top of the stack.
}
@Override
protected void onPause() {
super.onPause();
// Current Activity is loosing focus.
Either another Activity is
CSCI-300MobileApplicationDevelopment
// pushed on the stack or the entire task is stopped.
}
@Override
protected void onStop() {
super.onStop();
// The activity is no longer visible.
}
@Override
protected void onDestroy() {
super.onDestroy();
// The activity is about to be destroyed.
// call onSaveInstanceState() to save user information like username.
}
}
Note:Youmustcallthesuperclass’callbackmethodfirstinthesecallbackmethods.
LayoutsandWidgets
TheGUIthatisdisplayedinanActivityisdefinedinalayoutxmlfileresidingintheres/layoutdirectory.Byclickingona
layoutfile,AndroidStudiodisplaysaprototypeonthescreen.WecanseetheactualtextofthexmlfilebyclickingontheText
tabunderinprototypedisplay.
CSCI-300MobileApplicationDevelopment
AnActivity’slayoutfilespecifieswidgetswithinalayout.Eachwidgethaspropertiesthataremodifiable.
Let’sgobacktotheprototypebyclickingtheDesigntab.Awidgetisacomponentthattheusercansee:TextViews,Buttons,
CheckBoxes,etc.AndroidStudioallowsustodraganddropthemintoourlayout.
Alayoutcontrolshowthewidgetsaredisplayedwithrespecttooneanother.Youmayhavenestedlayoutsinonesinglexml
file.
Exercise:AddasecondTextViewtoyourHelloWorldlayout.Experimentwiththedifferentwidgets.
CSCI-300MobileApplicationDevelopment