Android Activity CS 4720 Mobile Application Development Source: - - PowerPoint PPT Presentation

CS 4720

Android Activity

CS 4720 – Mobile Application Development Source: developer.android.com

CS 4720

Activity

• Conceptually, an Activity is a single screen of

your application

• In other words, an App really is a collection of

related Activities

• Consider each Activity both a screen and a

feature

• Apps can activate Activities in other Apps

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The App Lifecycle

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The Activity

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The Lifecycle

• An Activity can be:

– Resumed – active and in the foreground with user focus – Paused – active, but is “covered up” by another activity that currently has the user focus – Stopped – currently residing in the background and is not attached to the window manager and is not visible; can be killed by system if memory is needed

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Lifecycle Callbacks

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Lifecycle Callbacks

• Make sure to make the correct calls in the

correct methods!

• If you do everything in onCreate(), then

problems could occur when you switch

• On a device rotation, onDestroy() / onCreate()

is called

• Consider saving state using
• nSaveInstanceState() to retain App state

between switching

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• nCreate()
• Called when App is starting fresh
• This is where setContentView() (setting the UI)

is called

• This is why onCreate() is called on a rotation –

you may be using a different layout

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Next Steps

• onStart() - Called right before Activity is shown

to user

• onResume() – Called just before Activity will

start accepting user input

• onPause() – Called when another Activity will

call onResume() – used to save remaining changes

• onStop() – Called when Activity is no longer

visible

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UI

• UI components come from the View class
• A View controls a set of rectangular space and

can respond to user interaction

• Can be the whole screen, or just a small

portion

• Technically, a “widget” (like a button) is a very

small View

• A Layout (and it’s xml) is a group of Views with

location instructions

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Adding to the Manifest

• All Activities must be defined in the

androidmanifest.xml file

• Intents that this Activity can respond to are

also defined here

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Launching a new Activity

• To launch a new Activity, create an Intent for

that Activity and send it out

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Fragments

• Consider a Fragment a sort of “sub-Activity”
• An Activity can be made up of numerous

Fragments, each with their own features

• This is useful when you want to move/reorder

components for different display sizes/rotations

• A Fragment obeys the Activity’s lifecycle state

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Fragments

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Fragments

• A Fragment has:

– Its own lifecycle calls – Its own layouts – Its own views

• In fact, a Fragment looks almost exactly like an

Activity!

• To add a Fragment, instantiate it in the layout
• f the Activity

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FragmentManager

• The main benefits Fragments are reusability

and addressability

• Fragments can be accessed through the

FragmentManager and can be swapped on the fly while the App is running

• Fragments can address the Activity that

contains them, allowing for sharing of data between Fragments

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