The Lifecycle

Android vs Java

If you use Android Studio on the lab machines: Update Java JDK Location

• Right click on app folder
• “Open Module Settings”
• Un-check “Use Embedded JDK (recommended)”
• Change location to /usr/local/oracle_jdk
• No way to configure this automatically, unfortunately

HW 1 Recap

• Who got it working?
• Who had errors?
• Common Errors:
• App could not be loaded without app.iml file
• Platform hash string android-22 could not be found
• Build tools 22.0.0.1 could not be found
• Unsupported major minor version 52.0.0

Running a Java Program

Compiling and Running Java Programs

What is “main” all about?

• public static void main(String[] args) ?

$ man java

Where is main in our MainActivity.java?

The Android Lifecycle

Android Runs and Drives our Code

• No single entry point for a program
• Android drives your app via callbacks
• Every piece of code that runs on Android must respect the lifecycle
• Goal is resource management
• Give the platform control

android.app.Activity

• Building block of user-facing Android programs
• “Single, focused thing a user can do”
• Can set a UI
• setContentView()
• Usually is full-screen

Generally speaking, an Activity can be thought of as a single screen of an app.

Activity Lifecycle

• Resumed
• Paused
• Stopped

Activity Lifecycle States

• Resumed
• Foreground, interactive
• Paused
• Foreground but blocked, not interactive
• Stopped
• In the background, can be killed

Things that affect the Lifecycle

• Apps coming to the foreground (e.g. phone call)
• Rotating the screen
• System killing apps for resources
• In-app navigation (e.g. clicking on an email)
• Outside navigation (e.g. back and home button)
• Etc: everything on the phone affects the lifecycle

Other Lifecycles. Common Paradigm

• Services
• Fragments
• Views (a bit less so)
• etc

Deep Dive on Lifecycle

onCreate(Bundle bundle)

• Most initialization goes here
• Every Activity has to implement it
• Set UI via setContentView(int resourceId)
• Set touch listeners for UI elements
• Start loading data
• bundle only non-null if restoring from previous Activity
• More later

onStart()

• Called when Activity becoming visible

onResume()

• Your Activity is at the top of the Activity stack
• User input going into activity
• Always followed by onPause()

onPause()

• Called when system about to start resuming previous Activity
• Commit unsaved changes
• Stop animations
• Stop loaders, tasks, other things consuming CPU
• Respect the lower priority your Activity is taking
• Recommended to be quick, as next Activity can’t start

onStop()

• Activity no longer visible
• Activity is KILLABLE
• activity may be killed by the system at any time without another line of its code being executed
• No guarantee this will be invoked
• ….so what good is it?

onDestroy()

• Last callback you receive
• Activity is KILLABLE
• activity may be killed by the system at any time without another line of its code being executed
• No guarantee this will be invoked
• ….so what good is it?

Saving Instance State

• Android tries to save Activity state
• Provides better user experience during normal use
• Phone call, reclaim memory without losing state/progress
• Only saves state if expected to return
• Back button does not save state
• This makes sense
• But this also increases the number of states you have to worry about

Saving State in Lifecycle

android.os.Bundle

• Typed Map
• Messaging object
• Passes data all around the Android OS
• putInt(String key, value int); getInt(String key)
• putString, putByte, putChar, putIntegerArray, put*(String key, * value), etc

onSaveInstanceState(Bundle outState)

• Save UI state not handled by system
• When restoring, onCreate() and onRestoreInstanceState() receive same bundle
• Typically onCreate() will be enough, and you don’t need onRestoreInstanceState()

Lifecycle in Action: log statements

($ git checkout lifecycle-logs)

• onCreate
• onStart
• onResume
• BACK
• onPause
• onStop
• onDestroy

Now that we understand the lifecycle, how we you use it?

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How do we tell Android to start an Activity for us?

Creating an Activity

• MainActivity mainActivity = new MainActivity();
• WRONG WRONG WRONG

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• Instead we tell Android what we want
• Android giveth.

android.content.Intent

• Intent is a message
• Android handles the intent and delivers it to the appropriate app
• An Intent can:
• Start an Activity
• Start a Service
• Broadcast a message to all apps configured to listen

Creating an Intent

• Explicit: Start a specific Activity
• Via a ComponentName(“org.cse390.playground”, “MainActivity”)
• Implicit: Let Android resolve the Activity
• Eg ACTION_EDIT, content://contacts/people/
• Android will open a picker for Contacts apps

Programmatic Intents ($ git checkout intent-basics)

Intents are handled by the system

Specifying an Initial Activity

• Tell the system what to do when your app icon is clicked
• Update AndroidManifest.xml
• Add <intent-filter>
• action = android.intent.action.MAIN
• category = android.intent.category.LAUNCHER

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AndroidManifest.xml

Questions?

• A lot of info to throw at you
• Have to move through the basics quickly

HW 2: Build public Android repo

• Complete HW 1!!! (i.e. set up Android Studio)
• I am going to be tricky with this in future assignments
• Best to get it sorted now
• Find an open source Android project
• Build it locally and run on device/emulator
• A lot of variation here. No real success/failure
• Look for email with submission details
• Play around with playground repo if you’re so inclined
• https://github.com/srsudar/cse390-android-playground