Activity, Lifecycle, Intents

Application: collection of components that can be instantiated and ran

Activity

Activity implements a single focused task a user can do
Navigation: activity launching via Intents / using buttons or gestures

Activity lifecycle

Activity states:

• active/running: visible and interactible
• paused: lost focus but still visible, maintains state and member information
• stopped: obscured by another activity, retains state and member information but can be terminated by OS

• Activity exists: onCreate() $\to$ onDestroy()
• Activity visible: onStart() $\to$ onStop()
• Activity visible in the foreground: onResume() $\to$ onPause()

onCreate(): Activity is first created $\to$ initial state setup in super.onCreate(), passed Bundle contains Activity’s previous state
onStart(): Activity is becoming visible $\to$ setup visible-only behaviour, load persistent application state, …
onRestart(): Activity is becoming visible after being stopped $\to$ special processing
onResume(): Activity is visible and becoming interactable $\to$ foreground-only activities
onPause(): Activity loses focus - another Activity is about to start $\to$ fast commit of unsaved changes (or new Thread)
onStop(): Activity is no longer visible but still exists $\to$ release not-needed resources
onDestroy(): Activity is being destroyed (called by finish() in app / back button is pressed), isn’t called when OS kills the application

Saving Activity state

Not a part of the lifecycle
Bundle stores: Acivity’s view hierarchy state + custom key-value data
onSaveInstanceState()/onRestoreInstanceState() and onCreate()

Starting activities

Create an Intent specifying the activity to start, pass to either startActivity() or startActivityForResult (expects result set by called Activity)

Task

Task: collection of Activities (not necessary from some application) user interacts with
Manages the Activity backstack: stack in order of being opened

• more instances of same Activity can be on it (Intent options)
• HOME pressed $\to$ current activity stopped, task goes into the background

Intent

Intent: operation to be performed - request from one component, received by Activity that can fulfill it
Intent fields:

• Action: string representing desired operation (eg. ACTION_DIAL, …)
• Data: data in URI format the Intent operates on
• Category: additional info about component that can handle Intent (eg. CATEGORY_BROWSABLE, …)
• Type: MIME type of Intent data, otherwise Android tries to infer it (eg. image/*, text/html, …)
• Component: explicit name of a component class for Intent use, if specified other fields are optional
• Extras: additional information for Intent as name-value pairs (eg. EXTRA_EMAIL, …)

Intent Resolutions: if Component is not specified $\to$ Android uses IntentFilters of installed apps (ties resolved by user choice)

Flags

Flag: Intent handling specification (eg. backstack behaviour with FLAG_ACTIVITY_NEW_TASK, …)