Lifecycle

Question 1

Reproduce the diagrams for:

• Activity lifecycle
• Fragment lifecycle

Answer 1

Activity
---------------------------
RESUMED (focus, visible)
onResume / onPause
STARTED (visible)
onRestoreInstanceState / 
onStart / onStop
 / **onSaveInstanceState
*onRestart / 
CREATED
onCreate / onDestroy
INITIALISED / DESTROYED

• onRestart is called instead of onCreate when app is not starting for the first time

**onSaveInstanceState - a safety measure used to save a small amount of information to a bundle as the activity exits the foreground (in case app is killed by OS later - it will not call onDestroy( ))

Fragment
---------------------------
RESUMED (focus, visible)
onResume / onPause
STARTED (visible)
onStart / onStop
onViewCreated / onDestroyView
onCreateView / 
CREATED
onCreate / onDestroy
onAttach / onDetach
INITIALISED / DESTROYED

Question 2

What must all overridden lifecycle functions do immediately?

Answer 2

Call super

Question 3

What must you remember to do if you set up a resource in a lifecycle method?

Answer 3

Tear it down in its corresponding lifecycle method.

eg resources set up in onStart( ) should be torn down in onStop( )

Question 4

What er the three main parts of the Lifecycle library?

Answer 4

Lifecycle OWNERS

• Activity and Fragment are lifecycle owners
• They implement the LifecycleOwner interface

Lifecycle CLASS
- Holds the actual state of a lifecycle owner and triggers events when lifecycle changes happen

Lifecycle OBSERVERS

• Observe the lifecycle state and perform tasks when the lifecycle changes
• They implement the LifecycleObserver interface

Question 5

Instead of setting up and tearing down a resource object in the lifecycle methods of an Activity / Fragment, how can you modify the resource itself so it can be managed by the Lifecycle library?

Answer 5

1. Have the resource’s class extend Lifecycle Observer
2. Pass a lifecycle into the resource’s class constructor
3. Add init { lifecycle.addObserver(this) } to the resource’s class
4. Annotate resource’s relevant class functions with @OnLifecycleEvent(Lifecycle.Event.LIFECYCLE_METHOD)
5. When initialising the resource in Fragment / Activity, pass in (this.lifecycle)

Question 6

Which lifecycle method is used to save a bundle before the app is killed?

which lifecycle methods take this bundle as a parameter for handling?

Answer 6

onSaveInstanceState( )

onCreate( )
onRestoreInstanceState( ) // useful if required when app has not just started

Question 7

How to add / extract values from a Bundle?

Answer 7

1. Define String constants for bundle keys

2. PUT k-v pairs into the bundle
override fun onSaveInstanceState(outState: Bundle) {
  super.onSaveInstanceState(outState)
  outState.putInt(KEY_CONST, value)
}

3. Following null check, GET k-v pairs from savedInstanceState (which is a Bundle) in onCreate / onRestoreInstanceState:
   myVal = savedInstanceState.getInt(KEY_CONST, defaultValue)

Question 8

What might trigger a configuration change (causing OS to shut down and re build the Activity)?

How do we avoid having to handle configuration changes manually?

Answer 8

• user changes language
• plugging device into a dock
• user adds a physical keyboard
• orientation change

onDestroy( ) and onCreate( ) will be called in these instances

ViewModels survive configuration changes. Using Architecture Components (ViewModels, ViewModelProviders, LiveData etc) obviates the need to use onSaveInstanceState( ) etc