Section one

• when configuration changes occurs(ex, rotation ) activity is destroyed and created again this leads to -> state stored directly in activity will be lost.

Examples

CASE 1

• when onCreate() used to display list of data for example listX and there is 3 nav tabs (X,Y,Z)
• so by default u load X and when click on Y,Z u load them but you do not save that state
• so when activity onCreate() called again it will listX while nav is selected on Y or Z which is wrong to User.
• Solution: save nav state and at onCreate() load lists based on saved state

CASE2

• when you have list of data(for example List<User>()) and is filled at runtime with method addUser().
• after rotate device (or any action that activity be recreated upon it), this list will be empty again!!.
• Solution: save that list at ViewModel and addUser() be at ViewModel.

Section two

• when activity is destroyed there two possible expectation from user

  • State is maintained: in case of configuration changes, system free recources.
  • State is not maintained: in case of kill app, call finish().

• in case of fully destroy of activity, ViewModel fails to maintain state.

• Solution: save state at onSaveInstanceState(outState:Bundle?) and at onCreate() get that state and store it at ViewModel

• This technique used to have a durabale persisitance.

Scetion three

• when storing complex state such aslist of real objectsas we knew that VM will be destroyed also and it is not effiecent to store real objects into bundle so IDEA is to save IDS or info that help to restore that list from DataStore in app then use these ids (will be stored in bundle) to restore objects from DataStore.

  Two-State-Managment

  • is to used VM and Durable persistance to manage state
    • VM: maintain state during configuration changes.
    • Durable persistance: maintain state during fully destroy activity(ex system free up resources).
      How?
  • first we should add isFirstCreatedVM = true field at VM and at oc we set it false
  • before we call restoreState we add check to both values.
  • CODE

    class MyVm:viewModel(){
        var isFirstCreatedVM = false 
    
    }
    
    
    class View {
    
        onCreate(bundle:Bundle?){
    
    
            if (viewmodel.isFirstCreatedVM && bundle != null ){
                viewmodel.restoreSaved(bundle)
            }
    
            viewmodel.isFirstCreatedVM = false 
    
        }
    }
    
    

  • Scenarios
  • when configuration changes ViewModel will not be destroyed so no need to restore state as it is already mantained in it
  • when system free up resorces ViewModel will be destroyed so isFirstCreatedVM be true as ViewModel is recreated and bundle be not null so now we can restore state.

Section four

• Lifecycle: some tasks(such as listen to location updates, call backend services) takes recources from android and needed to be cleared up so no leak occurs.

• also logic of register and unregister to these taks should be done at LifecyclerObserver so Activity be for User Interface tasks.

• so we will need two component

• LifecyclerOwner: own lifecycle and publish his events (such as Activity/Fragment)

• LifecyclerObserver: listen to lifecycle events

• for LifecyclerObserver should implement LifecyclerObserver and use OnLifecycleEvent annotations with specify what event such as

  @OnLifecycleEvent(Lifecycler.Event.ON_START)
  fun startHandler(){
      // start yout work
  }
  

• There are all events (ON_CREATE,ON_START, ON_RESUME, ON_PASUE, ON_STOP, ON_DESTROY)

• Steps:

  • Create Observer by extend LifecyclerObserver and annotate your methods with EVENT type
  • send LifecyclerOwner to that observer (Hint: Activity/Fragment has property called lifecycle this is LifecyclerOwner)
  • at init of Oberver add it to lifecyclerOwner lifecycler.addObserver(this)

Section Five

• sometimes we may leak state(i.e make action at wrong state of activity).
• for exmaple we can start a connection using handler with some delays, so in case of user rapdially close screen so that connect we be established after screen is off and it will remain live and app do not know about it.
• Lifecycler STATE comes to solve this, you can check state using lifecycle.currentState property.
• states are (INITIALIZED,CREATED,STARTED,RESUMED,DESTROYED), also you can use atLeast(state) and send minimum state u want.