Activity state and onSaveInstanceState()

Activity state and onSaveInstanceState()

Activity state and ejection from system’s memory:

The basic need to kill an activity by the system is to free up RAM in the system and the likelihood of the system to kill process totally, depend on the state of the process at that time. Then, Process state, in turn, depends on the state of the activity running in the process (means the callback methods that you have covered earlier).

By the table below one can easily understand the Relationship between process lifecycle and activity state.

Note: The system not kills an activity directly to free up memory or space. Instead, it kills the process in which the activity is running. It destroys not only the activity but everything else running in the process, as well.

What is onSaveInstanceState() ? Why we use it?

When the activity stopped, the system calls the onSaveInstanceState() method to check what the system have to do in that situation. If the system don’t found that method or the method is null, it means there is no data present which system have to be restored. Then, the data that was present before app is stopped is destroyed forever.

It is used because the system destroys the activity by default when the configuration changes temporarily (means when the orientation is changed or when the app is not focused and is on pause state) this causes wipe of UI state stored in activity instance.

So, it’s necessary to use onSaveInstanceState() to correctly match users expectations with system behavior(to save that UI state).

It is collection of key-value pairs stored in a Bundle (you will see this  in onCreate method).

Bundle- It is by default instance state to save information about each view object in activity layout such as EditText, TextView and sonon.

To preserve more than a very small amount of data we use onSaveInstanceState() .

The saved data that the system uses to restore the previous state is called the instance state.

How this all works?

                 If the activity destroyed due to system constraints 

 

    Actual Activity instance is gone but system remembers its existence

                              If the user navigate back to the activity

                          System creates a new instance of that activity.

                          (using a set of saved data in onSaveInstanceState())

// Simple code to use SavedInstance state method in Oncreate method.

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState); // Always call the superclass first

    // Check whether we're recreating a previously destroyed instance
    if (savedInstanceState != null) {
        // Restore value of members from saved state
        }

else {
        // Probably initialize members with default values for a new instance
    }

    // ...

 if (savedInstanceState != null) :- If it is null, then the system is creating a new instance of the activity, instead of restoring a previous one that was destroyed.

// onSaveInstanceState() method

@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
    // Always call the superclass so it can save the view hierarchy state
    super.onSaveInstanceState(savedInstanceState);

   //code
}

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