Android Activity Lifecycle Handling

Question

I am new to android programming and would like to make clear the concept of Activity Lifecycle.

To make it simple, I have shortened the case as below:

There are 2 activities (MainActivity and Main2Activity).
In MainActivity, a button is created to open Main2Activity.
In Main2Activity, in order to simulate user returning to MainActivity once Main2Activity is shown, it calls finish() and return to MainActivity in onCreate().

When the button is clicked, I found that there seems to have race condition occurred.

MainActivity::onCreate() <-- 1st MainActivity instance
MainActivity::onStart() <-- 1st MainActivity instance
MainActivity::onResume() <-- 1st MainActivity instance
MainActivity::onPause() <-- 1st MainActivity instance
MainActivity::onCreate() <-- 2nd MainActivity instance
MainActivity::onStart() <-- 2nd MainActivity instance
MainActivity::onResume() <-- 2nd MainActivity instance
MainActivity::onStop() <-- 1st MainActivity instance
MainActivity::onDestroy() <-- 1st MainActivity instance

The last onDestory() from the 1st MainActivity instance will run after the onCreate(), onStart(), onResume() from the 2nd MainActivity instance.

How can I ensure the onDestroy() is called before starting another activity?

Thanks very much for everyone's help.

MainActivity.java

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    final Button btnClick = findViewById(R.id.btnClick);
    btnClick.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            // finish self
            finish();
            // start Main2Activity
            Intent intent = new Intent(v.getContext(), Main2Activity.class);
            startActivity(intent);
        }
    });
}

Main2Activity.java

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main2);

    // return to MainActivity
    Intent intent = new Intent(this, MainActivity.class);
    startActivity(intent);
    // finish self
    finish();
}

Why do you think this is a problem? (AFAIK you can't do much about it, the runtime finishes/destroys Activities at its own pace. It depends on the Android version and the hardware as well as other factors like how many other apps are currently requesting resources) — Bö macht Blau, Aug 27 '18 at 18:33
What I plan is to release resources when the activity is destroyed. If the onDestroy() is not guaranteed to be called before a new instance of the same activity is created, it will accidentally release singleton resources being using by another instance. — hkckfs340, Aug 28 '18 at 04:12

score 0 · Answer 1 · answered Aug 27 '18 at 17:27

Not sure its the solution but give it a try..

on Main2Activity.java

@Override
protected void onCreate(Bundle savedInstanceState) {
   super.onCreate(savedInstanceState);
   setContentView(R.layout.activity_main2);

// return to MainActivity

new Handler().postDelayed(new Runnable() {

     @Override
     public void run() {
       Intent intent = new Intent(this, MainActivity.class);    
       startActivity(intent);
     }
},5000);

// finish self
finish();
}

this would call the startActivity after 5 seconds .. that would give for the finish enough time to call all the life cycle

Using a time delayed approach seems not to be a good solution for race condition. Thanks anyway. — hkckfs340, Aug 27 '18 at 17:45

score -1 · Accepted Answer · answered Aug 27 '18 at 17:28

-1

You can call isFinishing() on your first MainActivity instance.

Here is the documentation for this method.

This would look like :

if (isFinishing()) {
  //onDestroy() has not been called yet
}
else {
  //onDestroy() has been called already
}

answered Aug 27 '18 at 17:28

ArcDexx

453
5
15

no, this is not what `isFinishing()` is checking. Read the docs you linked to. – Marcin Orlowski Aug 27 '18 at 17:35
Thanks. I will try this approach to check the isFinishing() value in onPause() and to release resources if true. – hkckfs340 Aug 27 '18 at 17:42
@MarcinOrlowski `finish()` is explicitly called here. A call to this method will eventually trigger `onDestroy()`. See this answer for details : https://stackoverflow.com/questions/19891969/android-will-finish-always-call-ondestroy – ArcDexx Aug 29 '18 at 13:31
`isFinishing()` is in no way telling if `onDestroy()` was called as your comments suggest. In fact if it was, then your instance is already dead. – Marcin Orlowski Aug 29 '18 at 13:53

Android Activity Lifecycle Handling

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