当Android销毁活动以节省内存但不杀死App时，是否总是调用onDestroy？

Question

I want to use Application.ActivityLifecycleCallbacks to monitor how many activities are there in the back-stack. Can I increment/decrement counter in onCreate/onDestroy to handle this?

Answer 1

onDestroy is NOT guaranteed to be called every time an activity is destroyed.

If the user clicks back to destroy it, onDestroy will be called.
If the user swipes the application from the recent app menu, onDestroy will NOT be called.
If the application crashes, it's undetermined if it'll be called (from my experience, it isn't called).

Answer 2

onDestroy() is the final method that is called on an Activity instance before it's destroyed and completely removed from memory. In extreme situations Android may kill the application process that is hosting the Activity, which will result in OnDestroy not being invoked. Apparently most of the Activities will not implement this method because most clean up and shut down has been done in the OnPause and OnStop methods. For more details please visit Android Developers Portal. ( https://developer.android.com/reference/android/app/Activity.html "Android Developers")

Answer 3

Is onDestroy always called when android destroys activity to save memory?

Yes

Documentation:

The final call you receive before your activity is destroyed. This can happen either because the activity is finishing (someone called finish() on it, or because the system is temporarily destroying this instance of the activity to save space. You can distinguish between these two scenarios with the isFinishing() method.

I want to use Application.ActivityLifecycleCallbacks to monitor how many activities are there in the back-stack. Can I increment/decrement counter in onCreate/onDestroy to handle this?

Better to counter in the onStart() and onStop() methods, onCreate() doesn't guarantee visibility. For example if somehow something stopped onStart() from happening.