在方向改变时调用onDestroy被调用？

Question

I have a stock Nexus 5 running 4.4.2 (using ART if it matters) and I've found an interesting scenario. I have this as my onDestroy() :

@Override
protected void onDestroy() {
    super.onDestroy();
    t.setText("onDestroy");
    t.show();
}

It's a GPS oriented app so I'm up and walking around. I am using the technique mentioned in this question to show a lot of debug toast messages.

Anyway, when I rotate my app, the toast appears. I understand that the activity is destroyed and recreated for the new orientation, but how can I know what's really going on? How can I tell when my app is REALLY getting destroyed and not just being rotated? Similar to this question , I want to log out when a particular activity is destroyed.

Answer 1

Since Honeycomb, the isChangingConfigurations() method can be queried to check whether the Activity is being recreated due to configuration changes. Alternatively, the isFinishing() method can be queried on any API level to check whether the Activity is actually being finished, or is only being destroyed temporarily by the system.

As far as I can determine, the two methods should always return mutually consistent results in practice. The only point where they might have diverged is when the system kills the process to clear memory, but there are no callbacks or interaction with the app at that point.

The documentation of the onDestroy() method mentions the use of the isFinishing() method:

Perform any final cleanup before an 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.

Answer 2

You can put it in a fragment with setRetainInstanceState(true) set. Place your code in the onDestroy() method of the fragment. Then, the fragment will not be destroyed on orientation changes.

Answer 3

First of all, you should not use onDestroy() to do anything because its not guaranteed to be called. I would put things on the onPause() method; I wouldn't even put things in onStop().

Also, Im not sure why you want to log out a user when they navigate away from the app. I would rather implement some kind of timer on the app or server to log out after x time.

Now, the answer lies in the documentation: http://developer.android.com/reference/android/app/Activity.html#ConfigurationChanges

You might want to override onConfigurationChanged so that your activity is not restarted.

Answer 4

I found a couple of solutions which are really just patterns to detect when the screen rotates. Alternatively, you can determine that the device was actually destroyed by checking some static data member to see if it was initialized or not.

Configuration changed solutions:

The first one involves handling all of the configuration changes in the onConfigurationChanged callback.

"Note that this will only be called if you have selected configurations you would like to handle with the configChanges attribute in your manifest."

The second involves listening for Display.getRotation() which returns a Surface.ROTATION_* object. Which is the new orientation of your screen relative to the natural state of the device orientation.

Again, you can use the configuration changes along with the static member.