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Usually one of the biggest obstacles for the newcommers to android programming is the correct understanding of the activity's lifecycle, for example what is the difference in the event's chain when hitting "Back" button compared to activity being closed when phone is rotated. You cannot obtain this information by looking just at the activity lifecycle diagram in the API ref. In the following post I will use sample project that uses logging to demonstrate exactly what happens in the different cases. It is ment to be used as complementary tool for understanding how activities work.

Before proceeding you may want to download the sample project: Activity Lifecycle Demo. It contains simple logging of the activity and method name in each of the methods related to the lifecycle.

1. "Cold" start

Cold start is the case when the activity was a) started for the first time OR b) started after stopped with hitting "Back" button (the hardware one not <Button> that may exists in some activity's layout).

Logcat will show something like (timestamps will differ):

08-09 17:37:20.500: INFO/TEST(10107): A onCreate
08-09 17:37:20.507: INFO/TEST(10107): A onStart
08-09 17:37:20.507: INFO/TEST(10107): A onResume

Important to note here is that when onCreate(Bundle savedInstanceState) is called savedInstanceState is null. That is because there is no saved state because this is "cold" start (obviously).

2. Hitting "Back" button

08-09 17:50:19.050: INFO/TEST(10107): A onPause
08-09 17:50:19.433: INFO/TEST(10107): A onStop
08-09 17:50:19.433: INFO/TEST(10107): A onDestroy | isFinishing: true

Important to note here is isFinishing: true which means that call to isFinishing() in the onDestroy() returns true, i.e. which happens when:

• user hits "back" button OR
• activity's code calls it's finish()

(isFinishing() returns false when activity is geting closed after phone rotaion in order to be started again)

3. Another activity comes in front of the our activity

There are two variations here:

3.1. Fullscreen non-transparant activity comes in front (i.e normal activity)

In the sample project this happens when:

• "home" button is pressed, i.e. home screen activity is shown OR
• "home" button is longpressed and user switches to another activity OR
• user clicks on "Start activity B" (logcat shown bellow, includes log messages from activity B)

08-09 18:30:37.085: INFO/TEST(10107): A onSaveInstanceState
08-09 18:30:37.085: INFO/TEST(10107): A onPause
08-09 18:30:37.109: INFO/TEST(10107): B onCreate
08-09 18:30:37.109: INFO/TEST(10107): B onStart
08-09 18:30:37.109: INFO/TEST(10107): B onResume
08-09 18:30:37.382: INFO/TEST(10107): A onStop

Important to note here is that there is no guarantee if onSaveInstanceState will be called before or after onPause. onSaveInstanceState is the place where you may want to save some data in order to restore it when (and if) activity is restarted.

Next time out activity "A" comes in front logcat will show:

08-09 18:36:27.320: INFO/TEST(10107): A onRestart
08-09 18:36:27.320: INFO/TEST(10107): A onStart
08-09 18:36:27.320: INFO/TEST(10107): A onResume

i.e. this is the so called "long" path of reshowing which includes calls to onRestart and onStart.

3.2. Non-fullscreen transparant activity comes in front

Example for non-fullscreen transparant activity is activity that have android:theme="@android:style/Theme.Dialog" in the manifest. In the sample project you can start such activity by clicking on "Start activity D" button.

08-09 18:29:49.023: INFO/TEST(10107): A onSaveInstanceState
08-09 18:29:49.023: INFO/TEST(10107): A onPause
08-09 18:29:49.043: INFO/TEST(10107): D onCreate
08-09 18:29:49.043: INFO/TEST(10107): D onStart
08-09 18:29:49.043: INFO/TEST(10107): D onResume

Important to note here is that there is no call to A onStop which means that activity "A" is not stopped because it is seen trough the transparant activity "D".

When in activity "D" if you hit the "Back" button you will see:

08-09 18:34:43.105: INFO/TEST(10107): D onPause
08-09 18:34:43.121: INFO/TEST(10107): A onResume
08-09 18:34:43.140: INFO/TEST(10107): D onStop
08-09 18:34:43.140: INFO/TEST(10107): D onDestroy | isFinishing: true

This is so called "short" path of reshowing which includes just call to onResume. One other example of the "short" path is when we try to start another activity but it fails to get in front because it force-closes or calls finish() before get in front (like in onCreate). You can go trough this case by clicking on "Start activity C" button.

4. "Warm" start

"Warm" start happens when your activity was first "Cold" started, some other activity (fullscreen, non-transparent) got moved in front and then your activity was moved back in front.

08-09 18:46:03.636: INFO/TEST(10107): A onRestart
08-09 18:46:03.636: INFO/TEST(10107): A onStart
08-09 18:46:03.636: INFO/TEST(10107): A onResume

Main difference between "Cold" and "Warm" start is that with the "Warm" start onCreate is not called but onRestart is called instead.

5. Stopping and restarting on orientation change

By default when the device is rotated your activity will be stopped and then started again. Logcat looks like this:

08-09 22:32:05.628: INFO/TEST(10107): A onSaveInstanceState
08-09 22:32:05.628: INFO/TEST(10107): A onPause
08-09 22:32:05.628: INFO/TEST(10107): A onStop
08-09 22:32:05.628: INFO/TEST(10107): A onDestroy | isFinishing: false
08-09 22:32:05.730: INFO/TEST(10107): A onCreate
08-09 22:32:05.734: INFO/TEST(10107): A onStart
08-09 22:32:05.734: INFO/TEST(10107): A onRestoreInstanceState
08-09 22:32:05.734: INFO/TEST(10107): A onResume

Important to note here are three things:

• onSaveInstanceState is called that gives you a chance to save activity's state
• isFinishing returns false
• when activity is recreated the onRestoreInstanceState is called and it receives the bundle saved in onSaveInstanceState

Common mistake made by new android programmers (including me) is to expect that onSaveInstanceState and onRestoreInstanceState are paired, i.e. if onSaveInstanceState is called on stopping then onRestoreInstanceState will be called too on starting. This is NOT the case. onRestoreInstanceState is called only if application is beign killed by the OS. Most of the time you will do the state recovery in onCreate (which receives the same bundle saved in onSaveInstanceState) and not in onRestoreInstanceState. As stated in the documentation of onRestoreInstanceState:

Most implementations will simply use onCreate(Bundle) to restore their state, but it is sometimes convenient to do it here after all of the initialization has been done or to allow subclasses to decide whether to use your default implementation.

6. Application started after being killed by the OS

This may happen because of  two reasons:

• android OS needs more memory at some point and kills you app in order to reclaim it's memory;
• you use app like Advanced Task Killer to kill the app.

This entry was posted by Ognyan Bankov on 2011-08-09 at 11:00:51 and is filed under Android. You can follow any responses to this entry through the RSS 2.0 feed. Responses are currently closed, but you can trackback from your own site.