Activities in the system are managed as an activity stack. When a new activity is started, it is placed on the top of the stack and becomes the running activity — the previous activity always remains below it in the stack, and will not come to the foreground again until the new activity exits.

An activity has essentially four states:

The following diagram shows the important state paths of an Activity. The square rectangles represent callback methods you can implement to perform operations when the Activity moves between states. The colored ovals are major states the Activity can be in.

There are three key loops you may be interested in monitoring within your activity:

Your activity monitors and reacts to these events by instantiating methods that override the Activity class methods for each event:

1. onCreate()

Called when your activity is first created. This is the place you normally create your views, open any persistent datafiles your activity needs to use, and in general initialize your activity. When calling onCreate(), the Android framework is passed a Bundle object that contains any activity state saved from when the activity ran before.

2. onStart()

Called just before your activity becomes visible on the screen. Once onStart() completes, if your activity can become the foreground activity on the screen, control will transfer to onResume(). If the activity cannot become the foreground activity for some reason, control transfers to the onStop() method.

3. onResume()

Called right after onStart() if your activity is the foreground activity on the screen. At this point your activity is running and interacting with the user. You are receiving keyboard and touch inputs, and the screen is displaying your user interface. onResume() is also called if your activity loses the foreground to another activity, and that activity eventually exits, popping your activity back to the foreground. This is where your activity would start (or resume) doing things that are needed to update the user interface.

4. onPause()

Called when Android is just about to resume a different activity, giving that activity the foreground. At this point your activity will no longer have access to the screen, so you should stop doing things that consume battery and CPU cycles unnecessarily. If you are running an animation, no one is going to be able to see it, so you might as well suspend it until you get the screen back. Your activity needs to take advantage of this method to store any state that you will need in case your activity gains the foreground again—and it is not guaranteed that your activity will resume. If the mobile device you are running on runs out of memory, there is no virtual memory on disk to use for expansion, so your activity may have to make way for a system process that needs memory. Once you exit this method, Android may kill your activity at any time without returning control to you.

5. onStop()
Called when your activity is no longer visible, either because another activity has taken the foreground or because your activity is being destroyed.

6. onDestroy()
The last chance for your activity to do any processing before it is destroyed. Normally you’d get to this point because the activity is done and the framework called its finish method. But as mentioned earlier, the method might be called because Android has decided it needs the resources your activity is consuming.
It is important to take advantage of these methods to provide the best user experience possible.

Android Service Lifecycle

The lifecycle for a service is similar to that for an activity, but different in a few important details:

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