To navigate transitions between stages of the activity lifecycle, the Activity class provides a core set of six callbacks: onCreate(), onStart(), onResume(), onPause(), onStop(), and onDestroy(). The system invokes each of these callbacks as an activity enters a new state.
Figure 1 presents a visual representation of this paradigm.
Figure 1. A simplified illustration of the activity lifecycle.
As the user begins to leave the activity, the system calls methods to dismantle the activity. In some cases, this dismantlement is only partial; the activity still resides in memory (such as when the user switches to another app), and can still come back to the foreground. If the user returns to that activity, the activity resumes from where the user left off. With a few exceptions, apps are restricted from starting activities when running in the background.
The system’s likelihood of killing a given process—along with the activities in it—depends on the state of the activity at the time. Activity state and ejection from memory provides more information on the relationship between state and vulnerability to ejection.
Depending on the complexity of your activity, you probably don't need to implement all the lifecycle methods. However, it's important that you understand each one and implement those that ensure your app behaves the way users expect.
The next section of this document provides detail on the callbacks that you use to handle transitions between states.
If you have worked with C, C++ or Java programming language then you must have seen that your program starts from main() function. Very similar way, Android system initiates its program with in an Activity starting with a call on onCreate() callback method.
| Sr.No | Callback & Description |
|---|---|
| 1 | onCreate() This is the first callback and called when the activity is first created. |
| 2 | onStart() This callback is called when the activity becomes visible to the user. |
| 3 | onResume() This is called when the user starts interacting with the application. |
| 4 | onPause() The paused activity does not receive user input and cannot execute any code and called when the current activity is being paused and the previous activity is being resumed. |
| 5 | onStop() This callback is called when the activity is no longer visible. |
| 6 | onDestroy() This callback is called before the activity is destroyed by the system. |
| 7 | onRestart() This callback is called when the activity restarts after stopping it. |
