Pro Android with Kotlin

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CHAPTER 3: Activities

Paused: The activity loses focus but is still at least partly visible. Losing 
the focus, for example, happens when the user taps the Back or 
Recents button. The activity may continue to send updates to the UI 
or continue to produce sound, but in the majority of cases the activity 
will proceed to the stopped state. During the pausing, the onPause() 
callback gets called. The paused state is followed by the stopped state 
or the resumed state.

Stopped: The activity is invisible to the user. It later might be restarted, 
destroyed, and expunged from the active process list. During stopping, 
the onStop() callback gets called. After stopping, either destruction 
or starting happens. Here you can, for example, stop the service you 
started in onStart().

Destroyed: The activity is removed. The callback onDestroy() gets 
called, and you should implement it and do everything there to release 
resources and do other cleanup actions.
Table 
3-1
 lists the possible transitions between an activity’s states, which are illustrated in 
Figure 
3-1
.
Table 3-1. Activity State Transitions
From
To
Description
Implement
Shut Down
Created
An activity gets called the first time or 
after a destruction.
onCreate(): Call super.onCreate(), 
prepare the UI, start services.
Created
Started
An activity starts after creation.
onStart(): You can start services 
here that are needed only while the 
activity is visible.
Started
Resumed
The resumed state automatically 
follows a started state.
Use onResume.
Resumed
Running
The running state automatically 
follows a resumed state.
The activity’s functioning including 
UI activity happens here.
Running
Paused
The activity loses focus because the 
user tapped the Back or Recents button.
Use onPause.
Paused
Resumed
The activity has not stopped yet, 
and the user navigates back to the 
activity.
Use onResume().
Paused
Stopped
The activity is invisible to the user, for 
example, because another activity 
gets started.
onStop(): You can stop services 
here that are needed only while the 
activity is visible.
Stopped
Started
A stopped activity gets started again. onStart(): You can start services 
here that are needed only while the 
activity is visible.
Stopped
Destroyed A stopped activity gets removed.
onDestroy(): Release all resources, 
do a cleanup, and stop services 
that were started in onCreate.

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