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During this period, they may be in an active or inactive, visible and invisible state. This section discusses the lifecycle of activity, service, and broadcast receivers-including their possible states during the lifecycle, how to switch states, and how these states affect the termination or destruction of the program.

There are three main states of Android lifecycle activity

◆ when it is running in the foreground of the screen (at the top of the task stack), the status is active or running. At this time, the user's attention is focused on the activity.

◆ when it is still visible to the user, but loses focus, the status is paused. The acitivity on it is either translucent or does not fill the screen, so the paused acitivity is still visible.

A paused activity is completely active (holding state, membership information, binding to the window manager), but is turned off when the system runs out of memory.

When ◆ is completely obscured by other activity, the state is stopped, and it can still save state and membership information. But it can be destroyed at any time when other programs need memory.

If an activity is stopped or paused, the system may reclaim memory by calling its finish method or simply shutting down the process. But when the activity is visible again, it restarts and returns to its previous state.

When the state of activity changes, it is notified by the following protected methods:

TaskAffinity launchMode allowTaskReparenting clearTaskOnLaunch alwaysRetainTaskState finishOnTaskLaunch

These methods can be rewritten to meet specific requirements, just as activity must implement the onCreate () method when the object is initialized * times to do the initialization work. In many cases, you need to implement onPause () to save the data in case the program shuts down.

Together, Android lifecycle methods make up the lifecycle of activity, and you can monitor this recurring process in three ways: the entire lifecycle of activity begins with * calls to onCreate () and ends with calls to onDestroy (). Activity onCreate () completes initialization of the global state.

All resources should be freed in the onDestroy () method. For example, if you have a thread downloading data in the background, you should call the method to start the thread in the program's onCreate () and the method to close the thread in onDestroy (). The visible life cycle begins with onStart () and ends with the onStop () method. During this time, the user can see the acitvity on the screen (even if it is not interacting with the user in the foreground).

Between these two methods, you can save the resources that need to update the interface. For example, if you register a BroadcastReceiver in the onStart () method to monitor operations that affect UI, you need to log out in the onStop () method, because the user no longer needs the interface after calling this method.

As the user continues to switch between the visible and invisible of the program, the onStart () and onStop () methods are called many times. The foreground Android life cycle begins with the onResume () method and ends with the onPause () method. During this time, you can see from the screen that this acitivity is in front of all other activity.

And interact with the user. Because the user will frequently switch from the paused or resumed state, for example, when the device is on standby or a new activity is started, onPause () will be called. When a new intent is passed in, the onResume () method is called. Therefore, the code within these two methods should be lightweight. Shows the trajectory of activity switching between states. Colored ellipses represent the most common state of activity. The square represents the method that can be monitored when switching the state.

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