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This article mainly introduces the relevant knowledge of "what is the life cycle of Activity in Android". The editor shows you the operation process through an actual case. The operation method is simple, fast and practical. I hope this article "what is the life cycle of Activity in Android" can help you solve the problem.

Notice that there are a large number of function definitions in the form of onXXXX in Activity's API, including onStart,onStop and onPause in addition to the onCreate we used earlier. Literally, they are event callbacks, so what is the order? In fact, it is best to do an experiment on this kind of thing. Before doing this experiment, we need to find out how the Log in Android is output.

Obviously, we're going to use the android.util.log class, which is fairly easy to use because it provides all the static methods:

Log.v (String tag, String msg); / VERBOSE Log.d (String tag, String msg); / / DEBUG Log.i (String tag, String msg); / / INFO Log.w (String tag, String msg); / / WARN Log.e (String tag, String msg); / / ERROR

The previous tag is an identity defined by us, which can generally be defined as "class name _ method name".

The output LOG information, if developed in Eclipse+ADT, can be seen in LogCat, otherwise adb logcat is fine, but I have always relied on the IDE environment.

Okay, now let's modify the previous HelloThree code:

Public void onStart ()... {super.onStart (); Log.v (TAG, "onStart");} public void onStop ()... {super.onStop (); Log.v (TAG, "onStop");} public void onResume ()... {super.onResume (); Log.v (TAG, "onResume");} public void onRestart ()... {super.onRestart (); Log.v (TAG, "onReStart") } public void onPause ()... {super.onPause (); Log.v (TAG, "onPause");} public void onDestroy ()... {super.onDestroy (); Log.v (TAG, "onDestroy");} public void onFreeze (Bundle outState)... {super.onFreeze (outState); Log.v (TAG, "onFreeze");}

Add the same code to HelloThreeB, compile, run it, and analyze the output log from logcat.

When launching the Activity One of * interfaces, the order is:

OnCreate (ONE)-onStart (ONE)-onResume (ONE)

Although it is a * start, you have to go through the resume event. Then, we click goto to jump to the second Activity Two (the previous one is not closed), and the order is as follows:

OnFreeze (ONE)-onPause (ONE)-onCreate (TWO)-

OnStart (TWO)-onResume (TWO)-onStop (ONE)

Note: before the second Activity Two starts, the One will go through a process of freezing and pausing, and only after starting the Two will the One be stopped?

Then, we click back to return to the * * interface, and the order is as follows:

OnPause (TWO)-onActivityResult (ONE)-onStart (ONE)-

OnRestart (ONE)-onResume (ONE)-onStop (TWO)-onDestroy (TWO)

Note: on return, Two is paused without experiencing a freeze. After One receives parameters and restarts, Two is stopped and destroyed. *. Let's click Exit to exit the application. The order is as follows:

OnPause (ONE)-onStop (ONE)-onDestroy (ONE)

It means that if we use finish, there will be no freeze, but we will still go through pause-stop before it is destroyed.

The question here is: why is Start the first Restart when you come back? But the picture in the document is restart and then start ah? However, the description in the later table seems to be correct, and start is always followed by resume (if it is * *) or restart (if originally dropped by stop, a restart will be inserted in start and resume).

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Instead of examples, take a look at the documentation for the Android Activity lifecycle.

1.Android uses Activity Stack to manage multiple Activity, so only the top Activity is in the active or running state at a time. The rest of the Activity is pinned down.

two。 If the inactive Activity is still visible (that is, if it is pressed against a non-full-screen Activity or transparent Activity), it is in the paused state. In the case of insufficient memory in the system, the Activity in the paused state can be killed by the system. I just don't understand, what will the display on the interface look like if it is killed? It seems necessary to study this situation next time.

3. The pairing of several events can clearly understand their relationship. A pair of Create and Destroy is called entrie lifetime. If resources are allocated during creation, resources are released when they are terminated. On the top, there is a pair of Start and Stop, called visible lifetime, which expresses the process of visible and non-visible. At the top is the pair of Resume and Pause, called foreground lifetime, which expresses the process of whether it is active or not.

4. Therefore, the Activity derived class we implement overloads two important methods: onCreate () for initialization, and onPause () to save the results of the current operation.

In addition to Activity Lifecycle, Android also has a description of Process Lifecycle:

When there is insufficient memory, Android will take the initiative to clean the door, so how does it determine which process can be cleared? Its ranking of importance is also mentioned in the document:

1. Empty process is the easiest to clear. Empty processes are those that are not bound to Activity or any application components (such as Services or IntentReceiver), that is, there is no activity or service in this process, but only as a cache that can speed up when starting a new Activity. They will be cleared first. Therefore, it is suggested that our background operations, * *, should be in the form of Service, that is, a Service should be launched in Activity to perform these operations.

two。 Then there is background activity, that is, the process in which the activity is lost by stop. It is indeed safe for those invisible Activity to be removed. The system maintains a LRU list in which multiple activity in background are included. According to the LRU list, the system can determine which activity can be cleared and which one should be cleared. However, it is mentioned in the document that when the cleared Activity is recreated, its onCreate will be called with the same Bundle as the onFreeze. But what I don't understand here is, when the Activity is killed, will Android keep the Bundle for it?

3. Then it's service process, which is a process bound to Service and started by the startService method. Although they are not visible to the user, they are usually dealing with long-term operations (such as MP3 playback), and the system will protect it unless there is really no memory available.

4. Then it's the visible activity, or visible process. As mentioned earlier, the Activity of Paused may also be cleared by the system, but relatively speaking, it is already in a relatively safe position.

5. The safest thing should be that foreground activity, which will not be cleared until it is forced to. This process includes not only activity after resume, but also IntentReceiver instances after onReceiveIntent.

In the discussion of the Android Activity life cycle, the document also mentions some matters needing attention: because the lifetime of Android applications is not directly controlled by the application itself, but managed by the Android system platform, it is necessary for us developers to understand the lives of different components Activity, Service and IntentReceiver, keeping in mind that if the components are not selected properly It is quite possible that the system will kill a process that is doing important work.

This is the end of the content about "what is the life cycle of Activity in Android". Thank you for reading. If you want to know more about the industry, you can follow the industry information channel. The editor will update different knowledge points for you every day.

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