In Android, there are four major development components, which are the basis of Android programming, and Activity is the first and top priority. Heavy. Today we will learn about the life cycle of Activity.
Activity is the most important component in Android development. Almost all interactive operations are performed in Activity. It is mainly responsible for creating a window. In this window, you can add the UI controls you need by calling the setContentView function. Show it. Let’s first look at an activity life cycle diagram provided by the Android API. If you don’t understand it at first, you can read the entire blog post and then look back to deepen your understanding.
The following Activity methods show the complete life cycle of the Activity. When the status of the Activity changes, you can Override these methods to implement certain functionality.
<code class="language-none" style="-webkit-print-color-adjust:exact;margin:0px;padding:0px;border:none;border-radius:3px;background-color:transparent;"> public class Activity extends ApplicationContext { protected void onCreate(Bundle savedInstanceState); protected void onStart(); protected void onRestart(); protected void onResume(); protected void onPause(); protected void onStop(); protected void onDestroy();}</code>is called when the activity is created for the first time. The creation of views and initialization of data are all performed here. If you are sharp-eyed, you may have discovered that there is a Bundle savedInstanceState parameter in the onCreate method, which will save the previous state of the activity, which is very useful.
will be called after the onCreate method
会在onStart方法后调用。activity是使用栈结构来管理的，此时activity处于栈顶。
被stop的activity重新启动时调用此函数，后面会紧跟onResume。
当有新的activity进入栈顶，即当前activity被迫到后台运行，onPause就会被调用。
会在onPause方法后调用，此时activity对于用户来讲，已经是不可见的了(Invisible)。从栈的角度来说，就是有新的activity压栈，或者已经存在的activity回到了栈顶。
这是生命周期中的最后一个函数。一般情况下activity已经完成(调用finish函数)或者系统为了回收资源主动销毁activity时，onDestroy才会被调用，可以通过isFinishing()方法来区分这2种情况。
综上，从onCreate开始到onDestroy结束，称为activity的一个完整生命周期。这里还有2个概念需要区别一下。
<code class="language-none" style="-webkit-print-color-adjust:exact;margin:0px;padding:0px;border:none;border-radius:3px;background-color:transparent;">1. 可见的(Visible，从onStart到onStop，这里的可见，不仅仅是在屏幕上可见)2. 前台运行(Foreground，从onResume到onPause)</code>
为了弄明白这些回调函数在各种情况下的调用顺序，我们新建个工程，名叫ActivityLifecycle，在这些回调函数中加些打印，如下
<code class="language-none" style="-webkit-print-color-adjust:exact;margin:0px;padding:0px;border:none;border-radius:3px;background-color:transparent;">@Overrideprotected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); editText = (EditText)findViewById(R.id.edittext); Log.d(TAG,"=== onCreate ===");}@Overrideprotected void onStart() { super.onStart(); Log.d(TAG,"=== onStart ===");}@Overrideprotected void onResume() { super.onResume(); Log.d(TAG, "=== onResume ===");}@Overrideprotected void onPause() { super.onPause(); Log.d(TAG, "=== onPause ===");}@Overrideprotected void onRestart() { super.onRestart(); Log.d(TAG, "=== onRestart ===");}@Overrideprotected void onStop() { super.onStop(); Log.d(TAG, "=== onStop ===");}@Overrideprotected void onDestroy() { super.onDestroy(); Log.d(TAG, "=== onDestroy ===");}</code>运行一下程序，从Logcat中可以看到
说明了一个Activity从创建到显示出来需要执行onCreate-->onStart-->onResume三个方法。
While the App is running, if we want to open other Apps, we usually press the Home button to exit. What is the callback like at this time? Look at the picture below:
executes onPause-->onStop, indicating that the Activity has not been destroyed, but has been overwritten by other activities and placed in the background to run. , this situation is the same as when an emergency occurs (such as an incoming call or an App being installed), and when the emergency ends, the Activity will execute onRestart-->onStart-->onResume, and the entire process will be printed. As shown below
When the App is running, if the current Activity is the last Activity, when we press the Back key, the program will end. At this time, the three methods onPause-->onStop-->onDestroy will be executed one after another, as shown below
Next, modify the project slightly and add the second Activity. Called SecondActivity, enter SecondActivity by clicking a button on MainActivity. At this time, if you press the Back key, SecondActivity will execute onPause-->onStop-->onDestroy, and MainActivity will execute onRestart-->onStart- ->onResume, as shown below
There is a concept here, which is not mentioned above. After the onPause and onStop methods are called, the activity process may be killed at any time, so data saving operations should be performed in the onPause method to avoid data loss. The onSaveInstanceState method also performs a similar function, and the data is saved in the given bundle. Because onSaveInstanceState is not an activity life cycle function, the Android API recommends saving data in the onPause method.
No processing is done. Switch between horizontal and vertical screens and look at the print. You can find that the activity is first destroyed and then re-created
In order to solve the above problems, there are generally several methods
http://download.csdn.net/detail/djstavav/9147793
1. http://developer.android.com/intl/zh-cn/reference/android/app/Activity.html
2. http://developer.android.com/guide/topics/manifest/activity- element.html
3. http://developer.android.com/guide/topics/resources/runtime-changes.html
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