4、Android Activity的生命周期 Activity的生命周期

1、完整生命周期

上图是Android Activity的生命周期图，其中Resumed、Paused、Stopped状态是静态的，这三个状态下的Activity存在时间较长。

（1）Resumed：在此状态时，用户可以与Activity进行交互，Activity在最前端

（2）Paused：在此状态时，Activity被另外一个Activity遮盖，此Activity不可接受用户输入信息。另外一个Activity来到最前面，半透明的，但并不会覆盖整个屏幕。

（3）Stopped：在此状态时，Activity完全被隐藏，不可见。保留当前信息，Activity不执行任何代码。

（4）Created与Started：系统调用onCreate()后迅速调用onStart()，然后迅速执行onResume()。

以上就是Android的Activity整个生命周期。

2、主Activity

用户可以指定程序启动的主界面，此时被声明为“launcher或main”Activity的onCreate()方法被调用，成为程序的入口函数。该入口Activity可以在AndroidManifest.xml中定义主Activity。此时，主Activity必须使用以下标签声明：

<activity android:name=".MainActivity" android:label="@string/app_name">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>

3、一个新的Activity实例

系统首先调用新Activity的onCreate()方法，所以，我们必须实现onCreate()方法。如：声明UI元素、定义成员变量、配置UI等。但是事情不宜太多，避免启动程序太久而看不到界面。

TextView mTextView; // Member variable for text view in the layout

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    // Set the user interface layout for this Activity
    // The layout file is defined in the project res/layout/main_activity.xml
file
    setContentView(R.layout.main_activity);

    // Initialize member TextView so we can manipulate it later
    mTextView = (TextView) findViewById(R.id.text_message);

    // Make sure we're running on Honeycomb or higher to use ActionBar APIs
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
        // For the main activity, make sure the app icon in the action bar
        // does not behave as a button
        ActionBar actionBar = getActionBar();
        actionBar.setHomeButtonEnabled(false);
    }
}

4、销毁Activity

Activity的最后一个回调是onDestroy()，系统会执行这个方法做为你的Activity要从系统中完全删除的信号。大多数APP不需实现此方法，因为局部类的references会随着Activity的销毁而销毁。并且Activity应该在onPause()和onStop()方法中执行清楚Activity资源的操作。如果Activity在onCreate()时创建的后台线程，或者是其他有可能导致内存泄露的资源，你应该在onDestroy()时杀死它们。

@Override
public void onDestroy() {
    super.onDestroy();  // Always call the superclass

    // Stop method tracing that the activity started during onCreate()
    android.os.Debug.stopMethodTracing();
}

5、暂停与恢复

@Override
public void onPause() {
    super.onPause();  // Always call the superclass method first

    // Release the Camera because we don't need it when paused
    // and other activities might need to use it.
    if (mCamera != null) {
        mCamera.release()
        mCamera = null;
    }
}

@Override
public void onResume() {
    super.onResume();  // Always call the superclass method first

    // Get the Camera instance as the activity achieves full user focus
    if (mCamera == null) {
        initializeCamera(); // Local method to handle camera init
    }
}

static final String STATE_SCORE = "playerScore";
static final String STATE_LEVEL = "playerLevel";
...

@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
    // Save the user's current game state
    savedInstanceState.putInt(STATE_SCORE, mCurrentScore);
    savedInstanceState.putInt(STATE_LEVEL, mCurrentLevel);

    // Always call the superclass so it can save the view hierarchy state
    super.onSaveInstanceState(savedInstanceState);
}

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState); // Always call the superclass first

    // Check whether we're recreating a previously destroyed instance
    if (savedInstanceState != null) {
        // Restore value of members from saved state
        mCurrentScore = savedInstanceState.getInt(STATE_SCORE);
        mCurrentLevel = savedInstanceState.getInt(STATE_LEVEL);
    } else {
        // Probably initialize members with default values for a new instance
    }
    ...
}

public void onRestoreInstanceState(Bundle savedInstanceState) {
    // Always call the superclass so it can restore the view hierarchy
    super.onRestoreInstanceState(savedInstanceState);

    // Restore state members from saved instance
    mCurrentScore = savedInstanceState.getInt(STATE_SCORE);
    mCurrentLevel = savedInstanceState.getInt(STATE_LEVEL);
}