Android Application简析

本文主要是解析Android中的Application类：

/**
 * Base class for those who need to maintain global application state. You can
 * provide your own implementation by specifying its name in your
 * AndroidManifest.xml's <application> tag, which will cause that class
 * to be instantiated for you when the process for your application/package is
 * created.
 * 
 * <p class="note">There is normally no need to subclass Application.  In
 * most situation, static singletons can provide the same functionality in a
 * more modular way.  If your singleton needs a global context (for example
 * to register broadcast receivers), the function to retrieve it can be
 * given a {@link android.content.Context} which internally uses
 * {@link android.content.Context#getApplicationContext() Context.getApplicationContext()}
 * when first constructing the singleton.</p>
 */

维持全局应用状态的基类。你可以提供自己的实现，通过在你的AndroidManifest.xml文件中指定它的名字，当你的应用创建的时候，它将被实例化。
一般情况下，没有必要继承Application这个类，大多数情况下，静态单例可以以模块化的方式提供相同的功能。如果你的单例需要一个全局的Context(例如，注册广播接收器的时候)，在第一次构建单例的时候可以通过Context.getApplicationContext()获取。
Application中维持三个ArrayList

    private ArrayList<ComponentCallbacks> mComponentCallbacks =
            new ArrayList<ComponentCallbacks>();//配置改变、剩余内存低时被调用
    private ArrayList<ActivityLifecycleCallbacks> mActivityLifecycleCallbacks =
            new ArrayList<ActivityLifecycleCallbacks>();//Activity的生命周期
    private ArrayList<OnProvideAssistDataListener> mAssistCallbacks = null; //协助数据(不知道用来干嘛的)

public interface ActivityLifecycleCallbacks {
        void onActivityCreated(Activity activity, Bundle savedInstanceState);
        void onActivityStarted(Activity activity);
        void onActivityResumed(Activity activity);
        void onActivityPaused(Activity activity);
        void onActivityStopped(Activity activity);
        void onActivitySaveInstanceState(Activity activity, Bundle outState);
        void onActivityDestroyed(Activity activity);
 }

/**
 *在任何activity, service, receiver(除了Content Provider)被创建之前就被调用。这个不应该太耗时，否则会直接影响第一个Activity启动的时间。如果重写这个方法，记得首先调用super.onCreate()
 */
 public void onCreate() {
 }  

 /**
  * 这个方法只在模拟器环境中被调用，在实际手机上进程是直接被移除的，不会执行这个方法
  */
 public void onTerminate() {
 }

其他的代码都很简单，通过注册监听事件，然后在事件发生的时候发送一个消息给注册了监听的对象

public void onConfigurationChanged(Configuration newConfig) {
        Object[] callbacks = collectComponentCallbacks();
        if (callbacks != null) {
            for (int i=0; i<callbacks.length; i++) {
                ((ComponentCallbacks)callbacks[i]).onConfigurationChanged(newConfig);
            }
        }
    }

    public void onLowMemory() {
        Object[] callbacks = collectComponentCallbacks();
        if (callbacks != null) {
            for (int i=0; i<callbacks.length; i++) {
                ((ComponentCallbacks)callbacks[i]).onLowMemory();
            }
        }
    }

    public void onTrimMemory(int level) {
        Object[] callbacks = collectComponentCallbacks();
        if (callbacks != null) {
            for (int i=0; i<callbacks.length; i++) {
                Object c = callbacks[i];
                if (c instanceof ComponentCallbacks2) {
                    ((ComponentCallbacks2)c).onTrimMemory(level);
                }
            }
        }
    }

    public void registerComponentCallbacks(ComponentCallbacks callback) {
        synchronized (mComponentCallbacks) {
            mComponentCallbacks.add(callback);
        }
    }

    public void unregisterComponentCallbacks(ComponentCallbacks callback) {
        synchronized (mComponentCallbacks) {
            mComponentCallbacks.remove(callback);
        }
    }

    public void registerActivityLifecycleCallbacks(ActivityLifecycleCallbacks callback) {
        synchronized (mActivityLifecycleCallbacks) {
            mActivityLifecycleCallbacks.add(callback);
        }
    }

    public void unregisterActivityLifecycleCallbacks(ActivityLifecycleCallbacks callback) {
        synchronized (mActivityLifecycleCallbacks) {
            mActivityLifecycleCallbacks.remove(callback);
        }
    }

    public void registerOnProvideAssistDataListener(OnProvideAssistDataListener callback) {
        synchronized (this) {
            if (mAssistCallbacks == null) {
                mAssistCallbacks = new ArrayList<OnProvideAssistDataListener>();
            }
            mAssistCallbacks.add(callback);
        }
    }

    public void unregisterOnProvideAssistDataListener(OnProvideAssistDataListener callback) {
        synchronized (this) {
            if (mAssistCallbacks != null) {
                mAssistCallbacks.remove(callback);
            }
        }
    }

    /**
     * @hide
     */
    /* package */ final void attach(Context context) {
        attachBaseContext(context);
        mLoadedApk = ContextImpl.getImpl(context).mPackageInfo;
    }

    /* package */ void dispatchActivityCreated(Activity activity, Bundle savedInstanceState) {
        Object[] callbacks = collectActivityLifecycleCallbacks();
        if (callbacks != null) {
            for (int i=0; i<callbacks.length; i++) {
                ((ActivityLifecycleCallbacks)callbacks[i]).onActivityCreated(activity,
                        savedInstanceState);
            }
        }
    }

    /* package */ void dispatchActivityStarted(Activity activity) {
        Object[] callbacks = collectActivityLifecycleCallbacks();
        if (callbacks != null) {
            for (int i=0; i<callbacks.length; i++) {
                ((ActivityLifecycleCallbacks)callbacks[i]).onActivityStarted(activity);
            }
        }
    }

    /* package */ void dispatchActivityResumed(Activity activity) {
        Object[] callbacks = collectActivityLifecycleCallbacks();
        if (callbacks != null) {
            for (int i=0; i<callbacks.length; i++) {
                ((ActivityLifecycleCallbacks)callbacks[i]).onActivityResumed(activity);
            }
        }
    }

    /* package */ void dispatchActivityPaused(Activity activity) {
        Object[] callbacks = collectActivityLifecycleCallbacks();
        if (callbacks != null) {
            for (int i=0; i<callbacks.length; i++) {
                ((ActivityLifecycleCallbacks)callbacks[i]).onActivityPaused(activity);
            }
        }
    }

    /* package */ void dispatchActivityStopped(Activity activity) {
        Object[] callbacks = collectActivityLifecycleCallbacks();
        if (callbacks != null) {
            for (int i=0; i<callbacks.length; i++) {
                ((ActivityLifecycleCallbacks)callbacks[i]).onActivityStopped(activity);
            }
        }
    }

    /* package */ void dispatchActivitySaveInstanceState(Activity activity, Bundle outState) {
        Object[] callbacks = collectActivityLifecycleCallbacks();
        if (callbacks != null) {
            for (int i=0; i<callbacks.length; i++) {
                ((ActivityLifecycleCallbacks)callbacks[i]).onActivitySaveInstanceState(activity,
                        outState);
            }
        }
    }

    /* package */ void dispatchActivityDestroyed(Activity activity) {
        Object[] callbacks = collectActivityLifecycleCallbacks();
        if (callbacks != null) {
            for (int i=0; i<callbacks.length; i++) {
                ((ActivityLifecycleCallbacks)callbacks[i]).onActivityDestroyed(activity);
            }
        }
    }

    private Object[] collectComponentCallbacks() {
        Object[] callbacks = null;
        synchronized (mComponentCallbacks) {
            if (mComponentCallbacks.size() > 0) {
                callbacks = mComponentCallbacks.toArray();
            }
        }
        return callbacks;
    }

    private Object[] collectActivityLifecycleCallbacks() {
        Object[] callbacks = null;
        synchronized (mActivityLifecycleCallbacks) {
            if (mActivityLifecycleCallbacks.size() > 0) {
                callbacks = mActivityLifecycleCallbacks.toArray();
            }
        }
        return callbacks;
    }

    /* package */ void dispatchOnProvideAssistData(Activity activity, Bundle data) {
        Object[] callbacks;
        synchronized (this) {
            if (mAssistCallbacks == null) {
                return;
            }
            callbacks = mAssistCallbacks.toArray();
        }
        if (callbacks != null) {
            for (int i=0; i<callbacks.length; i++) {
                ((OnProvideAssistDataListener)callbacks[i]).onProvideAssistData(activity, data);
            }
        }
    }