Android设计模式之装饰者模式(Decorator Pattern)

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概论

在Android源码里很多时候会看到这个模式。下面来讲一些这个模式。
装饰者模式主要的优点是动态给一个对象添加一些额外的职责。使用Decorator模式相比用生成子类方式达到功能的扩充显得更为灵活。缺点是，要创建比继承更多的对象。
先来个热身，下面是装饰者模式的uml图。它大概就长得这样子的。

1. Component抽象组件，是一个接口或者是抽象类，就是定义我们最核心的对象，也就是最原始的对象。（注：在装饰模式中，必然有一个最基本、最核心、最原始的接口或者抽象类充当Component抽象组件）
2. ConcreteComponent具体组件，是最核心、最原始、最基本的接口或抽象类的实现，我们需要装饰的就是它。
3. Decorator装饰角色， 一般是一个抽象类，实现接口或者抽象方法，它的属性里必然有一个private变量指向Component抽象组件
4. 具体装饰角色，如上图中的ConcreteDecorator，我们要把我们最核心的、最原始的、最基本的东西装饰成其它东西。

Android源码中应用

再来一个Android的Context的uml图

两个图一对比，你这下你知道怎么回事了吧！

再看看context里面有什么。

public abstract class Context {
	......

    public abstract Resources getResources();

    /** Return PackageManager instance to find global package information. */
    public abstract PackageManager getPackageManager();

    /** Return a ContentResolver instance for your application's package. */
    public abstract ContentResolver getContentResolver();

    public abstract Looper getMainLooper();

	public abstract Context getApplicationContext();
	
	public abstract void sendBroadcast(@RequiresPermission Intent intent);
	......
	 @Nullable
    public abstract Intent registerReceiver(@Nullable BroadcastReceiver receiver,
                                            IntentFilter filter);
     ......
}

Context个抽象类，提供了一组通用的API，具体操作还没实现。 对应装饰者模式中的Componetnt。

ContextImpl类是下面这个样子的，它是Context的实现。对应ConcreteComponent。它会与Android框架层的各个服务（包括组件管理服务、资源管理服务、安装管理服务等）建立远程连接，通过对Android进程间的通信机制（IPC）和这些服务进行通信。

class ContextImpl extends Context {
	......
	@Override
    public void startActivity(Intent intent) {
        warnIfCallingFromSystemProcess();
        startActivity(intent, null);
    }

    /** @hide */
    @Override
    public void startActivityAsUser(Intent intent, UserHandle user) {
        startActivityAsUser(intent, null, user);
    }
    
	......

	 /** @hide */
    @Override
    public void startActivityAsUser(Intent intent, Bundle options, UserHandle user) {
        try {
            ActivityManagerNative.getDefault().startActivityAsUser(
                mMainThread.getApplicationThread(), getBasePackageName(), intent,
                intent.resolveTypeIfNeeded(getContentResolver()),
                null, null, 0, Intent.FLAG_ACTIVITY_NEW_TASK, null, options,
                user.getIdentifier());
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }

	@Override
    public void sendBroadcast(Intent intent) {
        warnIfCallingFromSystemProcess();
        String resolvedType = intent.resolveTypeIfNeeded(getContentResolver());
        try {
            intent.prepareToLeaveProcess(this);
            ActivityManagerNative.getDefault().broadcastIntent(
                    mMainThread.getApplicationThread(), intent, resolvedType, null,
                    Activity.RESULT_OK, null, null, null, AppOpsManager.OP_NONE, null, false, false,
                    getUserId());
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }
	......
}

这个是ContextWrapper ，它对应装饰者模式中的Decorator。在Android源码中有不少这样带Wrapper后缀的类，其实都可以往装饰者模式方面想。在外部调用时候需要把ContextImpl传进来赋值给mBase。像这样，ContextWrapper(new ContextImpl)。

public class ContextWrapper extends Context {
    Context mBase;

    public ContextWrapper(Context base) {
        mBase = base;
    }
    
	......
    @Override
    public void startActivity(Intent intent, Bundle options) {
        mBase.startActivity(intent, options);
    }
    ......
     @Override
    public void sendBroadcast(Intent intent) {
        mBase.sendBroadcast(intent);
    }
	......
	@Override
    public Intent registerReceiver(
        BroadcastReceiver receiver, IntentFilter filter) {
        return mBase.registerReceiver(receiver, filter);
    }
    .....
}

Android的界面组件Activity、服务组件Service以及应用基类Application都派生于ContextWrapper，它们可以通过重载来修改Context接口的实现。

ContextThemeWrapper

public class ContextThemeWrapper extends ContextWrapper {

	......

	public ContextThemeWrapper(Context base, @StyleRes int themeResId) {
        super(base);
        mThemeResource = themeResId;
    }
	......
	@Override
    public Object getSystemService(String name) {
        if (LAYOUT_INFLATER_SERVICE.equals(name)) {
            if (mInflater == null) {
                mInflater = LayoutInflater.from(getBaseContext()).cloneInContext(this);
            }
            return mInflater;
        }
        return getBaseContext().getSystemService(name);
    }
    ......
}

Activity

public class Activity extends ContextThemeWrapper
        implements LayoutInflater.Factory2,
        Window.Callback, KeyEvent.Callback,
        OnCreateContextMenuListener, ComponentCallbacks2,
        Window.OnWindowDismissedCallback, WindowControllerCallback {
    private static final String TAG = "Activity";
    private static final boolean DEBUG_LIFECYCLE = false;

    /** Standard activity result: operation canceled. */
    public static final int RESULT_CANCELED    = 0;
    /** Standard activity result: operation succeeded. */
    public static final int RESULT_OK           = -1;
    /** Start of user-defined activity results. */
    public static final int RESULT_FIRST_USER   = 1;

    /** @hide Task isn't finished when activity is finished */
    public static final int DONT_FINISH_TASK_WITH_ACTIVITY = 0;
    /**
     * @hide Task is finished if the finishing activity is the root of the task. To preserve the
     * past behavior the task is also removed from recents.
     */
    public static final int FINISH_TASK_WITH_ROOT_ACTIVITY = 1;
    /**
     * @hide Task is finished along with the finishing activity, but it is not removed from
     * recents.
     */
    public static final int FINISH_TASK_WITH_ACTIVITY = 2;

    static final String FRAGMENTS_TAG = "android:fragments";

    private static final String WINDOW_HIERARCHY_TAG = "android:viewHierarchyState";
    private static final String SAVED_DIALOG_IDS_KEY = "android:savedDialogIds";
    private static final String SAVED_DIALOGS_TAG = "android:savedDialogs";
    private static final String SAVED_DIALOG_KEY_PREFIX = "android:dialog_";
    private static final String SAVED_DIALOG_ARGS_KEY_PREFIX = "android:dialog_args_";
    private static final String HAS_CURENT_PERMISSIONS_REQUEST_KEY =
            "android:hasCurrentPermissionsRequest";

    private static final String REQUEST_PERMISSIONS_WHO_PREFIX = "@android:requestPermissions:";

    private static final String KEYBOARD_SHORTCUTS_RECEIVER_PKG_NAME = "com.android.systemui";
    private static final String KEYBOARD_SHORTCUTS_RECEIVER_CLASS_NAME =
            "com.android.systemui.statusbar.KeyboardShortcutsReceiver";

    private static class ManagedDialog {
        Dialog mDialog;
        Bundle mArgs;
    }
    private SparseArray<ManagedDialog> mManagedDialogs;

    // set by the thread after the constructor and before onCreate(Bundle savedInstanceState) is called.
    private Instrumentation mInstrumentation;
    private IBinder mToken;
    private int mIdent;
    /*package*/ String mEmbeddedID;
    private Application mApplication;
    /*package*/ Intent mIntent;
    /*package*/ String mReferrer;
    private ComponentName mComponent;
    /*package*/ ActivityInfo mActivityInfo;
    /*package*/ ActivityThread mMainThread;
    Activity mParent;
    boolean mCalled;
    /*package*/ boolean mResumed;
    /*package*/ boolean mStopped;
    boolean mFinished;
    boolean mStartedActivity;
    private boolean mDestroyed;
    private boolean mDoReportFullyDrawn = true;
    /** true if the activity is going through a transient pause */
    /*package*/ boolean mTemporaryPause = false;
    /** true if the activity is being destroyed in order to recreate it with a new configuration */
    /*package*/ boolean mChangingConfigurations = false;
    /*package*/ int mConfigChangeFlags;
    /*package*/ Configuration mCurrentConfig;
    private SearchManager mSearchManager;
    private MenuInflater mMenuInflater;

    static final class NonConfigurationInstances {
        Object activity;
        HashMap<String, Object> children;
        FragmentManagerNonConfig fragments;
        ArrayMap<String, LoaderManager> loaders;
        VoiceInteractor voiceInteractor;
    }
    /* package */ NonConfigurationInstances mLastNonConfigurationInstances;

    private Window mWindow;

    private WindowManager mWindowManager;
    /*package*/ View mDecor = null;
    /*package*/ boolean mWindowAdded = false;
    /*package*/ boolean mVisibleFromServer = false;
    /*package*/ boolean mVisibleFromClient = true;
    /*package*/ ActionBar mActionBar = null;
    private boolean mEnableDefaultActionBarUp;

    private VoiceInteractor mVoiceInteractor;

    private CharSequence mTitle;
    private int mTitleColor = 0;

    // we must have a handler before the FragmentController is constructed
    final Handler mHandler = new Handler();
    final FragmentController mFragments = FragmentController.createController(new HostCallbacks());

    // Most recent call to requestVisibleBehind().
    boolean mVisibleBehind;

    private static final class ManagedCursor {
        ManagedCursor(Cursor cursor) {
            mCursor = cursor;
            mReleased = false;
            mUpdated = false;
        }

        private final Cursor mCursor;
        private boolean mReleased;
        private boolean mUpdated;
    }
    private final ArrayList<ManagedCursor> mManagedCursors =
        new ArrayList<ManagedCursor>();

    // protected by synchronized (this)
    int mResultCode = RESULT_CANCELED;
    Intent mResultData = null;

    private TranslucentConversionListener mTranslucentCallback;
    private boolean mChangeCanvasToTranslucent;

    private SearchEvent mSearchEvent;

    private boolean mTitleReady = false;
    private int mActionModeTypeStarting = ActionMode.TYPE_PRIMARY;

    private int mDefaultKeyMode = DEFAULT_KEYS_DISABLE;
    private SpannableStringBuilder mDefaultKeySsb = null;

    private ActivityManager.TaskDescription mTaskDescription =
            new ActivityManager.TaskDescription();

    protected static final int[] FOCUSED_STATE_SET = {com.android.internal.R.attr.state_focused};

    @SuppressWarnings("unused")
    private final Object mInstanceTracker = StrictMode.trackActivity(this);

    private Thread mUiThread;

    ActivityTransitionState mActivityTransitionState = new ActivityTransitionState();
    SharedElementCallback mEnterTransitionListener = SharedElementCallback.NULL_CALLBACK;
    SharedElementCallback mExitTransitionListener = SharedElementCallback.NULL_CALLBACK;

    private boolean mHasCurrentPermissionsRequest;
    private boolean mEatKeyUpEvent;

    private static native String getDlWarning();

    /** Return the intent that started this activity. */
    public Intent getIntent() {
        return mIntent;
    }

    /**
     * Change the intent returned by {@link #getIntent}.  This holds a
     * reference to the given intent; it does not copy it.  Often used in
     * conjunction with {@link #onNewIntent}.
     *
     * @param newIntent The new Intent object to return from getIntent
     *
     * @see #getIntent
     * @see #onNewIntent
     */
    public void setIntent(Intent newIntent) {
        mIntent = newIntent;
    }

    /** Return the application that owns this activity. */
    public final Application getApplication() {
        return mApplication;
    }

    /** Is this activity embedded inside of another activity? */
    public final boolean isChild() {
        return mParent != null;
    }

    /** Return the parent activity if this view is an embedded child. */
    public final Activity getParent() {
        return mParent;
    }

    /** Retrieve the window manager for showing custom windows. */
    public WindowManager getWindowManager() {
        return mWindowManager;
    }
    ......
}

Application

public class Application extends ContextWrapper implements ComponentCallbacks2 {
    private ArrayList<ComponentCallbacks> mComponentCallbacks =
            new ArrayList<ComponentCallbacks>();
    private ArrayList<ActivityLifecycleCallbacks> mActivityLifecycleCallbacks =
            new ArrayList<ActivityLifecycleCallbacks>();
    private ArrayList<OnProvideAssistDataListener> mAssistCallbacks = null;
	......

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

    @CallSuper
    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);
            }
        }
    }
	......
}

Service

public abstract class Service extends ContextWrapper implements ComponentCallbacks2 {
    private static final String TAG = "Service";
	......
	  public void onDestroy() {
    }

    public void onConfigurationChanged(Configuration newConfig) {
    }
    
    public void onLowMemory() {
    }

    public void onTrimMemory(int level) {
    }

	......
}

结尾附上Context的应用场景

tables	Application	Activity	Service	ContentProvider	BroadcastReceiver
显示Dialog	NO	YES	NO	NO	NO
启动Activity	NO1	YES	NO1	NO1	NO1
Layout Inflation	NO2	YES	NO2	NO2	NO2
启动Service	YES	YES	YES	YES	YES
绑定到Service	YES	YES	YES	YES	NO
发送Broadcast	YES	YES	YES	YES	YES
注册BroadcastReceiver	YES	YES	YES	YES	NO3
加载Resource	YES	YES	YES	YES	YES

注：
NO1 表示Application context的确可以开始一个Activity，但是它需要创建一个新的task。这可能会满足一些特定的需求，但是在你的应用中会创建一个不标准的回退栈（back stack），这通常是不推荐的或者不是是好的实践。
NO2 表示这是非法的，但是这个填充（inflation）的确可以完成，但是是使用所运行的系统默认的主题（theme），而不是你app定义的主题。
NO3 在Android4.2以上，如果Receiver是null的话（这是用来获取一个sticky broadcast的当前 值的），这是允许的。