【Android 内核研究】理解Context

写在前面的话

非常感谢柯元旦所赠的《Android内核剖析》一书。通过对本书的学习，让我对Android内核有了更深一层次的理解。本文是《Android内核剖析》的学习笔记。

Context是什么

一个Context意味着一个场景，一个场景就是用户和操作系统交互的一个过程。在广义上，这个所谓的过程应该包括前台界面和后台数据。

举个例子，比如当你打电话的时候，场景包括电话程序对应的界面以及隐藏在界面后的数据。

从程序的角度来看，一个Activity就是一个Context，一个Service也是一个Context。

从语义的角度来看一下Context。谷歌程序员把“场景”抽象为Context类，他们认为用户和操作系统的每一次交互都是一个场景，比如打电话，发短信。

从代码的角度来看，Activity类基于Context，而Service类也基于Context类。值得一提的是，Activity除了基于Context类外，还实现了一些其他重要接口。

从设计的角度来看，interface仅仅是某些功能的标记，而extends才是类的本质和实现。

Context相关类的继承关系

相关类介绍

Context

/frameworks/base/core/java/android/content/Context.java

抽象类，提供了一组通用的API。

/**
 * Interface to global information about an application environment.  This is
 * an abstract class whose implementation is provided by
 * the Android system.  It
 * allows access to application-specific resources and classes, as well as
 * up-calls for application-level operations such as launching activities,
 * broadcasting and receiving intents, etc.
 */
public abstract class Context { ... }

ContextIml.java

/frameworks/base/core/java/android/app/ContextImpl.java

ContextImpl是Context的具体实现类，该类实现了Context类的所有功能。注意，该函数的大部分功能都是直接调用其成员变量mPackageInfo去完成，它是一个delegate。

/**
 * Common implementation of Context API, which provides the base
 * context object for Activity and other application components.
 */
class ContextImpl extends Context {
    private final static String TAG = "ApplicationContext";
    private final static boolean DEBUG = false;
    private final static boolean DEBUG_ICONS = false;

    private static final Object sSync = new Object();
    private static AlarmManager sAlarmManager;
    private static PowerManager sPowerManager;
    private static ConnectivityManager sConnectivityManager;
    private static ThrottleManager sThrottleManager;
    private static WifiManager sWifiManager;
    private static LocationManager sLocationManager;
    private static final HashMap<String, SharedPreferencesImpl> sSharedPrefs =
            new HashMap<String, SharedPreferencesImpl>();

    private AudioManager mAudioManager;
    /*package*/ LoadedApk mPackageInfo;
    private Resources mResources;
    /*package*/ ActivityThread mMainThread;
    private Context mOuterContext;
    private IBinder mActivityToken = null;
    private ApplicationContentResolver mContentResolver;
    private int mThemeResource = 0;
    private Resources.Theme mTheme = null;
    private PackageManager mPackageManager;
    private NotificationManager mNotificationManager = null;
    private ActivityManager mActivityManager = null;
    private WallpaperManager mWallpaperManager = null;
    private Context mReceiverRestrictedContext = null;
    private SearchManager mSearchManager = null;
    private SensorManager mSensorManager = null;
    private StorageManager mStorageManager = null;
    private Vibrator mVibrator = null;
    private LayoutInflater mLayoutInflater = null;
    private StatusBarManager mStatusBarManager = null;
    private TelephonyManager mTelephonyManager = null;
    private ClipboardManager mClipboardManager = null;
    private boolean mRestricted;
    private AccountManager mAccountManager; // protected by mSync
    private DropBoxManager mDropBoxManager = null;
    private DevicePolicyManager mDevicePolicyManager = null;
    private UiModeManager mUiModeManager = null;
    private DownloadManager mDownloadManager = null;
    private NfcManager mNfcManager = null;

    private final Object mSync = new Object();

    private File mDatabasesDir;
    private File mPreferencesDir;
    private File mFilesDir;
    private File mCacheDir;
    private File mExternalFilesDir;
    private File mExternalCacheDir;

    private static long sInstanceCount = 0;

    private static final String[] EMPTY_FILE_LIST = {};
    
    ...
}

ContextWrapper类

\frameworks\base\core\java\android\content\ContextWrapper.java

该类只是对Context类的一种包装，该类的构造函数包含了一个真正的Context引用，即ContextImpl对象。

/**
 * Proxying implementation of Context that simply delegates all of its calls to
 * another Context.  Can be subclassed to modify behavior without changing
 * the original Context.
 */
public class ContextWrapper extends Context {
    Context mBase;

    public ContextWrapper(Context base) {
        mBase = base;
    }
    
    /**
     * Set the base context for this ContextWrapper.  All calls will then be
     * delegated to the base context.  Throws
     * IllegalStateException if a base context has already been set.
     * 
     * @param base The new base context for this wrapper.
     */
    protected void attachBaseContext(Context base) {
        if (mBase != null) {
            throw new IllegalStateException("Base context already set");
        }
        mBase = base;
    }

    /**
     * @return the base context as set by the constructor or setBaseContext
     */
    public Context getBaseContext() {
        return mBase;
    }

    @Override
    public AssetManager getAssets() {
        return mBase.getAssets();
    }

    @Override
    public Resources getResources()
    {
        return mBase.getResources();
    }

    @Override
    public PackageManager getPackageManager() {
        return mBase.getPackageManager();
    }

    @Override
    public ContentResolver getContentResolver() {
        return mBase.getContentResolver();
    }

    @Override
    public Looper getMainLooper() {
        return mBase.getMainLooper();
    }
    
    @Override
    public Context getApplicationContext() {
        return mBase.getApplicationContext();
    }
    
    @Override
    public void setTheme(int resid) {
        mBase.setTheme(resid);
    }
    ...
}

ContextThemeWrapper类

/frameworks/base/core/java/android/view/ContextThemeWrapper.java

该类内部包含了主题(Theme)相关的接口，即android:theme属性指定的。只有Activity需要主题，而Service不需要主题，所以Service直接继承自ContextWrapper类。

/**
 * A ContextWrapper that allows you to modify the theme from what is in the 
 * wrapped context. 
 */
public class ContextThemeWrapper extends ContextWrapper {
    private Context mBase;
    private int mThemeResource;
    private Resources.Theme mTheme;
    private LayoutInflater mInflater;

    public ContextThemeWrapper() {
        super(null);
    }
    
    public ContextThemeWrapper(Context base, int themeres) {
        super(base);
        mBase = base;
        mThemeResource = themeres;
    }

    @Override protected void attachBaseContext(Context newBase) {
        super.attachBaseContext(newBase);
        mBase = newBase;
    }
    
    @Override public void setTheme(int resid) {
        mThemeResource = resid;
        initializeTheme();
    }
    
    @Override public Resources.Theme getTheme() {
        if (mTheme != null) {
            return mTheme;
        }

        if (mThemeResource == 0) {
            mThemeResource = com.android.internal.R.style.Theme;
        }
        initializeTheme();

        return mTheme;
    }

    @Override public Object getSystemService(String name) {
        if (LAYOUT_INFLATER_SERVICE.equals(name)) {
            if (mInflater == null) {
                mInflater = LayoutInflater.from(mBase).cloneInContext(this);
            }
            return mInflater;
        }
        return mBase.getSystemService(name);
    }
    
    /**
     * Called by {@link #setTheme} and {@link #getTheme} to apply a theme
     * resource to the current Theme object.  Can override to change the
     * default (simple) behavior.  This method will not be called in multiple
     * threads simultaneously.
     *
     * @param theme The Theme object being modified.
     * @param resid The theme style resource being applied to <var>theme</var>.
     * @param first Set to true if this is the first time a style is being
     *              applied to <var>theme</var>.
     */
    protected void onApplyThemeResource(Resources.Theme theme, int resid, boolean first) {
        theme.applyStyle(resid, true);
    }

    private void initializeTheme() {
        final boolean first = mTheme == null;
        if (first) {
            mTheme = getResources().newTheme();
            Resources.Theme theme = mBase.getTheme();
            if (theme != null) {
                mTheme.setTo(theme);
            }
        }
        onApplyThemeResource(mTheme, mThemeResource, first);
    }
}

何时创建Context 

应用程序创建Context实例有如下几种情况：

• 创建Application对象的时候， 而且整个App共一个Application对象
• 创建Service对象的时候
• 创建Activity对象的时候

1. 创建Application对象的时机

每个应用程序在第一次启动时，都会首先创建Application对象。如果对应用程序启动一个Activity(startActivity)流程比较清楚的话，创建Application的时机在创建handleBindApplication()方法中，该函数位于 ActivityThread.java类中，代码如下：

2. 创建Activity对象的时机

通过startActivity()或startActivityForResult()请求启动一个Activity时，如果系统检测需要新建一个Activity对象时，就会回调handleLaunchActivity()方法，该方法继而调用performLaunchActivity()方法，去创建一个Activity实例，并且回调onCreate()，onStart()方法等， 函数都位于 ActivityThread.java类，代码如下：

3. 创建Service对象的时机

通过startService或者bindService时，如果系统检测到需要新创建一个Service实例，就会回调handleCreateService()方法，完成相关数据操作。handleCreateService()函数位于 ActivityThread.java类，代码如下：

另外，需要注意的是，通过对ContextImp的分析可知，其方法的大多数操作都是直接调用其属性mPackageInfo(该属性类型为PackageInfo)的相关方法而来。这说明ContextImpl是一个轻量级的类，而PackageInfo才是真正重量级的类。而一个App里的所有ContextIml实例，都对应同一个packageInfo对象。

参考资料

http://blog.csdn.net/qinjuning/article/details/7310620