为什么ContextThemeWrapper覆盖在ContextWrapper中定义的字段“mBase”？

Question

我是java的初学者。现在我正在学习android开发，并对 android.view.ContextThemeWrapper 的类定义感到困惑。它是 android.content.ContextWrapper 的子类。这两个类都包含一个字段mBase（和android.content.Context的实例）。在基类中，它声明没有访问修饰符，但在子类中它使用private修饰符声明。

因此，android.view.ContextThemeWrapper的一个实例确实有两个名为“mBase”的字段，只有派生部分中定义的字段是可见的，但是在基类部分中声明的那个字段是隐藏的。为什么在这里派生类的实例中需要两个“mBase”字段？在哪种情况下，我应该将基类中定义的字段覆盖为private？

我需要你的帮助。提前谢谢。

以下是两个类的定义：

android.content.ContextWrapper：

 /**
 * 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);
    }

    /** @hide */
    @Override
    public int getThemeResId() {
        return mBase.getThemeResId();
    }

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

    @Override
    public ClassLoader getClassLoader() {
        return mBase.getClassLoader();
    }

    @Override
    public String getPackageName() {
        return mBase.getPackageName();
    }

    @Override
    public ApplicationInfo getApplicationInfo() {
        return mBase.getApplicationInfo();
    }

    @Override
    public String getPackageResourcePath() {
        return mBase.getPackageResourcePath();
    }

    @Override
    public String getPackageCodePath() {
        return mBase.getPackageCodePath();
    }

    /** @hide */
    @Override
    public File getSharedPrefsFile(String name) {
        return mBase.getSharedPrefsFile(name);
    }

    @Override
    public SharedPreferences getSharedPreferences(String name, int mode) {
        return mBase.getSharedPreferences(name, mode);
    }

    @Override
    public FileInputStream openFileInput(String name)
        throws FileNotFoundException {
        return mBase.openFileInput(name);
    }

    @Override
    public FileOutputStream openFileOutput(String name, int mode)
        throws FileNotFoundException {
        return mBase.openFileOutput(name, mode);
    }

    @Override
    public boolean deleteFile(String name) {
        return mBase.deleteFile(name);
    }

    @Override
    public File getFileStreamPath(String name) {
        return mBase.getFileStreamPath(name);
    }

    @Override
    public String[] fileList() {
        return mBase.fileList();
    }

    @Override
    public File getFilesDir() {
        return mBase.getFilesDir();
    }

    @Override
    public File getExternalFilesDir(String type) {
        return mBase.getExternalFilesDir(type);
    }

    @Override
    public File getObbDir() {
        return mBase.getObbDir();
    }

    @Override
    public File getCacheDir() {
        return mBase.getCacheDir();
    }

    @Override
    public File getExternalCacheDir() {
        return mBase.getExternalCacheDir();
    }

    @Override
    public File getDir(String name, int mode) {
        return mBase.getDir(name, mode);
    }

    @Override
    public SQLiteDatabase openOrCreateDatabase(String name, int mode, CursorFactory factory) {
        return mBase.openOrCreateDatabase(name, mode, factory);
    }

    @Override
    public SQLiteDatabase openOrCreateDatabase(String name, int mode, CursorFactory factory,
            DatabaseErrorHandler errorHandler) {
        return mBase.openOrCreateDatabase(name, mode, factory, errorHandler);
    }

    @Override
    public boolean deleteDatabase(String name) {
        return mBase.deleteDatabase(name);
    }

    @Override
    public File getDatabasePath(String name) {
        return mBase.getDatabasePath(name);
    }

    @Override
    public String[] databaseList() {
        return mBase.databaseList();
    }

    @Override
    public Drawable getWallpaper() {
        return mBase.getWallpaper();
    }

    @Override
    public Drawable peekWallpaper() {
        return mBase.peekWallpaper();
    }

    @Override
    public int getWallpaperDesiredMinimumWidth() {
        return mBase.getWallpaperDesiredMinimumWidth();
    }

    @Override
    public int getWallpaperDesiredMinimumHeight() {
        return mBase.getWallpaperDesiredMinimumHeight();
    }

    @Override
    public void setWallpaper(Bitmap bitmap) throws IOException {
        mBase.setWallpaper(bitmap);
    }

    @Override
    public void setWallpaper(InputStream data) throws IOException {
        mBase.setWallpaper(data);
    }

    @Override
    public void clearWallpaper() throws IOException {
        mBase.clearWallpaper();
    }

    @Override
    public void startActivity(Intent intent) {
        mBase.startActivity(intent);
    }

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

    @Override
    public void startActivity(Intent intent, Bundle options) {
        mBase.startActivity(intent, options);
    }

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

    @Override
    public void startActivities(Intent[] intents) {
        mBase.startActivities(intents);
    }

    @Override
    public void startActivities(Intent[] intents, Bundle options) {
        mBase.startActivities(intents, options);
    }

    /** @hide */
    @Override
    public void startActivitiesAsUser(Intent[] intents, Bundle options, UserHandle userHandle) {
        mBase.startActivitiesAsUser(intents, options, userHandle);
    }

    @Override
    public void startIntentSender(IntentSender intent,
            Intent fillInIntent, int flagsMask, int flagsValues, int extraFlags)
            throws IntentSender.SendIntentException {
        mBase.startIntentSender(intent, fillInIntent, flagsMask,
                flagsValues, extraFlags);
    }

    @Override
    public void startIntentSender(IntentSender intent,
            Intent fillInIntent, int flagsMask, int flagsValues, int extraFlags,
            Bundle options) throws IntentSender.SendIntentException {
        mBase.startIntentSender(intent, fillInIntent, flagsMask,
                flagsValues, extraFlags, options);
    }

    @Override
    public void sendBroadcast(Intent intent) {
        mBase.sendBroadcast(intent);
    }

    @Override
    public void sendBroadcast(Intent intent, String receiverPermission) {
        mBase.sendBroadcast(intent, receiverPermission);
    }

    @Override
    public void sendOrderedBroadcast(Intent intent,
            String receiverPermission) {
        mBase.sendOrderedBroadcast(intent, receiverPermission);
    }

    @Override
    public void sendOrderedBroadcast(
        Intent intent, String receiverPermission, BroadcastReceiver resultReceiver,
        Handler scheduler, int initialCode, String initialData,
        Bundle initialExtras) {
        mBase.sendOrderedBroadcast(intent, receiverPermission,
                resultReceiver, scheduler, initialCode,
                initialData, initialExtras);
    }

    @Override
    public void sendBroadcastAsUser(Intent intent, UserHandle user) {
        mBase.sendBroadcastAsUser(intent, user);
    }

    @Override
    public void sendBroadcastAsUser(Intent intent, UserHandle user,
            String receiverPermission) {
        mBase.sendBroadcastAsUser(intent, user, receiverPermission);
    }

    @Override
    public void sendOrderedBroadcastAsUser(Intent intent, UserHandle user,
            String receiverPermission, BroadcastReceiver resultReceiver, Handler scheduler,
            int initialCode, String initialData, Bundle initialExtras) {
        mBase.sendOrderedBroadcastAsUser(intent, user, receiverPermission, resultReceiver,
                scheduler, initialCode, initialData, initialExtras);
    }

    @Override
    public void sendStickyBroadcast(Intent intent) {
        mBase.sendStickyBroadcast(intent);
    }

    @Override
    public void sendStickyOrderedBroadcast(
        Intent intent, BroadcastReceiver resultReceiver,
        Handler scheduler, int initialCode, String initialData,
        Bundle initialExtras) {
        mBase.sendStickyOrderedBroadcast(intent,
                resultReceiver, scheduler, initialCode,
                initialData, initialExtras);
    }

    @Override
    public void removeStickyBroadcast(Intent intent) {
        mBase.removeStickyBroadcast(intent);
    }

    @Override
    public void sendStickyBroadcastAsUser(Intent intent, UserHandle user) {
        mBase.sendStickyBroadcastAsUser(intent, user);
    }

    @Override
    public void sendStickyOrderedBroadcastAsUser(Intent intent,
            UserHandle user, BroadcastReceiver resultReceiver,
            Handler scheduler, int initialCode, String initialData,
            Bundle initialExtras) {
        mBase.sendStickyOrderedBroadcastAsUser(intent, user, resultReceiver,
                scheduler, initialCode, initialData, initialExtras);
    }

    @Override
    public void removeStickyBroadcastAsUser(Intent intent, UserHandle user) {
        mBase.removeStickyBroadcastAsUser(intent, user);
    }

    @Override
    public Intent registerReceiver(
        BroadcastReceiver receiver, IntentFilter filter) {
        return mBase.registerReceiver(receiver, filter);
    }

    @Override
    public Intent registerReceiver(
        BroadcastReceiver receiver, IntentFilter filter,
        String broadcastPermission, Handler scheduler) {
        return mBase.registerReceiver(receiver, filter, broadcastPermission,
                scheduler);
    }

    /** @hide */
    @Override
    public Intent registerReceiverAsUser(
        BroadcastReceiver receiver, UserHandle user, IntentFilter filter,
        String broadcastPermission, Handler scheduler) {
        return mBase.registerReceiverAsUser(receiver, user, filter, broadcastPermission,
                scheduler);
    }

    @Override
    public void unregisterReceiver(BroadcastReceiver receiver) {
        mBase.unregisterReceiver(receiver);
    }

    @Override
    public ComponentName startService(Intent service) {
        return mBase.startService(service);
    }

    @Override
    public boolean stopService(Intent name) {
        return mBase.stopService(name);
    }

    /** @hide */
    @Override
    public ComponentName startServiceAsUser(Intent service, UserHandle user) {
        return mBase.startServiceAsUser(service, user);
    }

    /** @hide */
    @Override
    public boolean stopServiceAsUser(Intent name, UserHandle user) {
        return mBase.stopServiceAsUser(name, user);
    }

    @Override
    public boolean bindService(Intent service, ServiceConnection conn,
            int flags) {
        return mBase.bindService(service, conn, flags);
    }

    /** @hide */
    @Override
    public boolean bindService(Intent service, ServiceConnection conn, int flags, int userHandle) {
        return mBase.bindService(service, conn, flags, userHandle);
    }

    @Override
    public void unbindService(ServiceConnection conn) {
        mBase.unbindService(conn);
    }

    @Override
    public boolean startInstrumentation(ComponentName className,
            String profileFile, Bundle arguments) {
        return mBase.startInstrumentation(className, profileFile, arguments);
    }

    @Override
    public Object getSystemService(String name) {
        return mBase.getSystemService(name);
    }

    @Override
    public int checkPermission(String permission, int pid, int uid) {
        return mBase.checkPermission(permission, pid, uid);
    }

    @Override
    public int checkCallingPermission(String permission) {
        return mBase.checkCallingPermission(permission);
    }

    @Override
    public int checkCallingOrSelfPermission(String permission) {
        return mBase.checkCallingOrSelfPermission(permission);
    }

    @Override
    public void enforcePermission(
            String permission, int pid, int uid, String message) {
        mBase.enforcePermission(permission, pid, uid, message);
    }

    @Override
    public void enforceCallingPermission(String permission, String message) {
        mBase.enforceCallingPermission(permission, message);
    }

    @Override
    public void enforceCallingOrSelfPermission(
            String permission, String message) {
        mBase.enforceCallingOrSelfPermission(permission, message);
    }

    @Override
    public void grantUriPermission(String toPackage, Uri uri, int modeFlags) {
        mBase.grantUriPermission(toPackage, uri, modeFlags);
    }

    @Override
    public void revokeUriPermission(Uri uri, int modeFlags) {
        mBase.revokeUriPermission(uri, modeFlags);
    }

    @Override
    public int checkUriPermission(Uri uri, int pid, int uid, int modeFlags) {
        return mBase.checkUriPermission(uri, pid, uid, modeFlags);
    }

    @Override
    public int checkCallingUriPermission(Uri uri, int modeFlags) {
        return mBase.checkCallingUriPermission(uri, modeFlags);
    }

    @Override
    public int checkCallingOrSelfUriPermission(Uri uri, int modeFlags) {
        return mBase.checkCallingOrSelfUriPermission(uri, modeFlags);
    }

    @Override
    public int checkUriPermission(Uri uri, String readPermission,
            String writePermission, int pid, int uid, int modeFlags) {
        return mBase.checkUriPermission(uri, readPermission, writePermission,
                pid, uid, modeFlags);
    }

    @Override
    public void enforceUriPermission(
            Uri uri, int pid, int uid, int modeFlags, String message) {
        mBase.enforceUriPermission(uri, pid, uid, modeFlags, message);
    }

    @Override
    public void enforceCallingUriPermission(
            Uri uri, int modeFlags, String message) {
        mBase.enforceCallingUriPermission(uri, modeFlags, message);
    }

    @Override
    public void enforceCallingOrSelfUriPermission(
            Uri uri, int modeFlags, String message) {
        mBase.enforceCallingOrSelfUriPermission(uri, modeFlags, message);
    }

    @Override
    public void enforceUriPermission(
            Uri uri, String readPermission, String writePermission,
            int pid, int uid, int modeFlags, String message) {
        mBase.enforceUriPermission(
                uri, readPermission, writePermission, pid, uid, modeFlags,
                message);
    }

    @Override
    public Context createPackageContext(String packageName, int flags)
        throws PackageManager.NameNotFoundException {
        return mBase.createPackageContext(packageName, flags);
    }

    /** @hide */
    @Override
    public Context createPackageContextAsUser(String packageName, int flags, UserHandle user)
            throws PackageManager.NameNotFoundException {
        return mBase.createPackageContextAsUser(packageName, flags, user);
    }

    @Override
    public Context createConfigurationContext(Configuration overrideConfiguration) {
        return mBase.createConfigurationContext(overrideConfiguration);
    }

    @Override
    public Context createDisplayContext(Display display) {
        return mBase.createDisplayContext(display);
    }

    @Override
    public boolean isRestricted() {
        return mBase.isRestricted();
    }

    /** @hide */
    @Override
    public CompatibilityInfoHolder getCompatibilityInfo(int displayId) {
        return mBase.getCompatibilityInfo(displayId);
    }
}

android.view.ContextThemeWrapper：

/**
 * 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;
    private Configuration mOverrideConfiguration;
    private Resources mResources;

    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;
    }

    /**
     * Call to set an "override configuration" on this context -- this is
     * a configuration that replies one or more values of the standard
     * configuration that is applied to the context.  See
     * {@link Context#createConfigurationContext(Configuration)} for more
     * information.
     *
     * <p>This method can only be called once, and must be called before any
     * calls to {@link #getResources()} are made.
     */
    public void applyOverrideConfiguration(Configuration overrideConfiguration) {
        if (mResources != null) {
            throw new IllegalStateException("getResources() has already been called");
        }
        if (mOverrideConfiguration != null) {
            throw new IllegalStateException("Override configuration has already been set");
        }
        mOverrideConfiguration = new Configuration(overrideConfiguration);
    }

    @Override
    public Resources getResources() {
        if (mResources != null) {
            return mResources;
        }
        if (mOverrideConfiguration == null) {
            mResources = super.getResources();
            return mResources;
        } else {
            Context resc = createConfigurationContext(mOverrideConfiguration);
            mResources = resc.getResources();
            return mResources;
        }
    }

    @Override public void setTheme(int resid) {
        mThemeResource = resid;
        initializeTheme();
    }

    /** @hide */
    @Override
    public int getThemeResId() {
        return mThemeResource;
    }

    @Override public Resources.Theme getTheme() {
        if (mTheme != null) {
            return mTheme;
        }

        mThemeResource = Resources.selectDefaultTheme(mThemeResource,
                getApplicationInfo().targetSdkVersion);
        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);
    }
}

Answer 1

  

为什么在派生的实例中需要两个“mBase”字段的副本   这里上课？

不需要其中两个，因为super.mBase中从不使用ContextThemeWrapper。所以我认为这些字段具有相同的名称只是巧合。

  

在这种情况下，我应该覆盖基类中定义的字段   私人？

您无法覆盖字段。子类中的字段与超类中的字段不同，即使它们都具有相同的名称。我认为mBase中的ContextThemeWrapper是私有的，因为它永远不应该从类外部更改（因此没有getter和setter）。

---

修改

  

但是这两个类中的mBase引用了Context的相同实例。如果   ContextThemeWrapper不希望客户端更改mBase字段，   从ConextWrapper继承它就足够了。为什么   ContextThemeWrapper包含Context实例的另一个引用   在ContextWrapper中？

是的，我之前没有看到这个。这很有意思，但让我们看一下文档：

  

“ContextThemeWrapper是一个允许你修改的ContextWrapper   来自包装环境中的主题。“

和

  

“ContextWrapper简单地是Context的代理实现   将其所有调用委托给另一个Context。

所以ContextWrapper例如由Service类扩展，并且ContextThemeWrapper通过例如Activity类扩展（以及更多......）。

ContextThemeWrapper可以修改Context ...所以当创建这个类的对象时，它会用它自己的mBase（在构造函数中传递）替换超类中的mBase。所以两个mBase都有相同的价值。它就是这样做的，因为ContextThemeWrapper可以修改传递给它的Context。当在ContextThemeWrapper中修改了上下文时，它也会在ContextWrapper中更新，因此ContextWrapper所有子类都可以访问此修改后的Context。

对不起凌乱的解释，但我现在没有太多时间，我希望你现在明白了：）