Context

public abstract class Context {

之间的继承关系：

Context 作用

//可以点击studio旁边的 Structure 结构示意图

/**
* 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 {
    
    // 四大组件相关
    public abstract void startActivity(@RequiresPermission Intent intent);
    public abstract void sendBroadcast(@RequiresPermission Intent intent);
    public abstract Intent registerReceiver(@Nullable BroadcastReceiver receiver,
                                            IntentFilter filter);
    public abstract void unregisterReceiver(BroadcastReceiver receiver);
    public abstract ComponentName startService(Intent service);
    public abstract boolean stopService(Intent service);
    public abstract boolean bindService(@RequiresPermission Intent service,
            @NonNull ServiceConnection conn, @BindServiceFlags int flags);
    public abstract void unbindService(@NonNull ServiceConnection conn);
    public abstract ContentResolver getContentResolver();
    
    // 获取系统/应用资源
    public abstract AssetManager getAssets();
    public abstract Resources getResources();
    public abstract PackageManager getPackageManager();
    public abstract Context getApplicationContext();
    public abstract ClassLoader getClassLoader();
    public final @Nullable <T> T getSystemService(@NonNull Class<T> serviceClass) { ... }
    
    public final String getString(@StringRes int resId) { ... }
    public final int getColor(@ColorRes int id) { ... }
    public final Drawable getDrawable(@DrawableRes int id) { ... }
    public abstract Resources.Theme getTheme();
    public abstract void setTheme(@StyleRes int resid);
    public final TypedArray obtainStyledAttributes(@StyleableRes int[] attrs) { ... }
    
    // 获取应用相关信息
    public abstract ApplicationInfo getApplicationInfo();
    public abstract String getPackageName();
    public abstract Looper getMainLooper();
    public abstract int checkPermission(@NonNull String permission, int pid, int uid);
    
    // 文件相关
    public abstract File getSharedPreferencesPath(String name);
    public abstract File getDataDir();
    public abstract boolean deleteFile(String name);
    public abstract File getExternalFilesDir(@Nullable String type);
    public abstract File getCacheDir();
    ...
    public abstract SharedPreferences getSharedPreferences(String name, @PreferencesMode int mode);
    public abstract boolean deleteSharedPreferences(String name);
    
    // 数据库相关
    public abstract SQLiteDatabase openOrCreateDatabase(...);
    public abstract boolean deleteDatabase(String name);
    public abstract File getDatabasePath(String name);
    ...
    
    // 其它
    public void registerComponentCallbacks(ComponentCallbacks callback) { ... }
    public void unregisterComponentCallbacks(ComponentCallbacks callback) { ... }
    ...
}

public interface ComponentCallbacks {
    void onConfigurationChanged(Configuration newConfig);
    void onLowMemory();
}

Context 就相当于 Application 的大管家，主要负责：

• 四大组件，启动 Activity , 广播，服务等
• 获取系统资源
• 文件操作相关
• 数据库操作相关

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;
    }
 }

ContextWrapper 实际上就是 Context 的代理类，所有的操作都是 mBase 完成。另外，Activity, Service 的 getBaseContext 返回的就是这个 mBase.

ContextThemeWrapper

/**
 * A context wrapper that allows you to modify or replace the theme of the
 * wrapped context.
 */
public class ContextThemeWrapper extends ContextWrapper {
    private int mThemeResource;
    private Resources.Theme mTheme;
    private LayoutInflater mInflater;
    private Configuration mOverrideConfiguration;
    private Resources mResources;

    /**
     * Creates a new context wrapper with no theme and no base context.
     * <p class="note">
     * <strong>Note:</strong> A base context <strong>must</strong> be attached
     * using {@link #attachBaseContext(Context)} before calling any other
     * method on the newly constructed context wrapper.
     */
    public ContextThemeWrapper() {
        super(null);
    }

    /**
     * Creates a new context wrapper with the specified theme.
     * <p>
     * The specified theme will be applied on top of the base context's theme.
     * Any attributes not explicitly defined in the theme identified by
     * <var>themeResId</var> will retain their original values.
     *
     * @param base the base context
     * @param themeResId the resource ID of the theme to be applied on top of
     *                   the base context's theme
     */
    public ContextThemeWrapper(Context base, @StyleRes int themeResId) {
        super(base);
        mThemeResource = themeResId;
    }

    /**
     * Creates a new context wrapper with the specified theme.
     * <p>
     * Unlike {@link #ContextThemeWrapper(Context, int)}, the theme passed to
     * this constructor will completely replace the base context's theme.
     *
     * @param base the base context
     * @param theme the theme against which resources should be inflated
     */
    public ContextThemeWrapper(Context base, Resources.Theme theme) {
        super(base);
        mTheme = theme;
    }

    @Override
    protected void attachBaseContext(Context newBase) {
        super.attachBaseContext(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()} or {@link #getAssets()} are made.
     */
    public void applyOverrideConfiguration(Configuration overrideConfiguration) {
        if (mResources != null) {
            throw new IllegalStateException(
                    "getResources() or getAssets() has already been called");
        }
        if (mOverrideConfiguration != null) {
            throw new IllegalStateException("Override configuration has already been set");
        }
        mOverrideConfiguration = new Configuration(overrideConfiguration);
    }

    /**
     * Used by ActivityThread to apply the overridden configuration to onConfigurationChange
     * callbacks.
     * @hide
     */
    public Configuration getOverrideConfiguration() {
        return mOverrideConfiguration;
    }

    @Override
    public AssetManager getAssets() {
        // Ensure we're returning assets with the correct configuration.
        return getResourcesInternal().getAssets();
    }

    @Override
    public Resources getResources() {
        return getResourcesInternal();
    }

    private Resources getResourcesInternal() {
        if (mResources == null) {
            if (mOverrideConfiguration == null) {
                mResources = super.getResources();
            } else {
                final Context resContext = createConfigurationContext(mOverrideConfiguration);
                mResources = resContext.getResources();
            }
        }
        return mResources;
    }

    @Override
    public void setTheme(int resid) {
        if (mThemeResource != 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(getBaseContext()).cloneInContext(this);
            }
            return mInflater;
        }
        return getBaseContext().getSystemService(name);
    }

    /**
     * Called by {@link #setTheme} and {@link #getTheme} to apply a theme
     * resource to the current Theme object. May be overridden to change the
     * default (simple) behavior. This method will not be called in multiple
     * threads simultaneously.
     *
     * @param theme the theme being modified
     * @param resId the style resource being applied to <var>theme</var>
     * @param first {@code 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();
            final Resources.Theme theme = getBaseContext().getTheme();
            if (theme != null) {
                mTheme.setTo(theme);
            }
        }
        onApplyThemeResource(mTheme, mThemeResource, first);
    }
}

结合注释及源码，可以发现，相比 ContextWrapper, ContextThemeWrapper 有自己的另外的Resource以及 Theme 成员，并且可以传入配置信息以初始化自己的 Resource 及 Theme.

ContextThemeWrapper 和它的mBase 成员在 Resource 以及 Theme 相关的行为上是不同的。

ContextImpl

ContextImpl 和 ContextThemeWrapper 最大的区别就是没有一个 Configuration. 其它的行为大致一样。 另外 ContextImpl 可以用于创建 Activity Service 的 mBase 成员，这个 mBase context 除了参数不同，它们的 Resource 也不同， 需要注意的是， createActivityContext等方法中 setResource 是 mBase 自己调用的， Activity, service 以及 Application 本身并没有执行 setResource.

总结

• ContextWrapper, ContextThemeWrapper 都是 Context 的代理类，二者的区别在于 ContextThemeWrapper 有自己的 Theme 以及 Resource,并且 Resource 可以传入自己配置初始化
• ContextImpl 是 Context 主要实现类， Activity, Service 和 Application 的 Base contex都是由他创建的，即 ContextWrapper 代理的就是 ContextImpl 对象本身