Carson带你学Android：如何理解 Context对于Activity的意义？

public abstract class Context {
    
    // 作用1：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);

    // 作用2：获取系统/应用资源
    // 如 AssetManager、PackageManager、Resources、System Service 以及 color、string、drawable 等
    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) { ... }

    // 作用3：四大组件之间的交互
    // 如启动 Activity、Broadcast、Service，获取 ContentResolver 等
    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();
    
    // 作用4：文件相关
    // 如：获取缓存文件夹、删除文件、SharedPreference 相关等
    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);

    // 作用5：数据库
    // 如打开数据库、删除数据库、获取数据库路径等
    public abstract SQLiteDatabase openOrCreateDatabase(...);
    public abstract boolean deleteDatabase(String name);
    public abstract File getDatabasePath(String name);
   
    ...
}

public class ContextThemeWrapper extends ContextWrapper {
    private int mThemeResource;
    private Resources.Theme mTheme;
    private LayoutInflater mInflater;
    private Configuration mOverrideConfiguration;
    private Resources mResources;

    public ContextThemeWrapper() {
        super(null);
    }

    public ContextThemeWrapper(Context base, @StyleRes int themeResId) {
        super(base);
        mThemeResource = themeResId;
    }

    public ContextThemeWrapper(Context base, Resources.Theme theme) {
        super(base);
        mTheme = theme;
    }
    
   // 包含了大量跟主题相关的接口方法
    @Override
    protected void attachBaseContext(Context newBase) {
        super.attachBaseContext(newBase);
    }

    public void applyOverrideConfiguration(Configuration overrideConfiguration) {...}

    public Configuration getOverrideConfiguration() {...}

    @Override
    public AssetManager getAssets() {...}

    @Override
    public Resources getResources() {...}

    private Resources getResourcesInternal() {...}

    @Override
    public void setTheme(int resid) {...}

    @Override
    public int getThemeResId() {...}

    @Override
    public Resources.Theme getTheme() {...}

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

    protected void onApplyThemeResource(Resources.Theme theme, int resId, boolean first) {

    private void initializeTheme() {...}
}