public class I18nBaseActivity extends AppCompatActivity {
    @Override
    protected void attachBaseContext(Context newBase) {
      	//切換多語言，然后將新生成的 context 覆蓋給 attachBaseContext()
        Context context = MultiLanguageUtils.changeContextLocale(newBase);
        super.attachBaseContext(context);
    }
}

public class I18nBaseActivity extends AppCompatActivity {
    @Override
    protected void attachBaseContext(Context newBase) {
      	//切換多語言，然后將新生成的 context 覆蓋給 attachBaseContext()
        Context context = MultiLanguageUtils.changeContextLocale(newBase);
       //兼容appcompat 1.2.0后切換語言失效問題
        final Configuration configuration = context.getResources().getConfiguration();
        final ContextThemeWrapper wrappedContext = new ContextThemeWrapper(context,
                R.style.Base_Theme_AppCompat_Empty) {
            @Override
            public void applyOverrideConfiguration(Configuration overrideConfiguration) {
                if (overrideConfiguration != null) {
                    overrideConfiguration.setTo(configuration);
                }
                super.applyOverrideConfiguration(overrideConfiguration);
            }
        };
        super.attachBaseContext(wrappedContext);
    }
}

@Override
protected void attachBaseContext(Context newBase) {
  super.attachBaseContext(getDelegate().attachBaseContext2(newBase));
}

/**
 * @return The {@link AppCompatDelegate} being used by this Activity.
*/
@NonNull
public AppCompatDelegate getDelegate() {
  if (mDelegate == null) {
    mDelegate = AppCompatDelegate.create(this, this);	//代理對象是通過 AppCompatDelegate create出來的，那繼續(xù)往下看
  }
  return mDelegate;
}

// 這里直接看 AppCompatDelegateImpl 類，該類是 AppCompatDelegate 類的實(shí)現(xiàn)類
@NonNull
@Override
@CallSuper
public Context attachBaseContext2(@NonNull final Context baseContext) {
  //......
  /**
  * 這段邏輯是：如果傳入的 context 是經(jīng)過 ContextThemeWrapper 封裝的，則直接使用該 context 配置進(jìn)行覆蓋
  */
  // If the base context is a ContextThemeWrapper (thus not an Application context)
  // and nobody's touched its Resources yet, we can shortcut and directly apply our
  // override configuration.
  if (sCanApplyOverrideConfiguration
      && baseContext instanceof android.view.ContextThemeWrapper) {
    final Configuration config = createOverrideConfigurationForDayNight(
      baseContext, modeToApply, null);
    if (DEBUG) {
      Log.d(TAG, String.format("Attempting to apply config to base context: %s",
                               config.toString()));
    }
    try {
      ContextThemeWrapperCompatApi17Impl.applyOverrideConfiguration(
        (android.view.ContextThemeWrapper) baseContext, config);
      return baseContext;
    } catch (IllegalStateException e) {
      if (DEBUG) {
        Log.d(TAG, "Failed to apply configuration to base context", e);
      }
    }
  }
  // ......
  /**
  * 下面這段邏輯是：通過 packageManger 獲取配置，然后和傳入的 context 配置進(jìn)行對比，覆蓋修改過的配置。
  * 這里有個(gè)關(guān)鍵因素，通過 packageManager 獲取的配置與 context 的配置進(jìn)行 diff 更新，并將 diff 結(jié)果賦值給新建的 configration。這就會導(dǎo)致，當(dāng)這一次切換成功后，殺死進(jìn)程下次啟動(dòng)時(shí)，由于 packageManager 配置的語言 與 context 配置的語言一致，而直接跳過，并沒有給新建的 configration進(jìn)行賦值。最終導(dǎo)致多語言切換失效。同理，從 ActivityA 設(shè)置了多語言，然后重啟 ActivityA，再從ActivityA 跳轉(zhuǎn)到 ActivityB，此時(shí)ActivityB 多語言并沒有生效。
  */
  // We can't trust the application resources returned from the base context, since they
  // may have been altered by the caller, so instead we'll obtain them directly from the
  // Package Manager.
  final Configuration appConfig;
  try {
    appConfig = baseContext.getPackageManager().getResourcesForApplication(
      baseContext.getApplicationInfo()).getConfiguration();
  } catch (PackageManager.NameNotFoundException e) {
    throw new RuntimeException("Application failed to obtain resources from itself", e);
  }
  // The caller may have directly modified the base configuration, so we'll defensively
  // re-structure their changes as a configuration overlay and merge them with our own
  // night mode changes. Diffing against the application configuration reveals any changes.
  final Configuration baseConfig = baseContext.getResources().getConfiguration();
  final Configuration configOverlay;
  if (!appConfig.equals(baseConfig)) {
    configOverlay = generateConfigDelta(appConfig, baseConfig);		//這里是關(guān)鍵
    if (DEBUG) {
      Log.d(TAG,
            "Application config (" + appConfig + ") does not match base config ("
            + baseConfig + "), using base overlay: " + configOverlay);
    }
  } else {
    configOverlay = null;
    if (DEBUG) {
      Log.d(TAG, "Application config (" + appConfig + ") matches base context "
            + "config, using empty base overlay");
    }
  }
  final Configuration config = createOverrideConfigurationForDayNight(
    baseContext, modeToApply, configOverlay);
  if (DEBUG) {
    Log.d(TAG, String.format("Applying night mode using ContextThemeWrapper and "
                             + "applyOverrideConfiguration(). Config: %s", config.toString()));
  }
  // Next, we'll wrap the base context to ensure any method overrides or themes are left
  // intact. Since ThemeOverlay.AppCompat theme is empty, we'll get the base context's theme.
  final ContextThemeWrapper wrappedContext = new ContextThemeWrapper(baseContext,
                                                                     R.style.Theme_AppCompat_Empty);
  wrappedContext.applyOverrideConfiguration(config);
  // ......
  return super.attachBaseContext2(wrappedContext);
}

@NonNull
private static Configuration generateConfigDelta(@NonNull Configuration base,
                                                 @Nullable Configuration change) {
  final Configuration delta = new Configuration();
  delta.fontScale = 0;
  //......
  //這里可以看到，如果兩個(gè)配置相等，則直接跳過了，并沒有給新創(chuàng)建的 delta 的 locale 賦值。
  if (Build.VERSION.SDK_INT >= 24) {
    ConfigurationImplApi24.generateConfigDelta_locale(base, change, delta); 
  } else {
    if (!ObjectsCompat.equals(base.locale, change.locale)) {	
      delta.locale = change.locale;
    }
  }
	//......
}