fix: Update current UI instance upon locale change

Fixes #11599

flow-server/src/main/java/com/vaadin/flow/component/UI.java

@@ -1008,6 +1008,12 @@ public ExecutionRegistration beforeClientResponse(Component component,
                     "The 'execution' parameter may not be null");
         }
 
+        if (component.getUI().isPresent() && component.getUI().get() != this) {
+            throw new IllegalArgumentException(
+                    "The given component doesn't belong to the UI the task to be executed on"
+            );
+        }
+
         return internals.getStateTree().beforeClientResponse(
                 component.getElement().getNode(), execution);
     }

flow-server/src/main/java/com/vaadin/flow/server/VaadinSession.java

@@ -395,7 +395,15 @@ public void setLocale(Locale locale) {
         checkHasLock();
         this.locale = locale;
 
-        getUIs().forEach(ui -> ui.setLocale(locale));
+        getUIs().forEach(ui -> {
+            Map<Class<?>, CurrentInstance> oldInstances = CurrentInstance
+            .setCurrent(ui);
+            try {
+                ui.setLocale(locale);
+            } finally {
+                CurrentInstance.restoreInstances(oldInstances);
+            }
+        });
     }
 
     /**

flow-tests/test-root-context/src/main/java/com/vaadin/flow/uitest/ui/LocaleChangeView.java

@@ -0,0 +1,148 @@
+/*
+ * Copyright 2000-2021 Vaadin Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package com.vaadin.flow.uitest.ui;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Locale;
+
+import com.vaadin.flow.component.Component;
+import com.vaadin.flow.component.HasComponents;
+import com.vaadin.flow.component.HtmlComponent;
+import com.vaadin.flow.component.Tag;
+import com.vaadin.flow.component.UI;
+import com.vaadin.flow.component.html.Div;
+import com.vaadin.flow.component.html.NativeButton;
+import com.vaadin.flow.component.html.Span;
+import com.vaadin.flow.i18n.LocaleChangeEvent;
+import com.vaadin.flow.i18n.LocaleChangeObserver;
+import com.vaadin.flow.router.Route;
+import com.vaadin.flow.server.VaadinSession;
+
+@Route("com.vaadin.flow.uitest.ui.LocaleChangeView")
+public class LocaleChangeView extends Div {
+
+    public static final String SAME_UI_RESULT_ID = "same-ui-result-id";
+    public static final String CHANGE_LOCALE_BUTTON_ID =
+            "change-locale-button-id";
+    public static final String SHOW_RESULTS_BUTTON_ID =
+            "show-results-button-id";
+
+    public LocaleChangeView() {
+        Locale defaultLocale = UI.getCurrent().getLocale();
+        NativeButton changeLocaleButton = new NativeButton("Change Locale",
+                click -> {
+                    if (defaultLocale.equals(Locale.ENGLISH)) {
+                        changeSessionLocale(Locale.FRANCE);
+                    } else {
+                        changeSessionLocale(Locale.ENGLISH);
+                    }
+                });
+        changeLocaleButton.setId(CHANGE_LOCALE_BUTTON_ID);
+
+        LocaleObserverComponent localeObserverComponent = new LocaleObserverComponent(
+                defaultLocale);
+
+        NativeButton showLocaleUpdates = new NativeButton("Show locale updates",
+                click -> localeObserverComponent.showLocaleUpdates());
+        showLocaleUpdates.setId(SHOW_RESULTS_BUTTON_ID);
+
+        add(changeLocaleButton, showLocaleUpdates,
+                new HtmlComponent(Tag.BR), localeObserverComponent);
+    }
+
+    private void changeSessionLocale(Locale france) {
+        UI.getCurrent().getSession().setLocale(france);
+    }
+
+    @Tag("Div")
+    private static class LocaleObserverComponent extends Component
+            implements HasComponents, LocaleChangeObserver {
+
+        private final Locale defaultLocale;
+
+        public LocaleObserverComponent(Locale defaultLocale) {
+            this.defaultLocale = defaultLocale;
+        }
+
+        @Override
+        public void localeChange(LocaleChangeEvent event) {
+            if (event.getLocale().equals(defaultLocale)) {
+                return;
+            }
+            final boolean sameUI = getUI().isPresent()
+                    && getUI().get() == UI.getCurrent();
+
+            registerLocaleUpdate(sameUI);
+        }
+
+        public void showLocaleUpdates() {
+            LocaleUpdates localeUpdates = getLocaleUpdates();
+            renderResults(localeUpdates);
+        }
+
+        private void renderResults(LocaleUpdates localeUpdates) {
+            for (int i = 0; i < localeUpdates.getUiCheckResults().size(); i++) {
+                Boolean checkResult = localeUpdates.getUiCheckResults().get(i);
+                Span sameUIResult = new Span();
+                sameUIResult.setId(String.format("%s-%d", SAME_UI_RESULT_ID,
+                        i));
+                sameUIResult.setText(Boolean.toString(checkResult));
+                Span caption = new Span(String.format(
+                        "Component %d uses current UI instance = ", i));
+                add(caption, sameUIResult, new HtmlComponent(Tag.BR));
+            }
+        }
+
+        private void registerLocaleUpdate(boolean sameUIcheckUp) {
+            VaadinSession session = VaadinSession.getCurrent();
+            session.lock();
+            try {
+                LocaleUpdates localeUpdates = session
+                        .getAttribute(LocaleUpdates.class);
+                if (localeUpdates == null) {
+                    localeUpdates = new LocaleUpdates();
+                }
+                localeUpdates.addUICheckResult(sameUIcheckUp);
+                session.setAttribute(LocaleUpdates.class, localeUpdates);
+            } finally {
+                session.unlock();
+            }
+        }
+
+        private LocaleUpdates getLocaleUpdates() {
+            VaadinSession session = VaadinSession.getCurrent();
+            session.lock();
+            try {
+                return session.getAttribute(LocaleUpdates.class);
+            } finally {
+                session.unlock();
+            }
+        }
+    }
+
+    static class LocaleUpdates {
+        private final List<Boolean> uiCheckResults = new ArrayList<>(3);
+
+        public void addUICheckResult(boolean uiCheckResult) {
+            uiCheckResults.add(uiCheckResult);
+        }
+
+        public List<Boolean> getUiCheckResults() {
+            return uiCheckResults;
+        }
+    }
+}

flow-tests/test-root-context/src/test/java/com/vaadin/flow/uitest/ui/LocaleChangeIT.java

@@ -0,0 +1,54 @@
+/*
+ * Copyright 2000-2021 Vaadin Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package com.vaadin.flow.uitest.ui;
+
+import java.util.stream.IntStream;
+
+import org.junit.Assert;
+import org.junit.Test;
+import org.openqa.selenium.By;
+import org.openqa.selenium.WebElement;
+
+import com.vaadin.flow.testutil.ChromeBrowserTest;
+
+import static com.vaadin.flow.uitest.ui.LocaleChangeView.CHANGE_LOCALE_BUTTON_ID;
+import static com.vaadin.flow.uitest.ui.LocaleChangeView.SAME_UI_RESULT_ID;
+import static com.vaadin.flow.uitest.ui.LocaleChangeView.SHOW_RESULTS_BUTTON_ID;
+
+public class LocaleChangeIT extends ChromeBrowserTest {
+
+    @Test
+    public void setSessionLocale_currentUIInstanceUpdatedUponEachLocaleUpdate() {
+        final int openedUI = 3;
+
+        IntStream.range(0, openedUI).forEach(i -> open());
+
+        waitForElementPresent(By.id(CHANGE_LOCALE_BUTTON_ID));
+        findElement(By.id(CHANGE_LOCALE_BUTTON_ID)).click();
+
+        waitForElementPresent(By.id(SHOW_RESULTS_BUTTON_ID));
+        findElement(By.id(SHOW_RESULTS_BUTTON_ID)).click();
+
+        IntStream.range(0, openedUI).forEach(i -> {
+            String id = String.format("%s-%d", SAME_UI_RESULT_ID, i);
+            waitForElementPresent(By.id(id));
+            WebElement result = findElement(By.id(id));
+            Assert.assertTrue(
+                    "Component's UI and current UI instances are different",
+                    Boolean.parseBoolean(result.getText()));
+        });
+    }
+}