feat: add ThemeChangeObserver

Close #110

pom.xml

@@ -5,7 +5,7 @@
 
   <groupId>com.flowingcode.vaadin.addons.demo</groupId>
   <artifactId>commons-demo</artifactId>
-  <version>4.1.1-SNAPSHOT</version>
+  <version>4.2.0-SNAPSHOT</version>
 
   <name>Commons Demo</name>
   <description>Common classes for add-ons demo</description>

src/main/java/com/flowingcode/vaadin/addons/demo/TabbedDemo.java

@@ -43,6 +43,7 @@
 import java.util.List;
 import java.util.Objects;
 import java.util.Optional;
+import java.util.stream.Stream;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -340,6 +341,15 @@ public void setOrientation(Orientation orientation) {
   public static void applyTheme(Element element, boolean useDarkTheme) {
     String theme = useDarkTheme ? "dark" : "";
     element.executeJs("document.documentElement.setAttribute('theme', $0);", theme);
+
+    Component c = element.getComponent().get();
+    collectThemeChangeObservers(c).forEach(observer -> observer.onThemeChange(theme));
+  }
+
+  private static Stream<ThemeChangeObserver> collectThemeChangeObservers(Component c) {
+    Stream<ThemeChangeObserver> children =
+        c.getChildren().flatMap(child -> collectThemeChangeObservers(child));
+    return c instanceof ThemeChangeObserver o ? Stream.concat(Stream.of(o), children) : children;
   }
 
   private void updateFooterButtonsVisibility() {

src/main/java/com/flowingcode/vaadin/addons/demo/ThemeChangeObserver.java

@@ -0,0 +1,12 @@
+package com.flowingcode.vaadin.addons.demo;
+
+/**
+ * Any attached component implementing this interface will receive an event when a new theme is
+ * applied.
+ */
+@FunctionalInterface
+public interface ThemeChangeObserver {
+
+  void onThemeChange(String themeName);
+
+}