Dropdown tab support for GwtBootstrap3

Example.java

    TabPanelWithDropdowns tabPanel = new TabPanelWithDropdowns();
    RootLayoutPanel.get().add(tabPanel);

    tabPanel.addItem("First", new Text("This is the content in pane 1"));
    tabPanel.addItem("Second", new Text("This is the content in pane 2"));
    tabPanel.addItem("Third", new Text("This is the content in pane 3"));
    tabPanel.addItem("Really really really really really long tab name", new Text("This is the content in pane 4"));
    tabPanel.addItem("Fifth", new Text("This is the content in pane 5"));

    DropDownContents dropDownContents = tabPanel.addDropdownTab("Dropdown");
    dropDownContents.addItem("Dropdown item 1", new Text("This is the content in dropdown item 1"));
    dropDownContents.addItem("Dropdown item 2", new Text("This is the content in dropdown item 2"));

TabPanelWithDropdowns.java

package ca.fuerth.gwt.playground.client;

import java.util.HashSet;
import java.util.Set;

import org.gwtbootstrap3.client.shared.event.TabShowEvent;
import org.gwtbootstrap3.client.shared.event.TabShowHandler;
import org.gwtbootstrap3.client.shared.event.TabShownEvent;
import org.gwtbootstrap3.client.shared.event.TabShownHandler;
import org.gwtbootstrap3.client.ui.AnchorListItem;
import org.gwtbootstrap3.client.ui.DropDownMenu;
import org.gwtbootstrap3.client.ui.NavTabs;
import org.gwtbootstrap3.client.ui.TabContent;
import org.gwtbootstrap3.client.ui.TabListItem;
import org.gwtbootstrap3.client.ui.TabPane;
import org.gwtbootstrap3.client.ui.TabPanel;
import org.gwtbootstrap3.client.ui.constants.IconPosition;
import org.gwtbootstrap3.client.ui.constants.IconType;
import org.gwtbootstrap3.client.ui.constants.Styles;
import org.gwtbootstrap3.client.ui.constants.Toggle;

import com.google.gwt.user.client.ui.Composite;
import com.google.gwt.user.client.ui.Widget;

public class TabPanelWithDropdowns extends Composite {

  private final NavTabs tabBar;
  private final TabContent tabContent;
  private final Set<Widget> activatableWidgets = new HashSet<Widget>();

  private final TabShowHandler clearActiveStyles = new TabShowHandler() {
    @Override
    public void onShow(TabShowEvent showEvent) {
      for (Widget w : activatableWidgets ) {
        w.removeStyleName(Styles.ACTIVE);
      }
    }
  };

  public TabPanelWithDropdowns() {
    tabBar = new NavTabs();
    tabContent = new TabContent();

    TabPanel root = new TabPanel();
    root.add(tabBar);
    root.add(tabContent);
    initWidget(root);
  }

  /**
   * Adds a normal tab (not a dropdown) with the given label and contents.
   *
   * @param label
   *          the label for the tab itself.
   * @param content
   *          the contents that should appear in the content area when the tab is selected.
   */
  public void addItem(String label, Widget content) {
    TabPane contentPane = new TabPane();
    contentPane.add(content);

    TabListItem tab = new TabListItem(label);
    tab.setDataTargetWidget(contentPane);
    tab.addShowHandler(clearActiveStyles);
    activatableWidgets.add(tab);

    tabBar.add(tab);
    tabContent.add(contentPane);
  }

  /**
   * Adds a new tab to this panel that doesn't have any contents itself, but can contain multiple items that appear in a
   * dropdown menu when the tab is clicked. This dropdown menu is initially empty. Items can be added and removed using
   * the {@link DropDownContents#addItem(String, Widget)} and {@link DropDownContents#removeItem(String, Widget)}
   * methods.
   *
   * @param label The text that should appear on the dropdown tab.
   * @return the container for the items that appear when the tab is clicked.
   */
  public DropDownContents addDropdownTab(String label) {

    AnchorListItem tab = new AnchorListItem(label);

    // gets set to active when one of the menu items is selected
    activatableWidgets.add(tab);

    // FIXME should actually subclass AnchorListItem and add a <b class=caret/> to the anchor elem
    tab.setIcon(IconType.CARET_DOWN);
    tab.setIconPosition(IconPosition.RIGHT);

    tab.addStyleName(Styles.DROPDOWN_TOGGLE);
    tab.setDataToggle(Toggle.DROPDOWN);

    DropDownContents dropDownContents = new DropDownContents();
    tab.add(dropDownContents);

    tabBar.add(tab);

    return dropDownContents;
  }

  /**
   * Container for the menu items that appear when the dropdown tab is clicked.
   * Normally, should only be created by {@link TabPanelWithDropdowns#addDropdownTab(String)}.
   */
  public class DropDownContents extends DropDownMenu {

    public void addItem(String label, Widget content) {
      TabPane contentPane = new TabPane();
      contentPane.add(content);

      TabListItem dropDownMenuItem = new DropDownTabListItem(label);
      activatableWidgets.add(dropDownMenuItem);
      dropDownMenuItem.setDataTargetWidget(contentPane);
      dropDownMenuItem.addShowHandler(clearActiveStyles);
      dropDownMenuItem.addShownHandler(new TabShownHandler() {
        @Override
        public void onShown(TabShownEvent event) {
          DropDownContents.this.getParent().addStyleName(Styles.ACTIVE);
        }
      });

      add(dropDownMenuItem);
      tabContent.add(contentPane);
    }
  }

  /**
   * Widget for an individual menu item entry that appears when the dropdown tab is clicked.
   * Normally, should only be created by {@link DropDownContents#addItem(String, Widget)}.
   */
  public static class DropDownTabListItem extends TabListItem {

    public DropDownTabListItem(String label) {
      super(label);
      anchor.setTabIndex(-1);
    }
  }

}