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Flamingo - breadcrumb bars

Breadcrumb bars facilitate quick navigation of multi-level hierarchies, such as file systems or XML documents. This functionality is commonly found in many web sites and is the main navigation tool in the Windows Explorer application.

The core library provides helper classes to expose a variety of navigation sources as breadcrumb bars, such as local and remote file systems, and tree models.

Supplying content to breadcrumb bar

Unlike command and other related content models (command strips, command panels, command popup menus), breadcrumb bar does not force ahead-of-time, exhaustive construction of the entire model at initialization time.

The org.pushingpixels.flamingo.api.bcb.BreadcrumbBarCallBack is used by the breadcrumb component at runtime to query the application code for the structure of the hierarchy being navigated. The two main methods are:

  • List<StringValuePair<T>> getPathChoices(List<BreadcrumbItem<T>> path)
  • List<StringValuePair<T>> getLeafs(List<BreadcrumbItem<T>> path)

What are these two used for? Let's take another look at the screenshot of a sample app that is using the breadcrumb bar for navigating the local file system:

The getPathChoices() should return the list of paths that can be "explored" from the specified path. In the screenshot above these are the subfolders of the currently activated "JavaVirtualMachines" path.

The getLeafs() should return the list of leaf elements that correspond to the specified path. In the screenshot above these are the individual files (not subfolders) of the currently selected "bin" path.

Working with breadcrumb bar

The org.pushingpixels.flamingo.api.bcb.JBreadcrumbBar is a Swing component. It is initialized with a BreadcrumbBarCallBack instance.

Use JBreadcrumbBar.setPath() to set the specified path to be the currently selected one - like the "Select and set path..." in the screenshot above that does this:

JButton setPath = new JButton("Select and set path...");
setPath.addActionListener((ActionEvent e) -> SwingUtilities.invokeLater(() -> {
    JFileChooser folderChooser = new JFileChooser();
    if (folderChooser.showOpenDialog(BreadCrumbTest.this) == JFileChooser.APPROVE_OPTION) {
        File selected = folderChooser.getSelectedFile();

Here, we are using the BreadcrumbFileSelector that is a core extension of JBreadcrumbBar that exposes the local file system. This subclass exposes setPath(File) API that constructs a full List<BreadcrumbItem<File>> that leads from the root of the file system to the specified file.

Observing breadcrumb events

Let's go back to our app screenshot again:

The top part of the app is BreadcrumbFileSelector (which is a subclass of JBreadcrumbBar). The main content is the panel that shows the list of files in the currently selected path. This panel is an extension of org.pushingpixels.flamingo.api.common.AbstractFileViewPanel class.

On the presentation side of things, it is implemented as a command button panel with CommandPanelContentModel content model, CommandPanelPresentationModel presentation model and CommandPanelProjection projection.

But how does a selection change in the breadcrumb bar (clicking any element of the path or selecting a new path) get reflected in this panel?

It is achieved by adding a path listener to the JBreadcrumbBar's model:
        (BreadcrumbPathEvent<File> event) -> SwingUtilities.invokeLater(() -> {
            final List<BreadcrumbItem<File>> newPath = event.getSource().getItems();
            System.out.println("New path is ");
            for (BreadcrumbItem<File> item : newPath) {
                System.out.println("\t" + item.getData().getAbsolutePath());

            if (newPath.size() > 0) {
                SwingWorker<List<StringValuePair<File>>, Void> worker = new SwingWorker<>() {
                    protected List<StringValuePair<File>> doInBackground() throws Exception {
                        return bar.getCallback().getLeafs(newPath);

                    protected void done() {
                        try {
                            List<StringValuePair<File>> leafs = get();
                        } catch (Exception exc) {

Let's take a look at what we're doing here:

  • Use JBreadcrumbBar.getModel() to get an instance of BreadcrumbBarModel.
  • Use BreadcrumbBarModel.addPathListener() to add a BreadcrumbPathListener.
  • The listener's only method (elided breadcrumbPathEvent) gets a BreadcrumbPathEvent object.
  • Use BreadcrumbPathEvent.getSource() to obtain the model (or use the same bar.getModel() above) and then BreadcrumbBarModel.getItems() to get the currently selected path.
  • Create a SwingWorker to load the list of files in the selected path in the background using BreadcrumbBarCallBack.getLeafs(). We want to run this off the UI thread so that the UI remains responsive during this potentially blocking I/O operation.
  • When the list of files is loaded, call AbstractFileViewPanel.setFolder() API to update the panel to show the files from the currently selected path. As that content is loaded, AbstractFileViewPanel calls into its abstract configureCommand() and getResizableIcon (which are implemented in our subclass) to get the application-specific visual representation of each file.

Core breadcrumb bar integrations

The org.pushingpixels.flamingo.api.bcb.core has helper classes to facilitate exposing existing hierarchical data sources as breadcrumb bar content.

  • BreadcrumbFileSelector subclasses the JBreadcrumbBar class to expose the local file system.
  • BreadcrumbTreeAdapterSelector subclasses the JBreadcrumbBar class to wrap the core Swing JTree and TreeModel structures.
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