--- title: Building the UI order: 2 layout: page --- [[application.architecture]] = Building the UI Vaadin Framework user interfaces are built hierarchically from components, so that the leaf components are contained within layout components and other component containers. Building the hierarchy starts from the top (or bottom - whichever way you like to think about it), from the [classname]#UI# class of the application. You normally set a layout component as the content of the UI and fill it with other components. [source, java] ---- public class MyHierarchicalUI extends UI { @Override protected void init(VaadinRequest request) { // The root of the component hierarchy VerticalLayout content = new VerticalLayout(); content.setSizeFull(); // Use entire window setContent(content); // Attach to the UI // Add some component content.addComponent(new Label("Hello! - How are you?", ContentMode.HTML)); Grid grid = new Grid<>(); grid.setCaption("My Grid"); grid.setItems(GridExample.generateContent()); grid.setSizeFull(); content.addComponent(grid); content.setExpandRatio(grid, 1); // Expand to fill } } ---- The component hierarchy is illustrated in <>. [[figure.application.architecture.schematic]] .Schematic diagram of the UI image::img/ui-schematic.png[width=80%, scaledwidth=100%] The actual UI is shown in <>. [[figure.application.architecture.example]] .Simple hierarchical UI image::img/ui-architecture-hierarchical.png[width=70%, scaledwidth=90%] Instead of building the layout in Java, you can also use a declarative design, as described later in <>. The examples given for the declarative layouts give exactly the same UI layout as built from the components above. The easiest way to create declarative designs is to use Vaadin Designer. The built-in components are described in <> and the layout components in <>. The example application described above just is, it does not do anything. User interaction is handled with event listeners, as described a bit later in <>. [[application.architecture.architecture]] == Application Architecture Once your application grows beyond a dozen or so lines, which is usually quite soon, you need to start considering the application architecture more closely. You are free to use any object-oriented techniques available in Java to organize your code in methods, classes, packages, and libraries. An architecture defines how these modules communicate together and what sort of dependencies they have between them. It also defines the scope of the application. The scope of this book, however, only gives a possibility to mention some of the most common architectural patterns in Vaadin applications. The subsequent sections describe some basic application patterns. For more information about common architectures, see <>, which discusses layered architectures, the Model-View-Presenter (MVP) pattern, and so forth. [[application.architecture.composition]] == Compositing Components User interfaces typically contain many user interface components in a layout hierarchy. Vaadin provides many layout components for laying contained components vertically, horizontally, in a grid, and in many other ways. You can extend layout components to create composite components. [source, java] ---- class MyView extends VerticalLayout { TextField entry = new TextField("Enter this"); Label display = new Label("See this"); Button click = new Button("Click This"); public MyView() { addComponent(entry); addComponent(display); addComponent(click); setSizeFull(); addStyleName("myview"); } } // Create an instance of MyView Layout myview = new MyView(); ---- While extending layouts is an easy way to make component composition, it is a good practice to encapsulate implementation details, such as the exact layout component used. Otherwise, the users of such a composite could begin to rely on such implementation details, which would make changes harder. For this purpose, Vaadin has a special [classname]#CustomComponent# wrapper, which hides the content representation. [source, java] ---- class MyView extends CustomComponent { TextField entry = new TextField("Enter this"); Label display = new Label("See this"); Button click = new Button("Click This"); public MyView() { Layout layout = new VerticalLayout(); layout.addComponent(entry); layout.addComponent(display); layout.addComponent(click); setCompositionRoot(layout); setSizeFull(); } } // Create an instance of MyView MyView myview = new MyView(); ---- For a more detailed description of the [classname]#CustomComponent#, see <>. [[application.architecture.navigation]] == View Navigation While the simplest applications have just one __view__ (or __screen__), most of them often require several. Even in a single view, you often want to have sub-views, for example to display different content. <> illustrates a typical navigation between different top-level views of an application, and a main view with sub-views. [[figure.application.architecture.navigation]] .Navigation Between Views image::img/view-navigation-hi.png[width=80%, scaledwidth=100%] The [classname]#Navigator# described in <> is a view manager that provides a flexible way to navigate between views and sub-views, while managing the URI fragment in the page URL to allow bookmarking, linking, and going back in the browser history. Often Vaadin application views are part of something bigger. In such cases, you may need to integrate the Vaadin applications with the other website. You can use the embedding techniques described in <>. [[application.architecture.accessing]] == Accessing UI, Page, Session, and Service You can get the UI and the page to which a component is attached to with [methodname]#getUI()# and [methodname]#getPage()#. However, the values are [literal]#++null++# until the component is attached to the UI, and typically, when you need it in constructors, it is not. It is therefore preferable to access the current UI, page, session, and service objects from anywhere in the application using the static [methodname]#getCurrent()# methods in the respective [classname]#UI#, [classname]#Page#, [classname]#VaadinSession#, and [classname]#VaadinService# classes. [source, java] ---- // Set the default locale of the UI UI.getCurrent().setLocale(new Locale("en")); // Set the page title (window or tab caption) Page.getCurrent().setTitle("My Page"); // Set a session attribute VaadinSession.getCurrent().setAttribute("myattrib", "hello"); // Access the HTTP service parameters File baseDir = VaadinService.getCurrent().getBaseDirectory(); ---- You can get the page and the session also from a [classname]#UI# with [methodname]#getPage()# and [methodname]#getSession()# and the service from [classname]#VaadinSession# with [methodname]#getService()#. The static methods use the built-in ThreadLocal support in the classes. ifdef::web[] The pattern is described in <>. endif::web[]