You'll see frequent reference to a Layout Component in Tapestry documentation, but you won't find such a component in the component reference. The Layout component is a component that you create to provide common elements across all of your pages.
In traditional servlet development, you may be familiar with the use of a JSP include to include a banner across the top of your page and a copyright message across the bottom. In Tapestry, you could implement those recurring page elements as components (a banner component, a copyright component, etc.) and then add those components to every page.
But there's an even better way. Just create a layout component that provides the overall structure and recurring content for your pages:
In a real-world example, the two <div> elements above might contain the typical recurring content you'll see across the pages of a web application: banners, menus, login forms and so forth. Often these layout components get very complex ... in fact, in many applications the Layout component can grow to be as complex as any other component.
Using the Layout in a Page
To use your layout component, just have each page in your application wrap itself in the layout, like this:
Note the "t:type="layout" part. That says, in effect, "wrap the layout component around my content".
The magic is in the <t:body/> element of the layout template; this will be replaced by each page's content, whatever that is.
To keep our Welcome.tml page template relatively preview-able, we are using an <html> element and the t:type attribute to specify that it is a component. At render time, the page's <html> tag will be removed, and replaced with the content from the Layout.tml template (which conveniently starts with an <html> element). The <t:body> element in Layout.tml will be replaced with the page-specific content here: the <h1> and <p> tags.
Any page in the application that follows this pattern, using the Layout component, will have the same look and feel.
Layout is a regular component like other, with an ordinary component template. Like all component templates, it will be stored on the classpath (typically under src/main/resources).
Components must always have a Java class. But in this trivial example, the Layout component doesn't need any logic:
We use the @Import annotation (in 5.2 or later), as opposed to directly adding the <link> element to the template, for significant performance benefits described elsewhere. (For 5.0 and 5.1, use the deprecated @IncludeStyleSheet annotation instead.)
You may find that your application has more than one look and feel: perhaps user registration pages have one look, while administrative pages have another. This can be accomplished by having multiple layout components (using any names you choose) and using those different layout types for different pages.
Layouts are really just ordinary components, so they can be nested to any level needed. You can have, for example, a "CommonLayout" component that provides the peripheral elements for all your pages, and then a more specialized "AdminLayout" component that provides the layout only for the administrative pages, and make the AdminLayout component wrap itself in the CommonLayout component. So then the administrative pages would start with
<html t:type="adminLayout" ...> and the other pages (and the AdminLayout component itself) would start with
<html t:type="commonLayout" ...>.
A more advanced example
Here's an example of a Layout component with a few more features. It has a "title" parameter, so that every page can pass in its own title to be rendered in the <title> tag and in an <h1> tag at the top of the HTML. There is also a "style" parameter that allows each page to pass in a block of CSS rules to be rendered in the <head> section of the page (useful for those few CSS rules that can't be put into a static CSS file). Notice the HTML5-style DOCTYPE declaration at the top, the charset definition as UTF-8, and the addition of an "alerts" component.
The Alerts component above is new in Tapestry 5.3; it allows the application to present alert messages to the client in a consistent way. If you want alerts to always appear in the banner of your web site, it may make sense to put it in the layout component's template, as above.
The corresponding component class is still very simple, adding support for the "title" and "style" parameters:
Here's how you might use the above layout component for a UserList page:
The <p:style> element (and its contents) are passed to the layout component as a
style parameter (a block parameter, in this case, so you must have the
xmlns:p="tapestry:parameter" namespace declared, as above).
The rendered HTML would look like the following (whitespace aside, and assuming UserList.java has a backgroundImage property whose value is the string "http://www.example.com/fuzzy.gif"):