Apache Tiles - Framework - Nesting and Extending Definitions

Nesting Definitions

Sometimes it is useful to have a structured page with, say, a structured body. Typically, there is a main layout (for example, the "classic" layout) and the body is made of certain number of sections. In this case, nesting a definition (the one for the body) inside another definition (the main layout) can be useful.

Named subdefinitions

Tiles supports nesting definitions natively. One way of using nested definitions is creating a named "subdefinition" and using it as an attribute. For example:

<definition name="myapp.homepage.body" template="/layouts/three_rows.jsp">
  <put-attribute name="one" value="/tiles/headlines.jsp" />
  <put-attribute name="two" value="/tiles/topics.jsp" />
  <put-attribute name="one" value="/tiles/comments.jsp" />
</definition>

<definition name="myapp.homepage" template="/layouts/classic.jsp">
  <put-attribute name="title" value="Tiles tutorial homepage" />
  <put-attribute name="header" value="/tiles/banner.jsp" />
  <put-attribute name="menu" value="/tiles/common_menu.jsp" />
  <put-attribute name="body" value="myapp.homepage.body" />
  <put-attribute name="footer" value="/tiles/credits.jsp" />
</definition>

The myapp.homepage.body definition will be put inside the myapp.homepage, by putting it inside its body attribute. You will be seeing the definition one inside the other.

Anonymous nested definitions

What you can do with named subdefinitions can be done with nested anonymous definitions. The above example can be rewritten in:

<definition name="myapp.homepage.body" template="/layouts/three_rows.jsp">
  <put-attribute name="one" value="/tiles/headlines.jsp" />
  <put-attribute name="two" value="/tiles/topics.jsp" />
  <put-attribute name="one" value="/tiles/comments.jsp" />
</definition>

<definition name="myapp.homepage" template="/layouts/classic.jsp">
  <put-attribute name="title" value="Tiles tutorial homepage" />
  <put-attribute name="header" value="/tiles/banner.jsp" />
  <put-attribute name="menu" value="/tiles/common_menu.jsp" />
  <put-attribute name="body">
    <definition template="/layouts/three_rows.jsp">
      <put-attribute name="one" value="/tiles/headlines.jsp" />
      <put-attribute name="two" value="/tiles/topics.jsp" />
      <put-attribute name="one" value="/tiles/comments.jsp" />
    </definition>
  </put-attribute>
  <put-attribute name="footer" value="/tiles/credits.jsp" />
</definition>

The anonymous definition put under the "body" attribute can be used only by the surrounding definition. Moreover, you can nest a definition into a nested definition, with the desired level of depth.

Cascaded attributes

Attributes defined into a definition can be cascaded to be available to all nested definitions and templates. For example the sample definition detailed above can be rewritten this way:

<definition name="myapp.homepage" template="/layouts/classic.jsp">
  <put-attribute name="title" value="Tiles tutorial homepage" />
  <put-attribute name="header" value="/tiles/banner.jsp" />
  <put-attribute name="menu" value="/tiles/common_menu.jsp" />
  <put-attribute name="body" value="/layouts/three_rows.jsp" />
  <put-attribute name="footer" value="/tiles/credits.jsp" />

  <put-attribute name="one" value="/tiles/headlines.jsp" cascade="true" />
  <put-attribute name="two" value="/tiles/topics.jsp" cascade="true" />
  <put-attribute name="one" value="/tiles/comments.jsp" cascade="true" />
</definition>

The template of myapp.homepage.body definitionhas been used as the body attribute in the myapp.homepage definition. All of the attributes of myapp.homepage.body has been then moved as attributes of myapp.homepage definition, but with the addition of the "cascade" flag.

Extending Definitions

You can extend definitions like a Java class. The concepts of abstract definition, extension and override are available.

Abstract definition: it is a definition in which the template attributes are not completely filled. They are useful to create a base page and a number of extending definitions, reusing already created layout. For example:

<definition name="myapp.page.common" template="/layouts/classic.jsp">
  <put-attribute name="header" value="/tiles/banner.jsp" />
  <put-attribute name="menu" value="/tiles/common_menu.jsp" />
  <put-attribute name="footer" value="/tiles/credits.jsp" />
</definition>

Definition extension: a definition can inherit from another definition, to reuse an already made (abstract or not) definition:
```
<definition name="myapp.homepage" extends="myapp.page.common">
  <put-attribute name="title" value="Tiles tutorial homepage" />
  <put-attribute name="body" value="myapp.homepage.body" />
</definition>
```
In this case, the header, menu and footer are inherited from the myapp.page.common definition, while the rest is defined inside the "concrete" definition.
Template and attribute override: when extending a definition, its template and attributes can be overridden.
- Overriding a template:
```
<definition name="myapp.homepage.alternate" extends="myapp.homepage"
    template="/layouts/alternate.jsp" />
```
  The definition has the same attributes, but its template changed. The result is that the content is the same, but the layout is different.
- Overriding attributes:
```
<definition name="myapp.homepage.customer" extends="myapp.homepage">
  <put-attribute name="menu" value="/tiles/common_menu_for_customers.jsp" />
</definition>
```
  In this case, the page will have the same appearance as the myapp.homepage definition, but its menu subpage is different.