~~ $Id$
~~
~~ Licensed to the Apache Software Foundation (ASF) under one
~~ or more contributor license agreements.  See the NOTICE file
~~ distributed with this work for additional information
~~ regarding copyright ownership.  The ASF licenses this file
~~ to you under the Apache License, Version 2.0 (the
~~ "License"); you may not use this file except in compliance
~~ with the License.  You may obtain a copy of the License at
~~
~~ http://www.apache.org/licenses/LICENSE-2.0
~~
~~ Unless required by applicable law or agreed to in writing,
~~ software distributed under the License is distributed on an
~~ "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
~~ KIND, either express or implied.  See the License for the
~~ specific language governing permissions and limitations
~~ under the License.
~~
         -----------
         Nesting and Extending Definitions
         -----------

Nesting Definitions

  Sometimes it is useful to have a structured page with, say, a structured body.
  Tipically, 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 usng 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
  {{{./nesting-extending.html#Named_subdefinitions}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.