VLS_README.txt - velocity-tools - Git at Google

 The quick and dirty guide to VLS (VelocityLayoutServlet)
 --------------------------------------------------------


 VLS - Objectives:

 1. Provide easy layout control for velocity-tools based projects
 2. Provide customizable error screens for said projects


 VLS - Setup:

 This is an extension of the VelocityViewServlet, so to use
 it instead of the VVS, just change the servlet-class value of
 your web.xml entry for the velocity servlet to:

 <servlet-class>org.apache.velocity.tools.view.VelocityLayoutServlet</servlet-class>

 That means the full entry will be something like:

   <!-- Define Velocity template compiler -->
   <servlet>
     <servlet-name>velocity</servlet-name>
     <servlet-class>org.apache.velocity.tools.view.VelocityLayoutServlet</servlet-class>
     <load-on-startup>10</load-on-startup>
   </servlet>


 If you enjoy configuring filenames and paths and would rather
 not conform to the defaults, then you'll
 want to know about these:

 tools.view.servlet.error.template
 tools.view.servlet.layout.directory
 tools.view.servlet.layout.default.template

 These are the velocity.properties keys
 for configuring the behaviour of this dservlet.
 the first specifies the filepath of the error template
 relative to your webapp's root directory.
 the second specifies the directory in which you will be placing
 your layout templates.
 the third specifies the filepath of the default layout template
 relative to the layout directory, NOT relative to the
 root directory of your webapp!

 If you do not add any of these keys/values to your velocity.properties,
 that will result in the equivalent of putting these default values in:

 tools.view.servlet.error.template = Error.vm
 tools.view.servlet.layout.directory = layout/
 tools.view.servlet.layout.default.template = Default.vm


 VLS - Layouts:

 In your layout templates, the only thing you really need is the
 screen content reference.  So an acceptable layout template could be
 just

 $screen_content

 ...but you'll probably want to do something along these lines:

 <html>
 <head>
   <title>$!page_title</title>
 </head>
 <body>

 $screen_content

 </body>
 </html>

 This saves you the trouble of doing the basic <html>,<head>, and <body>
 tags in every single screen.  That's the point of layouts: to save effort
 and eliminate redundancy.  Note that this still lets the inner screen
 control the title of the page.  This works because the layout template
 is blessed by the VLS with access to the same context as the screen *after*
 the screen is done with it. Just do a #set( $page_title = "Hello" ) in the
 screen.


                     Alternative Layouts

 VLS provides two ways to specify an alternate template for a requested page:

 1. Specify the layout in the request parameters

     Just add the query string "layout=MyOtherLayout.vm" to any request params
     and the VLS will find it and render your screen within that layout instead
     of the default layout.  It don't matter how you get the layout param into
     the query data, only that it's there.  If you're using the LinkTool, the
     most common will likely be
         <a href="$link.relative('MyScreen.vm').param('layout','MyOtherLayout.vm')">

     but form post data will work just as well.


 2. Specify the layout in the requested screen.

     In the requested screen, put a line like this:
     #set( $layout = "MyOtherLayout.vm" )

     This will direct the VLS to use "MyOtherLayout.vm" instead of
     "Default.vm".  *Setting the layout in this fashion will
     override any layout set by the request parameters.*


                     Navigations, Tiles, and How

 Those who are (or were) Turbine or Struts users will probably want to
 do more than just set the layout and screen content, and include
 arbitrary "tiles" or "navigations" in the layout.  Thanks to Velocity's built-in
 #parse directive, this is easy.

 First, create your "tile" as a separate template file like:

 <div id="footer">It's a footer!</div>

 Now, assuming that this code is in a file named "Footer.vm"
 located in the root of the webapp like any other non-layout templates.
 You can include the footer like this:

 <html>
 <head>
   <title>$!page_title</title>
 </head>
 <body>

 $screen_content

 #parse('Footer.vm')

 </body>
 </html>

 If you have a lot of different "footer" files and you want your screen
 to decide which one will be used, do something like this:

 <html>
 <head>
   <title>$!page_title</title>
 </head>
 <body>

 $screen_content

 #parse( $screen_footer )

 </body>
 </html>

 and in your screen, just do #set( $screen_header = 'FooFooter.vm' ).

 Remember, your #parsed footer template will have access to the same
 velocity context as your layout, which gets the screen's context
 once the screen is done with it.  This lets you set variables for
 the layout and footer to use from your screens.


 VLS - Error screen:

 If an uncaught exception or error is thrown
 at some point during the processing of your screen and layout, the error() method
 of the VLS is called.  This overrides the default error() method of the VelocityServlet
 to render a template instead of hardcoded html.

 This error screen will be rendered within a layout under the same rules as any other
 screen, and will have the following values placed in its context to help you debug
 the error:

 $error_cause

 and

 $stack_trace


 Their values are pretty much what you'd expect.  "$error_cause" is the java.lang.Throwable
 that was thrown.   "$stack_trace" is the captured output of $cause.printStackTrace().

 In the event that a MethodInvocationException is behind the calling of error(),
 the root cause is extracted from it and dealt with as described above.  But, since
 template reference behavior is partly at fault here, the VLS will also add the
 MethodInvocationException itself to the context as $invocation_exception.  This
 allows you to discover the reference and method call that triggered the root cause.
 To get those, do something like this in your error template:

 #if( $invocation_exception )
     oh joy! it's a MethodInvocationException!

     Message: $invocation_exception.message
     Reference name: $invocation_exception.referenceName
     Method name: $invocation_exception.methodName
 #end
	The quick and dirty guide to VLS (VelocityLayoutServlet)
	--------------------------------------------------------


	VLS - Objectives:

	1. Provide easy layout control for velocity-tools based projects
	2. Provide customizable error screens for said projects


	VLS - Setup:

	This is an extension of the VelocityViewServlet, so to use
	it instead of the VVS, just change the servlet-class value of
	your web.xml entry for the velocity servlet to:

	<servlet-class>org.apache.velocity.tools.view.VelocityLayoutServlet</servlet-class>

	That means the full entry will be something like:

	<!-- Define Velocity template compiler -->
	<servlet>
	<servlet-name>velocity</servlet-name>
	<servlet-class>org.apache.velocity.tools.view.VelocityLayoutServlet</servlet-class>
	<load-on-startup>10</load-on-startup>
	</servlet>


	If you enjoy configuring filenames and paths and would rather
	not conform to the defaults, then you'll
	want to know about these:

	tools.view.servlet.error.template
	tools.view.servlet.layout.directory
	tools.view.servlet.layout.default.template

	These are the velocity.properties keys
	for configuring the behaviour of this dservlet.
	the first specifies the filepath of the error template
	relative to your webapp's root directory.
	the second specifies the directory in which you will be placing
	your layout templates.
	the third specifies the filepath of the default layout template
	relative to the layout directory, NOT relative to the
	root directory of your webapp!

	If you do not add any of these keys/values to your velocity.properties,
	that will result in the equivalent of putting these default values in:

	tools.view.servlet.error.template = Error.vm
	tools.view.servlet.layout.directory = layout/
	tools.view.servlet.layout.default.template = Default.vm


	VLS - Layouts:

	In your layout templates, the only thing you really need is the
	screen content reference. So an acceptable layout template could be
	just

	$screen_content

	...but you'll probably want to do something along these lines:

	<html>
	<head>
	<title>$!page_title</title>
	</head>
	<body>

	$screen_content

	</body>
	</html>

	This saves you the trouble of doing the basic <html>,<head>, and <body>
	tags in every single screen. That's the point of layouts: to save effort
	and eliminate redundancy. Note that this still lets the inner screen
	control the title of the page. This works because the layout template
	is blessed by the VLS with access to the same context as the screen after
	the screen is done with it. Just do a #set( $page_title = "Hello" ) in the
	screen.


	Alternative Layouts

	VLS provides two ways to specify an alternate template for a requested page:

	1. Specify the layout in the request parameters

	Just add the query string "layout=MyOtherLayout.vm" to any request params
	and the VLS will find it and render your screen within that layout instead
	of the default layout. It don't matter how you get the layout param into
	the query data, only that it's there. If you're using the LinkTool, the
	most common will likely be
	<a href="$link.relative('MyScreen.vm').param('layout','MyOtherLayout.vm')">

	but form post data will work just as well.


	2. Specify the layout in the requested screen.

	In the requested screen, put a line like this:
	#set( $layout = "MyOtherLayout.vm" )

	This will direct the VLS to use "MyOtherLayout.vm" instead of
	"Default.vm". *Setting the layout in this fashion will
	override any layout set by the request parameters.*


	Navigations, Tiles, and How

	Those who are (or were) Turbine or Struts users will probably want to
	do more than just set the layout and screen content, and include
	arbitrary "tiles" or "navigations" in the layout. Thanks to Velocity's built-in
	#parse directive, this is easy.

	First, create your "tile" as a separate template file like:

	<div id="footer">It's a footer!</div>

	Now, assuming that this code is in a file named "Footer.vm"
	located in the root of the webapp like any other non-layout templates.
	You can include the footer like this:

	<html>
	<head>
	<title>$!page_title</title>
	</head>
	<body>

	$screen_content

	#parse('Footer.vm')

	</body>
	</html>

	If you have a lot of different "footer" files and you want your screen
	to decide which one will be used, do something like this:

	<html>
	<head>
	<title>$!page_title</title>
	</head>
	<body>

	$screen_content

	#parse( $screen_footer )

	</body>
	</html>

	and in your screen, just do #set( $screen_header = 'FooFooter.vm' ).

	Remember, your #parsed footer template will have access to the same
	velocity context as your layout, which gets the screen's context
	once the screen is done with it. This lets you set variables for
	the layout and footer to use from your screens.


	VLS - Error screen:

	If an uncaught exception or error is thrown
	at some point during the processing of your screen and layout, the error() method
	of the VLS is called. This overrides the default error() method of the VelocityServlet
	to render a template instead of hardcoded html.

	This error screen will be rendered within a layout under the same rules as any other
	screen, and will have the following values placed in its context to help you debug
	the error:

	$error_cause

	and

	$stack_trace


	Their values are pretty much what you'd expect. "$error_cause" is the java.lang.Throwable
	that was thrown. "$stack_trace" is the captured output of $cause.printStackTrace().

	In the event that a MethodInvocationException is behind the calling of error(),
	the root cause is extracted from it and dealt with as described above. But, since
	template reference behavior is partly at fault here, the VLS will also add the
	MethodInvocationException itself to the context as $invocation_exception. This
	allows you to discover the reference and method call that triggered the root cause.
	To get those, do something like this in your error template:

	#if( $invocation_exception )
	oh joy! it's a MethodInvocationException!

	Message: $invocation_exception.message
	Reference name: $invocation_exception.referenceName
	Method name: $invocation_exception.methodName
	#end