The template engine

时间：2024-06-16 18:37:26

Play has an efficient templating system which allows to dynamically generate HTML, XML, JSON or any text-based formatted document. The template engine uses Groovy as an expression language. A tag system allows you to create reusable functions.

Templates are stored in the app/views directory.

Template syntax

A template file is a text file, some parts of which have placeholders for dynamically generated content. The template’s dynamic elements are written using the Groovy language. Groovy’s syntax is very close to Java’s.

Dynamic elements are resolved during template execution. The rendered result is then sent as part of the HTTP response.

Expressions: ${…}

The simplest way to make a dynamic element is to declare an expression. The syntax used here is${…}. The result of evaluating the expression is inserted in place of the expression.

For example:

<h1>Client ${client.name}</h1>

If you can’t be sure of client being null, there is a Groovy shortcut:

<h1>Client ${client?.name}</h1>

Which will only display the client name if the client is not null.

Template decorators : #{extends /} and #{doLayout /}

Decorators provide a clean solution to share a page layout (or design) across several templates.

Use #{get} and #{set} tags to share variables between the template and the decorator.

Embedding a page in a decorator is the matter of a one liner:

#{extends 'simpledesign.html' /}

#{set title:'A decorated page' /}

This content will be decorated.

The decorator : simpledesign.html

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">

<head>

  <title>#{get 'title' /}</title>

  <link rel="stylesheet" type="text/css" href="@{'/public/stylesheets/main.css'}" />

</head>

<body>

  <h1>#{get 'title' /}</h1>

  #{doLayout /}

  <div class="footer">Built with the play! framework</div>

</body>

</html>

Tags: #{tagName /}

A tag is a template fragment that can be called with parameters. If the tag has only one parameter, by convention it is called “arg” and its name can be omitted.

For example, this tag inserts a SCRIPT tag to load a JavaScript file:

#{script 'jquery.js' /}

A tag has to be closed, either directly or by an end tag:

#{script 'jquery.js' /}

or

#{script 'jquery.js'}#{/script}

For example the list tag allows iteration over any collection. It takes two mandatory parameters:

<h1>Client ${client.name}</h1>

<ul>

    #{list items:client.accounts, as:'account' }

        <li>${account}</li>

    #{/list}

</ul>

All dynamic expressions are escaped by the template engine to avoid XSS security issues in your application. So the title variable containing <h1>Title</h1> is now escaped:

${title} --> &lt;h1&gt;Title&lt;/h1&gt;

If you really want to display it in an unescaped way, you need to explicitely call the raw() method:

${title.raw()} --> <h1>Title</h1>

Also, if you want to display a large part of raw HTML, you can use the #{verbatim /} tag:

#{verbatim}

    ${title} --> <h1>Title</h1>

#{/verbatim}

Actions: @{…} or @@{…}

You can use the Router to (reverse) generate a URL corresponding to a specified route. From a template you can use the special @{…} syntax to do that.

For example:

<h1>Client ${client.name}</h1>

<p>

   <a href="@{Clients.showAccounts(client.id)}">All accounts</a>

</p>

<hr />

<a href="@{Clients.index()}">Back</a>

The @@{…} syntax does the same but generates an absolute URL (notably useful for e-mail, …).

Messages: &{…}

If your application needs internationalization you can use the &{…} syntax to display an internationalized message:

For example in the files conf/messages we specify:

clientName=The client name is %s

To display this message in a template, simply use:

<h1>&{'clientName', client.name}</h1>

Comment: *{…}*

Comments aren’t evaluated by the template engine. They are just comments…

*{**** Display the user name ****}*

<div class="name">

    ${user.name}

</div>

Scripts: %{…}%

A script is a more complicated set of expressions. A script can declare some variables and define some statements. Use the {…} syntax to insert a script.

%{

   fullName = client.name.toUpperCase()+' '+client.forname;

}%

<h1>Client ${fullName}</h1>

A script can write dynamic content directly using the out object:

%{

   fullName = client.name.toUpperCase()+' '+client.forname;

   out.print('<h1>'+fullName+'</h1>');

}%

You can use a script to create a structure such as an iteration in your template:

<h1>Client ${client.name}</h1>

<ul>

%{

     for(account in client.accounts) {

}%

     <li>${account}</li>

%{

     }

}%

</ul>

Bear in mind that a template is not a place to do complex things. So, use a tag when you can, or move the computations into the controller or the model object.

Template inheritance

A template can inherit another template, i.e. it can be included as a part of an other template.

To inherit another template, use the extends tag:

#{extends 'main.html' /}

<h1>Some code</h1>

The main.html template is a standard template, but it uses the doLayout tag to include the content:

<h1>Main template</h1>

<div id="content">

    #{doLayout /}

</div>

Custom template tags

You can easily create specific tags for your application. A tag is a simple template file, stored in theapp/views/tags directory. The template’s file name is used as the tag name.

To create a hello tag, just create the app/views/tags/hello.html file.

Hello from tag!

No need to configure anything. You can use the tag directly:

#{hello /}

Retrieve tag parameters

Tag parameters are exposed as template variables. The variable names are constructed with the ‘_’ character prepended to the parameter name.

For example:

Hello ${_name} !

And you can pass the name parameter to the tag:

#{hello name:'Bob' /}

If your tag has only one parameter, you can use the fact than the default parameter name is arg and that its name is implicit.

Example:

Hello ${_arg}!

And you can call it easily using:

#{hello 'Bob' /}

Invoke tag body

If your tag supports a body, you can include it at any point in the tag code, using the doBody tag.

For example:

Hello #{doBody /}!

And you can then pass the name as tag body:

#{hello}

   Bob

#{/hello}

Format-specific tags

You can have different versions of a tag for different content types and Play will select the appropriate tag. For example, Play will use the app/views/tags/hello.html tag when request.format is html, andapp/views/tags/hello.xml when the format is xml.

Whatever the content type, Play will fall back to the .tag extension if a format-specific tag is not available, e.g. app/views/tags/hello.tag.

Custom Java tags

You can also define custom tags in Java code. Similarly to how JavaExtensions work by extending theplay.templates.JavaExtensions class, to create a FastTag you need to create a method in a class that extends play.templates.FastTags. Each method that you want to execute as a tag must have the following method signature.

public static void _tagName(Map<?, ?> args, Closure body, PrintWriter out,

   ExecutableTemplate template, int fromLine)

Note the underscore before the tag name.

To understand how to build an actual tag, let’s look at two of the built-in tags.

For example, the verbatim tag is implemented by a one-line method that simply calls the toString method on the JavaExtensions, and passes in the body of the tag.

public static void _verbatim(Map<?, ?> args, Closure body, PrintWriter out,

   ExecutableTemplate template, int fromLine) {

   out.println(JavaExtensions.toString(body));

}

The body of the tag would be anything between the open and close tag. So

<verbatim>My verbatim</verbatim>

The body value would be

My verbatim

The second example is the option tag, which is slightly more complex because it relies on a parent tag to function.

public static void _option(Map<?, ?> args, Closure body, PrintWriter out,

      ExecutableTemplate template, int fromLine) {

   Object value = args.get("arg");

   Object selection = TagContext.parent("select").data.get("selected");

   boolean selected = selection != null && value != null

      && selection.equals(value);

   out.print("<option value=\"" + (value == null ? "" : value) + "\" "

      + (selected ? "selected=\"selected\"" : "")

      + "" + serialize(args, "selected", "value") + ">");

   out.println(JavaExtensions.toString(body));

   out.print("</option>");

}

This code works by outputting an HTML option tag, and sets the selected value by checking which value is selected from the parent tag. The first three lines set variables for use in the output output. Then, the final three lines output the result of the tag.

There are many more examples in the source code for the built in tags, with varying degrees of complexity. See FastTags.java in github.

Tag namespaces

To ensure that your tags do not conflict between projects, or with the core Play tags, you can set up namespaces, using the class level annotation @FastTags.Namespace.

So, for a hello tag, in a my.tags namespace, you would do the following

@FastTags.Namespace("my.tags")

public class MyFastTag extends FastTags {

   public static void _hello (Map<?, ?> args, Closure body, PrintWriter out,

      ExecutableTemplate template, int fromLine) {

      ...

   }

}

and then in your templates, you would reference the hello tag as

#{my.tags.hello/}

Java object extensions in templates

When you use your Java object within the template engine, new methods are added to it. These methods don’t exist in the original Java class and are dynamically added by the template engine.

For example, to allow easy number formatting in a template, a format method is added tojava.lang.Number.

It’s then very easy to format a number:

<ul>

#{list items:products, as:'product'}

    <li>${product.name}. Price: ${product.price.format('## ###,00')} €</li>

#{/list}

</ul>

The same applies to java.util.Date.

The Java extensions manual page lists the available methods, by type.

Create custom extensions

Your project may have specific formatting needs, in which case you can provide your own extensions.

You only need to create a Java class extending play.templates.JavaExtensions.

For instance, to provide a custom currency formatter for a number:

package ext;

import play.templates.JavaExtensions;

public class CurrencyExtensions extends JavaExtensions {

  public static String ccyAmount(Number number, String currencySymbol) {

     String format = "'"+currencySymbol + "'#####.##";

     return new DecimalFormat(format).format(number);

  }

}

Each extension method is a static method and should return a java.lang.String to be written back in the page. The first parameter will hold the enhanced object.

Use your formatter like this:

<em>Price: ${123456.324234.ccyAmount()}</em>

Template extension classes are automatically detected by Play at start-up. You just have to restart your application to make them available.

Implicit objects available in a template

All objects added to the renderArgs scope are directly injected as template variables.

For instance, to inject a ‘user’ bean into the template from a controller:

renderArgs.put("user", user );

When you render a template from an action, the framework also adds implicit objects:

Variable Description API documentation See also
errors Validation errors play.data.validation.Validation.errors() Validating HTTP data
flash Flash scope play.mvc.Scope.Flash Controllers - Session & Flash Scope
lang The current language play.i18n.Lang Setting up I18N - Define languages
messages The messages map play.i18n.Messages Setting up I18N - Externalize messages
out The output stream writer java.io.PrintWriter  
params Current parameters play.mvc.Scope.Params Controllers - HTTP parameters
play Main framework class play.Play  
request The current HTTP request play.mvc.Http.Request  
session Session scope play.mvc.Scope.Session Controllers - Session & Flash Scope

In addition to the list above the names owner, delegate and it are reserved in Groovy and shouldn’t be used as variable names in templates.

Next: Form data validation

相关文章