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Hacking Twig

Twig is very extensible and you can easily hack it. Keep in mind that you should probably try to create an extension before hacking the core, as most features and enhancements can be done with extensions. This chapter is also useful for people who want to understand how Twig works under the hood.

How Twig works?

The rendering of a Twig template can be summarized into four key steps:

  • Load the template: If the template is already compiled, load it and go to the evaluation step, otherwise:
    • First, the lexer tokenizes the template source code into small pieces for easier processing;
    • Then, the parser converts the token stream into a meaningful tree of nodes (the Abstract Syntax Tree);
    • Eventually, the compiler transforms the AST into PHP code;
  • Evaluate the template: It basically means calling the display() method of the compiled template and passing it the context.

The Lexer

The Twig lexer goal is to tokenize a source code into a token stream (each token is of class Token, and the stream is an instance of Twig_TokenStream). The default lexer recognizes nine different token types:

  • Twig_Token::TEXT_TYPE
  • Twig_Token::BLOCK_START_TYPE
  • Twig_Token::VAR_START_TYPE
  • Twig_Token::BLOCK_END_TYPE
  • Twig_Token::VAR_END_TYPE
  • Twig_Token::NAME_TYPE
  • Twig_Token::NUMBER_TYPE
  • Twig_Token::STRING_TYPE
  • Twig_Token::OPERATOR_TYPE
  • Twig_Token::EOF_TYPE

You can manually convert a source code into a token stream by calling the tokenize() of an environment:

$stream = $twig->tokenize($source, $identifier);

As the stream has a __toString() method, you can have a textual representation of it by echoing the object:

echo $stream."\n";

Here is the output for the Hello {{ name }} template:

TEXT_TYPE(Hello )
VAR_START_TYPE()
NAME_TYPE(name)
VAR_END_TYPE()
EOF_TYPE()

You can change the default lexer use by Twig (Twig_Lexer) by calling the setLexer() method:

$twig->setLexer($lexer);

Lexer classes must implement the Twig_LexerInterface:

interface Twig_LexerInterface
{
    /**
     * Tokenizes a source code.
     *
     * @param  string $code     The source code
     * @param  string $filename A unique identifier for the source code
     *
     * @return Twig_TokenStream A token stream instance
     */
    function tokenize($code, $filename = 'n/a');
}

The Parser

The parser converts the token stream into an AST (Abstract Syntax Tree), or a node tree (of class Twig_Node_Module). The core extension defines the basic nodes like: for, if, ... and the expression nodes.

You can manually convert a token stream into a node tree by calling the parse() method of an environment:

$nodes = $twig->parse($stream);

Echoing the node object gives you a nice representation of the tree:

echo $nodes."\n";

Here is the output for the Hello {{ name }} template:

Twig_Node_Module(
  Twig_Node_Text(Hello )
  Twig_Node_Print(
    Twig_Node_Expression_Name(name)
  )
)

The default parser (Twig_TokenParser) can be also changed by calling the setParser() method:

$twig->setParser($parser);

All Twig parsers must implement the Twig_ParserInterface:

interface Twig_ParserInterface
{
    /**
     * Converts a token stream to a node tree.
     *
     * @param  Twig_TokenStream $stream A token stream instance
     *
     * @return Twig_Node_Module A node tree
     */
    function parser(Twig_TokenStream $code);
}

The Compiler

The last step is done by the compiler. It takes a node tree as an input and generates PHP code usable for runtime execution of the templates. The default compiler generates PHP classes to ease the implementation of the template inheritance feature.

You can call the compiler by hand with the compile() method of an environment:

$php = $twig->compile($nodes);

The compile() method returns the PHP source code representing the node.

The generated template for a Hello {{ name }} template reads as follows:

/* Hello {{ name }} */
class __TwigTemplate_1121b6f109fe93ebe8c6e22e3712bceb extends Twig_Template
{
    public function display($context)
    {
        $this->env->initRuntime();

        // line 1
        echo "Hello ";
        echo (isset($context['name']) ? $context['name'] : null);
    }
}

As for the lexer and the parser, the default compiler (Twig_Compiler) can be changed by calling the setCompiler() method:

$twig->setCompiler($compiler);

All Twig compilers must implement the Twig_CompilerInterface:

interface Twig_CompilerInterface
{
    /**
     * Compiles a node.
     *
     * @param  Twig_Node $node The node to compile
     *
     * @return Twig_Compiler The current compiler instance
     */
    function compile(Twig_Node $node);

    /**
     * Gets the current PHP code after compilation.
     *
     * @return string The PHP code
     */
    function getSource();
}
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