Extending the ExpressionLanguage
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Extending the ExpressionLanguage
The ExpressionLanguage can be extended by adding custom functions. For instance, in the Symfony Framework, the security has custom functions to check the user's role.
Note
If you want to learn how to use functions in an expression, read "The Expression Syntax".
Registering Functions
Functions are registered on each specific ExpressionLanguage instance.
That means the functions can be used in any expression executed by that
instance.
To register a function, use register(). This method has 3 arguments:
- name - The name of the function in an expression;
- compiler - A function executed when compiling an expression using the function;
- evaluator - A function executed when the expression is evaluated.
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use Symfony\Component\ExpressionLanguage\ExpressionLanguage;
$expressionLanguage = new ExpressionLanguage();
$expressionLanguage->register('lowercase', function ($str) {
return sprintf('(is_string(%1$s) ? strtolower(%1$s) : %1$s)', $str);
}, function ($arguments, $str) {
if (!is_string($str)) {
return $str;
}
return strtolower($str);
});
var_dump($expressionLanguage->evaluate('lowercase("HELLO")'));
// this will print: helloIn addition to the custom function arguments, the evaluator is passed an
arguments variable as its first argument, which is equal to the second
argument of compile() (e.g. the "values" when evaluating an expression).
Using Expression Providers
When you use the ExpressionLanguage class in your library, you often want
to add custom functions. To do so, you can create a new expression provider by
creating a class that implements
ExpressionFunctionProviderInterface.
This interface requires one method: getFunctions(), which returns an array of expression functions (instances of ExpressionFunction) to register:
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use Symfony\Component\ExpressionLanguage\ExpressionFunction;
use Symfony\Component\ExpressionLanguage\ExpressionFunctionProviderInterface;
class StringExpressionLanguageProvider implements ExpressionFunctionProviderInterface
{
public function getFunctions()
{
return array(
new ExpressionFunction('lowercase', function ($str) {
return sprintf('(is_string(%1$s) ? strtolower(%1$s) : %1$s)', $str);
}, function ($arguments, $str) {
if (!is_string($str)) {
return $str;
}
return strtolower($str);
}),
);
}
}You can register providers using registerProvider() or by using the second argument of the constructor:
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use Symfony\Component\ExpressionLanguage\ExpressionLanguage;
// using the constructor
$expressionLanguage = new ExpressionLanguage(null, array(
new StringExpressionLanguageProvider(),
// ...
));
// using registerProvider()
$expressionLanguage->registerProvider(new StringExpressionLanguageProvider());Tip
It is recommended to create your own ExpressionLanguage class in your
library. Now you can add the extension by overriding the constructor:
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use Symfony\Component\ExpressionLanguage\ExpressionLanguage as BaseExpressionLanguage;
use Symfony\Component\ExpressionLanguage\ParserCache\ParserCacheInterface;
class ExpressionLanguage extends BaseExpressionLanguage
{
public function __construct(ParserCacheInterface $parser = null, array $providers = array())
{
// prepends the default provider to let users override it easily
array_unshift($providers, new StringExpressionLanguageProvider());
parent::__construct($parser, $providers);
}
}
