Developing a custom ViewHelper

Developing a custom ViewHelper is often necessary in extension development. This chapter will demonstrate how to write a custom ViewHelper for the blog example extension.

The official documentation of Fluid for writing custom ViewHelpers can be found within Fluid documentation at https://github.com/TYPO3/Fluid/blob/master/doc/FLUID_CREATING_VIEWHELPERS.md.

The Example ViewHelper

"Avatar" images are pictures or icons that for example are dedicated to the author of an article in blogs or on forums. The photos of blog authors and forum moderators are mostly stored on the appropriate server. With users that only want to ask a question or to comment a blog post, this is not the case. To allow them to supply their article with an icon, a service called gravatar.com is available. This online service makes sure that an email address is assigned to a certain avatar picture.

A web application that wants to check if an avatar picture exists for a given email address has to send a checksum (with the hash function md5()) of the email address to the service and receives the picture for display.

This section explains how to write a ViewHelper that uses an email address as parameter and shows the picture from gravatar.com if it exists.

The custom ViewHelper is not part of the default distribution, therefore an own namespace import is necessary to use this ViewHelper. In the following example the namespace \MyVendor\BlogExample\ViewHelpers is imported with the prefix blog. Now, all tags starting with blog: are interpreted as ViewHelper from within this namespace:

{namespace blog=MyVendor\BlogExample\ViewHelpers}

For further information about namespace import, see Importing namespaces.

The ViewHelper should be given the name "gravatar" and only take an email address as parameter. The ViewHelper is called in the template as follows:

<blog:gravatar emailAddress="username@example.com" />

See importing-namespaces-globally for information how to import namespaces globally.

The implementation

Every ViewHelper is a PHP class. For the Gravatar ViewHelper the name of the class is \MyVendor\BlogExample\ViewHelpers\GravatarViewHelper.

Following the naming conventions for Extbase extensions the ViewHelper skeleton is created in the PHP file EXT:blog_example/Classes/ViewHelpers/GravatarViewHelper.php:

<?php
namespace MyVendor\BlogExample\ViewHelpers;

use TYPO3Fluid\Fluid\Core\Rendering\RenderingContextInterface;
use TYPO3Fluid\Fluid\Core\ViewHelper\Traits\CompileWithRenderStatic;
use TYPO3\CMS\Fluid\Core\ViewHelper\AbstractViewHelper;

class GravatarViewHelper extends AbstractViewHelper
{
   use CompileWithRenderStatic;

   public static function renderStatic(
       array $arguments,
       \Closure $renderChildrenClosure,
       RenderingContextInterface $renderingContext
   ) {
       // Implementation ...
   }
}

Every ViewHelper must inherit from the class \TYPO3Fluid\Fluid\Core\ViewHelper\AbstractViewHelper.

A ViewHelper can also inherit from subclasses of AbstractViewHelper, e.g. from \TYPO3Fluid\Fluid\Core\ViewHelper\AbstractTagBasedViewHelper. Several subclasses are offering additional functionality. The TagBasedViewHelper will be explained later on in this chapter in detail.

In addition every ViewHelper needs a method renderStatic(), which is called once the ViewHelper will be displayed in the template. The return value of the method is copied directly into the complete output. If the above example is extended like the following:

<?php
namespace MyVendor\BlogExample\ViewHelpers;

use TYPO3Fluid\Fluid\Core\Rendering\RenderingContextInterface;
use TYPO3Fluid\Fluid\Core\ViewHelper\Traits\CompileWithRenderStatic;
use TYPO3\CMS\Fluid\Core\ViewHelper\AbstractViewHelper;

class GravatarViewHelper extends AbstractViewHelper
{
   use CompileWithRenderStatic;

   public static function renderStatic(
       array $arguments,
       \Closure $renderChildrenClosure,
       RenderingContextInterface $renderingContext
   ) {
       return 'World';
   }
}

And called like this in the template:

{namespace blog=MyVendor\BlogExample\ViewHelpers}

Hello <blog:gravatar />

The displayed result will be Hello World.

Register arguments of ViewHelpers

The Gravatar ViewHelper must hand over the email address on which identifies the Gravatar. This is the last remaining piece before the implementation can be completed.

All arguments of a ViewHelper must be registered. Every ViewHelper has to declare explicitly which parameters are accepted. The registration happens inside initializeArguments():

public function initializeArguments()
{
    $this->registerArgument('emailAddress', 'string', 'The email address to resolve the gravatar for', true);
}

While this works for most use cases, there are some when arguments with a given prefix should be allowed, e.g. data- arguments. Therefore it's possible to handle additional arguments. For further information see Handle additional arguments.

This way the ViewHelper receives the argument emailAddress of type string. The type is given by the second argument of the registerTagAttribute(). The arguments are registered via $this->registerArgument($name, $type, $description, $required, $defaultValue). These arguments can be accessed through the array $arguments, which is passed into the method.

Tip

Sometimes arguments can take various types. In this case the type mixed should be used.

Finally the output of the img tag needs to be implemented:

<?php
namespace MyVendor\BlogExample\ViewHelpers;

use TYPO3Fluid\Fluid\Core\Rendering\RenderingContextInterface;
use TYPO3Fluid\Fluid\Core\ViewHelper\Traits\CompileWithRenderStatic;
use TYPO3\CMS\Fluid\Core\ViewHelper\AbstractViewHelper;

class GravatarViewHelper extends AbstractViewHelper
{
   use CompileWithRenderStatic;

   public function initializeArguments()
   {
       $this->registerArgument('emailAddress', 'string', 'The email address to resolve the gravatar for', true);
   }

   public static function renderStatic(
       array $arguments,
       \Closure $renderChildrenClosure,
       RenderingContextInterface $renderingContext
   ) {
      return '<img src="http://www.gravatar.com/avatar/' .
         md5($arguments['emailAddress']) .
         '" />';
   }
}

Escaping of Output

The above implementation still does not provide the expected result. The output for the following usage:

<blog:gravatar emailAddress="username@example.com" />

Does not result in:

<img src="http://www.gravatar.com/avatar/5f0efb20de5ecfedbe0bf5e7c12353fe" />

Instead the result is:

&lt;img src=&quot;http://www.gravatar.com/avatar/5f0efb20de5ecfedbe0bf5e7c12353fe&quot; /&gt;

By default all output is escaped by htmlspecialchars to prevent cross site scripting.

To allow unescaped HTML output, escaping has to be explicitly disabled. This is done by setting the class property $escapeOutput to false:

protected $escapeOutput = false;

To make the above implementation work, the property is set to false. The output is no longer escaped and the <img>-Tag is now rendered as expected.

If escaping of children is disabled, no nodes that are passed with inline syntax or values used as tag content will be escaped. Note that $escapeOutput takes priority: if it is disabled, escaping of child nodes is also disabled unless explicitly enabled.

protected $escapeChildren = false;

Passing in children is explained in Prepare ViewHelper for inline syntax.

Creating HTML/XML tags using TagBasedViewHelper

For ViewHelpers which create HTML/XML tags, Fluid provides an enhanced base class: \TYPO3Fluid\Fluid\Core\ViewHelper\AbstractTagBasedViewHelper. This base class provides an instance of \TYPO3Fluid\Fluid\Core\ViewHelper\TagBuilder that can be used to create HTML-tags. It takes care of the syntactically correct creation and for example escapes single and double quotes in attribute values.

Attention

Correctly escaping the attributes is mandatory as it affects security and prevents cross site scripting attacks.

The GravatarViewHelper will now make use of the TagBasedViewHelper. Because the Gravatar ViewHelper creates an img tag the use of the \TYPO3Fluid\Fluid\Core\ViewHelper\TagBuilder is advised:

<?php
namespace MyVendor\BlogExample\ViewHelpers;

use TYPO3\CMS\Fluid\Core\ViewHelper\AbstractTagBasedViewHelper;

class GravatarViewHelper extends AbstractTagBasedViewHelper
{
    protected $tagName = 'img';

    public function initializeArguments()
    {
        parent::initializeArguments();
        $this->registerUniversalTagAttributes();
        $this->registerTagAttribute('alt', 'string', 'Alternative Text for the image');
        $this->registerArgument('emailAddress', 'string', 'The email address to resolve the gravatar for', true);
    }

    public function render()
    {
        $this->tag->addAttribute(
            'src',
            'http://www.gravatar.com/avatar/' . md5($this->arguments['emailAddress'])
        );
        return $this->tag->render();
    }
}

What is different in this code?

First of all, the ViewHelper does not inherit directly from AbstractViewHelper but from TagBasedViewHelper, which provides and initializes the tag builder. Beyond that there is a class property $tagName which stores the name of the tag to be created. Furthermore the tag builder is available at $this->tag. It offers the method addAttribute() to add new tag attributes. In our example the attribute src is added to the tag, with the value assigned one line above it. Finally, the GravatarViewHelper creates an img tag builder, which has a method named render(). After configuring the tag builder instance, the rendered tag markup is returned.

Note

As $this->tag is an instance variable, render() is used to generate the output. renderStatic() would have no access. For further information take a look at The different render methods.

Notice that the attribute $escapeOutput is no longer necessary.

Furthermore the TagBasedViewHelper offers assistance for ViewHelper arguments that should recur directly and unchanged as tag attributes. These should be output directly must be registered in initializeArguments() with the method $this->registerTagAttribute($name, $type, $description, $required = false). If support for the <img> attribute alt should be provided in the ViewHelper, this can be done by initializing this in initializeArguments() in the following way:

public function initializeArguments()
{
    $this->registerTagAttribute('alt', 'string', 'Alternative Text for the image');
}

For registering the universal attributes id, class, dir, style, lang, title, accesskey and tabindex there is a helper method registerUniversalTagAttributes() available.

If support for universal attributes should be provided and in addition to the alt attribute in the Gravatar ViewHelper the following initializeArguments() method will be necessary:

public function initializeArguments()
{
    parent::initializeArguments();
    $this->registerUniversalTagAttributes();
    $this->registerTagAttribute('alt', 'string', 'Alternative Text for the image');
}

Insert optional arguments

An optional size for the image can be provided to the Gravatar ViewHelper. This size parameter will be used to determine the height and width in pixels of the image and can range from 1 to 512. When no size is given, an image of 80px is generated.

The render() method can be improved like this:

public function initializeArguments()
{
    $this->registerArgument('emailAddress', 'string', 'The email address to resolve the gravatar for', true);
    $this->registerArgument('size', 'integer', The size of the gravatar, ranging from 1 to 512', false, 80);
}

public function render()
{
    $this->tag->addAttribute(
       'src',
       'http://www.gravatar.com/avatar/' .
           md5($this->arguments['emailAddress']) .
           '?s=' . urlencode($this->arguments['size'])
    );
    return $this->tag->render();
}

With this setting of a default value and setting the fourth argument to false, the size attribute becomes optional.

Prepare ViewHelper for inline syntax

So far the Gravatar ViewHelper has focused on the tag structure of the ViewHelper. The call to render the ViewHelper was written with tag syntax which seemed obvious because it itself returns a tag:

<blog:gravatar emailAddress="{post.author.emailAddress}" />

Alternatively this expression can be written using inline notation:

{blog:gravatar(emailAddress: post.author.emailAddress)}

One should see the Gravatar ViewHelper as a kind of post processor for an email address and would allow the following syntax:

{post.author.emailAddress -> blog:gravatar()}

This syntax places focus on the variable that is passed to the ViewHelper as it comes first.

The syntax {post.author.emailAddress -> blog:gravatar()} is an alternative syntax for <blog:gravatar>{post.author.emailAddress}</blog:gravatar>. To support this the email address comes either from the argument emailAddress or, if it is empty, the content of the tag should be interpreted as email address.

This was in particular used with formatting ViewHelpers. These ViewHelpers all support both tag mode and inline syntax.

Depending on the implemented method for rendering, the implementation is different:

With renderStatic()

To fetch the content of the ViewHelper the argument $renderChildrenClosure is available. This returns the evaluated object between the opening and closing tag.

Lets have a look at the new code of the renderStatic() method:

<?php
namespace MyVendor\BlogExample\ViewHelpers;

use TYPO3Fluid\Fluid\Core\Rendering\RenderingContextInterface;
use TYPO3Fluid\Fluid\Core\ViewHelper\Traits\CompileWithRenderStatic;
use TYPO3\CMS\Fluid\Core\ViewHelper\AbstractViewHelper;

class GravatarViewHelper extends AbstractViewHelper
{
    use CompileWithContentArgumentAndRenderStatic;

    protected $escapeOutput = false;

    public function initializeArguments()
    {
        $this->registerArgument('emailAddress', 'string', 'The email address to resolve the gravatar for');
    }

    public static function renderStatic(
        array $arguments,
        \Closure $renderChildrenClosure,
        RenderingContextInterface $renderingContext
    ) {
        $emailAddress = $renderChildrenClosure();

        return '<img src="http://www.gravatar.com/avatar/' .
            md5($emailAddress) .
            '" />';
    }
}

With render()

To fetch the content of the ViewHelper the method renderChildren() is available in the AbstractViewHelper. This returns the evaluated object between the opening and closing tag.

Lets have a look at the new code of the render() method:

<?php
namespace MyVendor\BlogExample\ViewHelpers;

use TYPO3\CMS\Fluid\Core\ViewHelper\AbstractTagBasedViewHelper;

class GravatarViewHelper extends AbstractTagBasedViewHelper
{
    protected $tagName = 'img';

    public function initializeArguments()
    {
        $this->registerArgument('emailAddress', 'string', 'The email address to resolve the gravatar for', false, null);
    }

    public function render()
    {
        $emailAddress = $this->arguments['emailAddress'] ?? $this->renderChildren();

        $this->tag->addAttribute(
            'src',
            'http://www.gravatar.com/avatar/' . md5($emailAddress)
        );

        return $this->tag->render();
    }
}

Importing namespaces

Three different ways exist to import a ViewHelper namespace into Fluid. These three ways are explained in the following section.

Local namespace import via <>-Syntax

To import a ViewHelper namespace into Fluid, the following Syntax can be used:

<html xmlns:f="http://typo3.org/ns/TYPO3/CMS/Fluid/ViewHelpers"
      xmlns:blog="http://typo3.org/ns/MyVendor/BlogExample/ViewHelpers"
      data-namespace-typo3-fluid="true"
>
    <!-- Content of Fluid Template -->
</html>

In above example blog is the namespace available within the Fluid template and MyVendor\BlogExample\ViewHelpers is the PHP namespace to import into Fluid.

All ViewHelper which start with blog: will be looked up within the PHP namespace.

The <html>-Tags will not be part of the output, due to data-namespace-typo3-fluid="true" attribute.

One benefit of this approach is auto completion within modern IDEs and the possibility to lint the Fluid files.

Local namespace import via {}-Syntax

To import a ViewHelper namespace into Fluid, the following Syntax can be used:

{namespace blog=MyVendor\BlogExample\ViewHelpers}

In above example blog is the namespace available within the Fluid template and MyVendor\BlogExample\ViewHelpers is the PHP namespace to import into Fluid.

All ViewHelper which start with blog: will be looked up within the PHP namespace.

Each of the rows will result in an blank line. Multiple import statements can go into a single, or multiple lines.

Global namespace import

Fluid allows for registering namespaces. This is already done for typo3/cms-fluid and typo3fluid/fluid ViewHelpers. Therefore they are always available via the f namespace.

Custom ViewHelpers, e.g. for a Sitepackages, can be registered the same way. Namespaces are registered within $GLOBALS['TYPO3_CONF_VARS']['SYS']['fluid']['namespaces'] in the form of:

$GLOBALS['TYPO3_CONF_VARS']['SYS']['fluid']['namespaces']['blog'] => [
    'MyVendor\BlogExample\ViewHelpers',
];

In above example blog is the namespace within Fluid templates, which is resolved to the PHP namespace \MyVendor\BlogExample\ViewHelpers.

Handle additional arguments

If a ViewHelper allows further arguments then the configured one, see Register arguments of ViewHelpers, the handleAdditionalArguments() method can be implemented.

The \TYPO3Fluid\Fluid\Core\ViewHelper\AbstractTagBasedViewHelper makes use of this, to allow setting any data- argument for tag based ViewHelpers.

The method will receive an array of all arguments, which are passed in addition to the registered arguments. The array uses the argument name as key and the argument value as the value. Within the method these arguments can be handled.

E.g. the AbstractTagBasedViewHelper implements the following behaviour:

public function handleAdditionalArguments(array $arguments)
{
    $unassigned = [];
    foreach ($arguments as $argumentName => $argumentValue) {
        if (strpos($argumentName, 'data-') === 0) {
            $this->tag->addAttribute($argumentName, $argumentValue);
        } else {
            $unassigned[$argumentName] = $argumentValue;
        }
    }
    parent::handleAdditionalArguments($unassigned);
}

To keep the default behaviour, all unwanted arguments should be passed to the parent method call parent::handleAdditionalArguments($unassigned);, to throw accordingly exceptions.

The different render methods

ViewHelper can have one or multiple of the following three methods for implementing the rendering. The following section will describe the differences between all three implementations.

compile()-Method

This method can be overwritten to define how the ViewHelper should be compiled. That can make sense if the ViewHelper itself is a wrapper for another native PHP function or TYPO3 function. In that case the method can return the call to this function and remove the need to call the ViewHelper as a wrapper at all.

The compile() has to return the compiled PHP code for the ViewHelper. Also the argument $initializationPhpCode can be used to add further PHP code before the execution.

Note

The renderStatic() method has still to be implemented for the non compiled version of the ViewHelper. In the future, this should no longer be necessary.

Example implementation:

<?php
namespace MyVendor\BlogExample\ViewHelpers;

use TYPO3Fluid\Fluid\Core\Compiler\TemplateCompiler;
use TYPO3Fluid\Fluid\Core\Parser\SyntaxTree\ViewHelperNode;
use TYPO3Fluid\Fluid\Core\Rendering\RenderingContextInterface;
use TYPO3Fluid\Fluid\Core\ViewHelper\Traits\CompileWithRenderStatic;
use TYPO3\CMS\Fluid\Core\ViewHelper\AbstractViewHelper;

class StrtolowerViewHelper extends AbstractViewHelper
{
    use CompileWithRenderStatic;

    public function initializeArguments()
    {
        $this->registerArgument('string', 'string', 'The string to lowercase.', true);
    }

    public static function renderStatic(
        array $arguments,
        \Closure $renderChildrenClosure,
        RenderingContextInterface $renderingContext
    ) {
        return strtolower($arguments['string']);
    }

    public function compile(
        $argumentsName,
        $closureName,
        &$initializationPhpCode,
        ViewHelperNode $node,
        TemplateCompiler $compiler
    ) {
        return 'strtolower(' . $argumentsName. '[\'string\'])';
    }
}

renderStatic()-Method

Most of the time this method is implemented. It's the one that is called by default from within the compiled Fluid.

It is however not called on AbstractTagBasedViewHelper implementations - on such classes you still need to use the render() method since that is the only way you can access $this->tag which contains the tag builder that generates the actual XML tag.

As this method has to be static, there is no access to instance attributes, e.g. $this->tag within an subclass of AbstractTagBasedViewHelper.

Note

This method can not be used when access to child nodes is necessary. This is the case for ViewHelpers like if or switch which need to access their childs like then or else. In that case, render() has to be used.

render()-Method

This method is the slowest one. As the compiled version always calls the renderStatic() method, this will call further PHP code which, in the end, will call the original render() method.

By using this method, the surrounding logic does not get compiled.

Note

This way is planned to be removed in the future.