Add proposal for template renderer

proposed/template-renderer.md

@@ -0,0 +1,209 @@
+Common Interface for Rendering Templates
+========================================
+
+This document describes a common interface for template renderers.
+
+The goal set by `TemplateRendererInterface` is to standardize how frameworks, libraries and CMSs
+render their templates so that projects are more free to use the template renderer they prefer.
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
+"SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
+interpreted as described in [RFC 2119][].
+
+The word `implementor` in this document is to be interpreted as someone
+implementing the `TemplateRendererInterface` in a [template engine][] library.
+Users of template renderer are referred to as `user`.
+The word `enduser` in this document is to be interpreted as someone using
+a library, framework, CMS created by a `user`.
+
+[RFC 2119]: http://tools.ietf.org/html/rfc2119
+
+## Goal
+
+Having common interfaces for rendering templates allows developers to create libraries that can interact with many frameworks and other libraries in a common fashion.
+
+Some examples may use the Interface:
+
+ - Content Management Systems (Sulu CMS, Drupal, Typo3, Contao CMS, ...)
+ - E-Commerce Platforms (Sylius, Spryker)
+ - Newsletter/Mail Libraries (Symfony Mailer, Abstractions over Mailchimp and other Newsletter tools) 
+ - Anything following ["Separating content from presentation"](https://en.wikipedia.org/wiki/Separation_of_content_and_presentation) where the presentation is project specific.
+
+Some examples may implement the Interface or providing a bridge:
+
+ - Blade (Laravel Framework)
+ - Laminas View
+ - Latte (Nette Framework)
+ - Smarty
+ - Sulu CMS
+ - Twig (Symfony Framework)
+ - Typo3
+
+## Definitions
+
+* **Template** - A string representation of a template.
+* **Context** - An array of context data given to the rendered template.
+* **TemplateRenderer** - A renderer which will render the template with the given context and return the rendered content as string.
+
+### Template
+
+A template MUST be a string representation of a given template supported by the template renderer. It MAY be a file path
+to the template file, but it can also be a virtual name or path supported only by a specific template renderer. The
+template is not limited by specific characters by definition but a template renderer MAY support only specific one.
+
+### Context
+
+A context MUST be an array of the available variables given to the template renderer. The array keys represent the
+variable names and MUST be strings. The array values represent the variable values and can be anything and
+.
+
+### TemplateRenderer
+
+A template renderer is a service implementing the `TemplateRendererInterface`. It MUST be responsible of rendering a 
+supported template given an OPTIONAL context. It MUST return the rendered content of the template as a string.
+If the template was not found by the template renderer, an Exception implementing the `TemplateNotFoundExceptionInterface`
+MUST be thrown. The template renderer SHALL NOT output/print/stream directly.
+
+## Usage
+
+While the implementor of `TemplateRendererInterface` MUST make sure that the template renderer behave like defined
+above. The user of `TemplateRendererInterface` MUST allow when providing a library, application, cms for an enduser
+using the Interface to give or configure the `template` used by the template renderer. This can be achieved with any
+type of injection or configuration.
+
+Basic implementor implementation:
+
+```php
+<?php
+
+use Psr\TemplateRenderer\TemplateRendererInterface;
+
+class TemplateRenderer implements TemplateRendererInterface
+{
+    public function render(string $template, array $context = []): string
+    {
+        switch ($template) {
+            case 'home.tpl';
+
+                return $this->renderTemplate('<p>{{name}}</p>', $context);
+            default: 
+                throw new TemplateNotFoundException($template);
+        }
+    }
+
+    /**
+     * @param array<string, mixed> $context
+     * @return string
+     */
+    private function renderTemplate(string $content, array $context): string
+    {
+        foreach ($context as $key => $value) {
+            $content = str_replace('{{' . $key . '}}', $context[$key], $content);
+        }
+
+        return $content;
+    }
+}
+
+use Psr\TemplateRenderer\TemplateNotFoundExceptionInterface;
+
+class TemplateNotFoundException extends \RuntimeException implements TemplateNotFoundExceptionInterface
+{
+    public function __construct(private string $template, ?\Throwable $previous = null)
+    {
+        parent::__construct('Template not found: "' . $template . '"', 0 , $previous);
+    }
+
+    public function getTemplate(): string
+    {
+        return $this->template;
+    }
+}
+```
+
+Basic user implementation:
+
+```php
+<?php
+
+use Psr\TemplateRenderer\TemplateRendererInterface;
+
+class SomeController
+{
+    public function __construct(
+        private TemplateRendererInterface $templateRenderer,
+        private string $template,
+    )
+
+    public function someAction(): string
+    {
+        $context = [/* load something */]; 
+
+        return $this->templateRenderer->render($this->template, $context);
+    }
+}
+```
+
+Example enduser implementation A:
+
+```php
+<?php
+
+$controller = new Controller($twig, '@Context/pages/mail.html.twig');
+$controller->someAction();
+```
+
+Example enduser implementation B:
+
+```php
+<?php
+
+$controller = new Controller($blade, 'mail');
+$controller->someAction();
+```
+
+Example enduser implementation C:
+
+```php
+<?php
+
+$controller = new Controller($latte, 'mail.latte');
+$controller->someAction();
+```
+
+## Interfaces
+
+### TemplateRendererInterface
+
+```php
+<?php
+
+namespace Psr\TemplateRenderer;
+
+interface TemplateRendererInterface {
+    /**
+     * Render the template with the given context data.
+     *
+     * @param string $template
+     * @param array<string, mixed> $context
+     *
+     * @return string
+     */
+    public function render(string $template, array $context = []): string;
+}
+```
+
+### TemplateNotFoundExceptionInterface
+
+```php
+<?php
+
+namespace Psr\TemplateRenderer;
+
+use Throwable;
+
+interface TemplateNotFoundExceptionInterface extends Throwable
+{
+    public function getTemplate(): string;
+}
+```