Smarty plugins

To create plugin Smarty in Thelia, just create a new class that extend TheliaSmarty\Template\AbstractSmartyPlugin, then create function called getPluginDescriptors that lists and describes all the plugin you want to create (you can create as many plugins as you want in one class).
A smarty plugin is described like this :

new SmartyPluginDescriptor(
    'function',
    'myFunction',
    $this,
    'doMyFunction'
)

First parameter is the type of the plugin, more information here.
Second parameter is the name you have to put in your templates to call your plugin.
Third parameter is location of the Class of your function, in general it's $this because you write function in the same class as the declaration.
Fourth parameter is the name of the function you want to call.

Declaration of your plugins :

class MyProjectPlugin extends AbstractSmartyPlugin
{
    public function getPluginDescriptors()
    {
        return [
            new SmartyPluginDescriptor(
                'function',
                'lower',
                 $this,
                'lowerString'
            ),
            new SmartyPluginDescriptor(
                'function',
                'bestProduct',
                $this,
                'getBestProduct'
            )
        ];
    }
    
    public function lowerString($params)
    {
        return strtolower($params['string']);
    }
    
    public function getBestProduct($params)
    {
        // Do a query to find your best product and return it
    }
}

Usage :

    {lower string="A String THAT need to be LOWERCASE"} // Some plugins needs params
    
    {bestProduct} // Other don't