Create a post builder skin in a plugin

Question

Currently I'm working on a plugin that will display a grid based on post type with many settings to customize it. One of the settings will be to choose between different skins available to display inside the grid.

I would like to offer the possibility to developers to include their own skins with a custom HOOK/Filter.

What is the right way to add this functionality in a plugin?

I was thinking about something like this but I'm not sure if it's the right way and if it will work:

// register skin class in plugin
class skinclass {
    //existing skins
    function myclass() {

    }
    do_action('add_custom_skin');
}

// developers register action to include it's own skins (in function.php)
add_action('add_custom_skin','my_custom_skins')
function my_custom_skins($post) {

    function skin1($post) {
        $skin1  = '<article class="skin1-holder">';
        $skin1 .= '<h2 class="skin1-title">'. get_the_title() .'</h2>';
        $skin1 .= '<div class="skin1-excerpt">'. get_the_excerpt() .'</div>';
        $skin1 .= '</article>';
        return $skin1;
    }

    function skin2($post) {
        $skin2  = '<article class="skin2-holder">';
        $skin2 .= '<h2 class="skin2-title">'. get_the_title() .'</h2>';
        $skin2 .= '<div class="skin2-excerpt">'. get_the_excerpt() .'</div>';
        $skin2 .= '</article>';
        return $skin2;
    }

}

// get skin function in plugin
$skins = get_class_methods(new skinclass());
// to output skin in a drop down list
foreach ($skins as $skin_name) {
    echo "$skin_name\n";
}

// output skin in front end
$func = 'skin1';
$func();

The skin to output will be in a query_posts loop.

Answer 1

You cannot insert the do_action() inside the class like that, it's gonna give you a fatal error.

Instead you could use an array $skins to store all the skin functions (as closures), and the magic method _call() to call them:

class skinclass {

    private $skins = array();

    function __construct() {
        $this->skins = apply_filters( 'add_skin', $this->skins );
    }

    function __call( $name, $post ) {
        if(isset($this->skins[$name]))
            return $this->skins[$name]( $post );
    }

}

As you can see I used apply_filters() to be able to add skins on initializazion, and then the__call is gonna check if the skin exists and, if so, execute it.

So now to add a skin you add a filter that modifies the $skins property:

// add skin1
add_filter( 'add_skin', function( $skins ){
    $skins['skin1'] = function( $post ) {
        $skin1  = '<article class="skin1-holder">';
        $skin1 .= '<h2 class="skin1-title">'. get_the_title() .'</h2>';
        $skin1 .= '<div class="skin1-excerpt">'. get_the_excerpt() .'</div>';
        $skin1 .= '</article>';
        return $skin1;
    };
    return $skins;
});

// add skin2
add_filter( 'add_skin', function( $skins ){
    $skins['skin2'] = function( $post ) {
        $skin2  = '<article class="skin2-holder">';
        $skin2 .= '<h2 class="skin2-title">'. get_the_title() .'</h2>';
        $skin2 .= '<div class="skin2-excerpt">'. get_the_excerpt() .'</div>';
        $skin2 .= '</article>';
        return $skin2;
    };
    return $skins;
});

Wait, but what about retrieving the skins list? Well, we can add a method to the class to do that, so inside the class you can add:

function get_skins() {
    return array_keys($this->skins);
}

---

Ok so now to use the class you need to get an instance:

$skinObj = new skinclass();

And the rest is very similar to your code:

// get skin function in plugin
$skins = $skinObj->get_skins();
// to output skin in a drop down list
foreach ($skins as $skin_name) {
    echo "$skin_name\n";
}

// output skin in front end
$func = 'skin1';
echo $skinObj->$func( $post );

Answer 2

I understand, that you add the possibilties for developers to change the html, typical a template part. That is the point, that I think you should use the default functionality for this job. get_page_template() can be overridden via the page_template filter.

A simple example for the dev to change your templates from your plugin.

add_filter( 'page_template', 'wpse185512_page_template' );
function wpse185512_page_template( $page_template ) {

    if ( is_page( 'my-custom-page-slug' ) ) {
        $page_template = dirname( __FILE__ ) . '/custom-page-template.php';
    }

    return $page_template;
}

For the include in your plugin use a custom function, that include the file from a path. You find a good reference on this question. I think is not helpful to copy this here, much better you see the answers and understand the solutions.