Underpin WordPress Framework

The goal of Underpin is to provide a pattern that makes building scaleable WordPress plugins and themes easier. It provides support for useful utilities that plugins need as they mature, such as a solid error logging utility, a batch processor for upgrade routines, and a decision tree class that makes extending and debugging multi-layered decisions way easier than traditional WordPress hooks.

Installation

Underpin can be installed in any place you can write code for WordPress, including:

As a part of a WordPress plugin.
As a part of a WordPress theme.
As a part of a WordPress must-use plugin.

Via Composer

composer require underpin/underpin

Note This will add Underpin as a mu-plugin, but due to how WordPress handles must-use plugins, this does not actually add the plugin to your site. You must also manually require the file in a mu-plugin PHP file:

<?php

if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

// Load Underpin, and its dependencies.
$autoload = plugin_dir_path( __FILE__ ) . 'vendor/autoload.php';

require_once( $autoload );

Manually

If you're developing Underpin directly, or simply don't want to use Composer, follow these steps to use:

Clone this repository, preferably in the mu-plugins directory.
Require Underpin.php, preferably as a mu-plugin.

Boilerplates

Check out the Theme and Plugin boilerplates that use Underpin. This will give you some examples on how Underpin can be used, and also provide you with a good starting point for your next project.

Local Dev Environment With Underpin

Along with these options, I also put together a local development environment using Docker Compose and Composer with Underpin. We use this for our custom site builds at DesignFrame, and it works pretty well. You can learn more about that here.

Minimum Requirements

WordPress 5.1 or better.
PHP 7.0 or better.

Upgrading From 1.*

Change `Middleware` to `Observer` pattern

Middleware has been changed to use the Observer pattern API.

// Underpin 1.*
plugin_name()->scripts()->add( 'test', [
	'handle'      => 'test',
	'src'         => 'path/to/script/src',
	'name'        => 'test',
	'description' => 'The description',
	'middlewares' => [
		'Underpin_Rest_Middleware\Factories\Rest_Middleware', // Will localize script params.
		'Underpin_Scripts\Factories\Enqueue_Script',          // Will enqueue the script on the front end all the time.
		[                                                     // Will instantiate an instance of Script_Middleware_Instance using the provided arguments
			'do_actions_callback' => function ( \Underpin_Scripts\Abstracts\Script $loader_item ) {
				// Do actions
			},
		],
	],
] );

// Underpin 2.*
plugin_name()->scripts()->add( 'test', [
	'handle'      => 'test',
	'src'         => 'path/to/script/src',
	'name'        => 'test',
	'description' => 'The description',
	'middlewares' => [
		'Underpin_Rest_Middleware\Factories\Rest_Middleware', // Will localize script params.
		'Underpin_Scripts\Factories\Enqueue_Script',          // Will enqueue the script on the front end all the time.
         new Observer( 'custom_middleware', [                 // A custom middleware action
             'update'   => function ( $class, $accumulator ) {
                 // Do an action when this script is set-up.
             },
         ] ),
	],
] );

Decision List Changes

The Decision List loader is no-longer compatible with Underpin. Instead, and has been replaced by the observer pattern.

// Underpin 1.*
plugin_name()->decision_lists()->add( 'example_decision_list', [
	// Decision one
	[
		'valid_callback'         => '__return_true',
		'valid_actions_callback' => '__return_empty_string',
		'name'                   => 'Test Decision',
		'description'            => 'A single decision',
		'priority'               => 500,
	],

	// Decision two
	[
		'valid_callback'         => '__return_true',
		'valid_actions_callback' => '__return_empty_array',
		'name'                   => 'Test Decision Two',
		'description'            => 'A single decision',
		'priority'               => 1000,
	],
] );

// Underpin 2.*
plugin_name()->loader_name()->attach( 'decision_id', [
	// Decision one
    new Observer( 'decision', [                 // A custom middleware action
        'update'   => function ( $class, $accumulator ) {
            // Condition in-which this should run
            if($condition){
              // Update the accumulator state. This sets the value when the decision is returned
              $accumulator->set_state()
            }
        },
        'priority' => 10, // Optionally set a priority to determine when this runs. Observers are sorted by deps, and then priority after.
        'deps'     => ['observer_key', 'another_observer_key'] // list of decisions that should be checked BEFORE this one.
    ] ),

	// Decision two
    new Observer( 'decision_two', [                 // A custom middleware action
        'update'   => function ( $class, $accumulator ) {
            // Condition in-which this should run
            if($condition){
              // Update the accumulator state. This sets the value when the decision is returned
              $accumulator->set_state()
            }
        },
        'priority' => 10, // Optionally set a priority to determine when this runs. Observers are sorted by deps, and then priority after.
        'deps'     => ['observer_key', 'another_observer_key'] // list of decisions that should be checked BEFORE this one.
    ] ),
] );

All Underpin-specific Hooks now use `apply`

Any of the internal Underpin hooks, like underpin\init, have been replaced with a notify call. Due to this, you must migrate existing calls to add_action('underpin_call') to use apply('new_hook')

If this is impractical, or impossible, you can connect a hook to the legacy action like so:

// Apply legacy action to new observer pattern
plugin_name()->attach('new_hook_id', new Observer( 'init_action_call', [
        'update'   => function ( $class, $accumulator ) {
          do_action('old_hook_id', $args, $passed, $to, $original );
        },
        'priority' => 1
    ] ),)

Replace any custom loaders to use `class` instead of `registry`

Any loader call that used the registry argument now must use class

// Underpin 1.*
plugin_name()->loaders()->add('name',['registry' => 'Loader_Class']);

// Underpin 2.*
plugin_name()->loaders()->add('name',['class' => 'Loader_Class']);

Replace all calls to `plugin_name()->logger()` with `Logger::`

The logger was originally built into each instance as a loader, but this caused a lot of race-condition issues that made it impractical to keep it that way. Because of this, the logger loader has been moved into its own static instance, and all logger commands can be accessed statically.

// Underpin 1.*
plugin_name()->logger()->log();

// Underpin 2.*
Logger::log();

This does mean that the logged events for all plugins exist within the different event types. If you want to separate your logged events, you'll need to register your own logger event type and use that. This is just another Loader Registry so you can treat it exactly like you do any other registry item. See #Loaders for more info.

Logger::instance()->add('custom_event_type','Event_Type');

`underpin()` function removed

The underpin() function has been removed entirely. If you're extending all plugins, you can use attach

Underpin::attach('init',new Observer([
  'update' => function(Underpin $instance, Accumulator $args){
    // Do an action. $instance is the current plugin's Underpin instance.
  }
]));

Namespace Changes

All Underpin_* namespaces have changed to use Underpin\* to be more PSR4 compliant, and nudge Underpin toward using PSR-based compilers to make distributing plugins easier.

// Underpin 1.*
use Underpin_Scripts\Loaders\Scripts;

// Underpin 2.*
use Underpin\Scripts\Loaders\Scripts;

The Bootstrap

Underpin's bootstrap class encapsulates everything a singleton-instance service provider. This class has a number of key purposes.

It serves as a service provider. It autoloads your namespaced files, and only loads necessary components of the plugin on each server request.
It has a series of preflight checks to ensure that the environment the plugin is running on meets the minimum requirements.
It includes a place to retrieve plugin-wide values, like the plugin URL, the text translation domain.

Autoloader

This boostrap includes a basic autoloading system. By default, the namespace will represent the subdirectories within the lib directory of the plugin.

For Example, any file with namespace Example_Plugin\Cron would need to be located in lib/cron/.

As long as your namespaces line up, and you utilize the registries in the manners detailed in this document, you should never need to manually require a file.

Create your own Bootstrap

While it is possible to work directly with the underpin function as your bootstrap, it's considered a best practice to make your own bootstrap function that creates its own instance of Underpin. This keeps each plugin's registries separate from one-another, and helps prevent code collisions and other unexpected problems.

Basic Example

The simplest example of the bootstrap makes use of the make_class function. This handy function is used throughout Underpin, and behind the scenes it spins up a pre-determined PHP class from the arguments provided. In this case, an instance of Underpin\Factories\Underpin_Instance is created.

/**
 * Fetches the instance of the plugin.
 * This function makes it possible to access everything else in this plugin.
 * It will automatically initiate the plugin, if necessary.
 * It also handles autoloading for any class in the plugin.
 *
 * @since 1.0.0
 *
 * @return \Underpin\Factories\Underpin_Instance The bootstrap for this plugin.
 */
function plugin_name_replace_me() {
	return Underpin\Abstracts\Underpin::make_class( [
		'root_namespace'      => 'Plugin_Name_Replace_Me',
		'text_domain'         => 'plugin_name_replace_me',
		'version'             => '1.0.0',
		'minimum_wp_version'  => '5.1',
		'minimum_php_version' => '7.0',
		'setup_callback'      => function( $instance ){
           // Actions that happen when this plugin is started up.
		}
	] )->get( __FILE__ );
}

Extending The Bootstrap

There are many circumstances in-which you will need to extend your bootstrap class. Perhaps you want to change how minimum requirements behave, or maybe you want to add a custom method to the class. To-do this, you must create your own instance of Underpin\Abstracts\Underpin.

The example above could be converted into a class that looks like this:


class Plugin_Name_Replace_Me extends Underpin\Abstracts\Underpin{

  
	/**
	 * The namespace for loaders. Used for loader autoloading.
	 *
	 * @since 1.0.0
	 *
	 * @var string Complete namespace for all loaders.
	 */
	protected $root_namespace = "Plugin_Name_Replace_Me";

	/**
	 * Translation Text domain.
	 *
	 * Used by translation method for translations.
	 *
	 * @since 1.0.0
	 *
	 * @var string
	 */
	protected $text_domain = 'plugin_name_replace_me';

	/**
	 * Minimum PHP Version.
	 *
	 * @since 1.0.0
	 *
	 * @var string
	 */
	protected $minimum_php_version = '7.0';

	/**
	 * Current Version
	 *
	 * @since 1.0.0
	 *
	 * @var string
	 */
	protected $version = '1.0.0';

	/**
	 * Minimum WordPress Version.
	 *
	 * @since 1.0.0
	 *
	 * @var string
	 */
	protected $minimum_wp_version = '5.1';

    protected function _setup(){
      // Actions that happen when this plugin is started up.
    }
}

From there, you can create function like so:

/**
 * Fetches the instance of the plugin.
 * This function makes it possible to access everything else in this plugin.
 * It will automatically initiate the plugin, if necessary.
 * It also handles autoloading for any class in the plugin.
 *
 * @since 1.0.0
 *
 * @return \Underpin\Factories\Underpin_Instance The bootstrap for this plugin.
 */
function plugin_name_replace_me() {
	return ( new Plugin_Name_Replace_Me )->get( __FILE__ );
}

Extending From a Factory

A common theme in WordPress is to have several add-ons that all use the same copy-pasted bootstrap, however the bootstrap contents are identical between each plugin. In these cases, you would want to use one extended class for all plugins.

Fortunately, this is trivial with Underpin::make_class.

Instead of extending the abstract class, extend the Underpin_Instance class. This is the default class that is used by Underpin::make_class, and by extending it you can customize what arguments get passed in your bootstrap.


class Plugin_Name_Replace_Me extends Underpin\Factories\Underpin_Instance{
  use Underpin\Traits\Instance_Setter;
  // Add any public, or protected paramaters you wish to be override-able with Underpin::make_class 
  protected $custom_param;
  
  protected $custom_function_callback;
  
  // If a function needs to be override-able, you can use set_callable
  protected function custom_function( $args ){
    return $this->set_callable( $this->custom_function_callback, $args );
  }
}

Now, simply call this class using Underpin::make_class.

/**
 * Fetches the instance of the plugin.
 * This function makes it possible to access everything else in this plugin.
 * It will automatically initiate the plugin, if necessary.
 * It also handles autoloading for any class in the plugin.
 *
 * @since 1.0.0
 *
 * @return \Underpin\Factories\Underpin_Instance The bootstrap for this plugin.
 */
function plugin_name_replace_me() {
	return Underpin\Abstracts\Underpin::make_class( [
	    'class' => 'Plugin_Name_Replace_Me',
	    'args'  => [
	        'custom_param'        => 'Custom paramater value',
            'root_namespace'      => 'Plugin_Name_Replace_Me',
            'text_domain'         => 'plugin_name_replace_me',
            'version'             => '1.0.0',
            'minimum_wp_version'  => '5.1',
            'minimum_php_version' => '7.0',
            'custom_callback'     => function( $args ){
              // Action that fires on the custom_callback function.
            },
            'setup_callback'      => function( $instance ){
               // Actions that happen when this plugin is started up.
            }
		]
	] )->get( __FILE__ );
}

Loaders

A frustrating thing about WordPress is the myriad number of ways things get "added". Everything works just a little differently, and this means a lot of time is spent looking up "how do I do that, again?"

Loaders make it so that everything uses an identical pattern to add items to WordPress. With this system, all of these things use nearly exact same set of steps to register.

Currently, there are several different loaders that can be installed alongside Underpin, and used to extend its core functionality.

Admin Bar Menu Loader Create custom menus on the WP Admin Bar.
Admin Notice Loader Loader That assists with adding admin notices to a WordPress website.
Admin Pages Quickly spin up admin settings pages.
Background Process Loader Run slow processes in a separate, asynchornous thread.
Batch Task Loader Create, register, and implement batch tasks.
Block Loader Create, register, and manage WordPress blocks.
Cron Job Loader Create, manage, and execute cron jobs.
Custom Post Type Loader Loader That assists with adding custom Post Types to a WordPress website.
CLI Loader Create WP CLI commands.
Eraser Loader Loader That assists with adding GDPR-compliant erasers to a WordPress website.
Exporter Loader Loader That assists with adding GDPR-compliant exporters to a WordPress website.
Menu Loader Register, and manage custom theme nav menus
Meta Loader Manage custom meta to store in various meta tables
Option Loader Register , and manage values to store in wp_options
Rest Endpoint Loader Create, register, and manage REST endpoints
Role Loader Create, and register custom roles
Script Loader Create, and enqueue scripts
Shortcode Loader Create, and render custom shortcodes
Sidebar Loader Create, and manage WordPress sidebars
Style Loader Create, and enqueue styles
Taxonomy Loader Create, and manage custom taxonomies
Underpin BerlinDB Register, and manage custom database tables with BerlinDB
Widget Loader Create widgets, complete with admin settings.

Creating Custom Loaders

It is also fairly straightforward to create custom loaders, so if you have your own extend-able registry of items, you can add those as well.

Registering Things

Everything is registered with Underpin::make_class, and can be registered in one of three ways:

A string reference to a class name
An anonymous class
An array containing the class name and the constructor arguments
An array containing constructor arguments.

The class name you register must be an instance of the loader's abstraction_class value, so if you wanted to register a shortcode, you must make a class that extends Underpin\Abstracts\Shortcode.

The examples below work with any loader class, and work in basically the same way. The extended class houses all of the logic necessary to tie everything together.

EXAMPLE: Register A Shortcode

Expanding on this example, let's say you wanted to register a new shortcode. It might look something like this:


class Hello_World extends \Underpin_Shortcodes\Abstracts\Shortcode {
	
	protected $shortcode = 'hello_world';
	
	public function shortcode_actions() {
		// TODO: Implement shortcode_actions() method.
	}
}

Extended Class

First you would create your Shortcode class. This class happens to have an abstract method, shortcode_actions.

Looking at the Shortcode abstract, we can see that our shortcode atts are stored in $this->atts, so we could access that directly if we needed. Since this is a simple example, however, we're simply going to return 'Hello world!"


Namespace Underpin\Shortcodes;

class Hello_World extends \Underpin_Shortcodes\Abstracts\Shortcode {
	
	protected $shortcode = 'hello_world';
	
	public function shortcode_actions() {
		return 'Hello world!';
	}
}

Now that our class has been created, we need to register this shortcode. This is done like this:

plugin_name()->shortcodes()->add( 'hello_world','Underpin\Shortcodes\Hello_World' );

Register Inline

Alternatively, you can register the class inline. This will automatically use a default instance of the Shortcode with no customizations.

plugin_name()->shortcodes()->add( 'hello_world', [
	'shortcode'                  => 'hello_world',                   // Required. Shortcode name.
	'shortcode_actions_callback' => function ( $parsed_atts ) {      // Required. Shortcode action.
		return 'Hello world!'; // 'value'
	},
] );

Register Inline With Factory

Finally, you can register the class inline, using a different class for the factory. This makes it possible to customize the factory that is used.

This is particualrly useful in cases where multiple registered items need similar treatment. It provides a way to extend classes without creating unique classes in the process.

First, extend the instance in whatever way you want.


Namespace Underpin\Factories;

class Hello_World_Instance extends \Underpin_Shortcodes\Factories\Shortcode_Instance {

  /* Cusotmize the class */

}

Finally, instruct Underpin to use a different class.

plugin_name()->shortcodes()->add( 'hello_world', [
    'class' => 'Underpin\Factories\Hello_World_Instance',
    'args'  => [
	  'shortcode'                  => 'hello_world',                   // Required. Shortcode name.
	  'shortcode_actions_callback' => function ( $parsed_atts ) {      // Required. Shortcode action.
	  	  return 'Hello world!';                                       // 'value'
	  },
	]
] );

Either way, this shortcode can be accessed using do_shortcode('hello_world');, or you can access the class, and its methods directly with underpin()->shortcodes()->get( 'hello_world' );

Example With Constructor

Sometimes, it makes more sense dynamically register things using a constructor. This pattern works in the same manner as above, the only difference is how you pass your information to the add() method.

Let's say you want to register a shortcode for every post type on the site. You could do with the help of a constructor. something like:


class Post_Type_Shortcode extends \Underpin\Abstracts\Shortcode {

	public function __construct( $post_type ) {
		$this->shortcode = $post_type . '_is_the_best';

		$this->post_type = $post_type;
	}

	public function shortcode_actions() {
		echo $this->post_type . ' is the best post type';
	}
}

And then register each one like so:


add_action( 'init', function() {
	$post_types    = get_post_types( [], 'objects' );

	foreach ( $post_types as $post_type ) {
         $this->shortcodes()->add( $post_type->name . '_shortcode', [
             'class' => 'Flare_WP\Shortcodes\Post_Type_Shortcode',
             'args'  => [ $post_type ],
         ] );
     }
} );

The key part here is how differently we handled the add method. Instead of simply providing a instance name, we instead provide an array containing the class, and an array of ordered args to pass directly into the contstructor. As a result, we register this class to be constructed if it is ever needed.

The Observer Pattern

Instead of using add_action, do_action, add_filter and apply_filters, Underpin has a robust observer pattern built-in. This standardizes the extending process for plugins, provides more-robust priority settings, and encapsulates the action that runs in a class that can be extended.

All items in this pattern use the Observer class.

new Observer( 'action_id', [
  'name'   => 'Action Name', //Used for debugging purposes
  'description' => 'Custom action that runs', //Used for debugging purposes
  'update' => function($instance, Accumulator $accumulator){
    // Do actions here. 
  },
  'priority' => 10, // Sorts items by dependency. Items are sorted by deps first, and then priority.
  'deps' => ['list_of_deps'] // List of dependencies for this item. Items are sorted by dependency, and if all dependencies are not set, this item is skipped.
] );

Filters

Filters are always provided with 2 params, the current class instance, and an Accumulator object. When all hooked filters finish running, the state of the Accumulator is returned. This can be changed with Accumulator::set_state inside the update action of the Observer instance. Like this:

plugin_name()->loader_name()->filter('hook_name', new Observer('unique_action_id', [
  'update' => function($instance, Accumulator $accumulator){
    $accumulator->set_state('new value');
  }
  'deps' => ['list_of_deps'] // List of filters that must run before this. If the dependency doesn't exist, this filter does not run.
]));

Middleware

Unlike Filters and notifications, this pattern always runs when a loader item is registered, and only runs once. This pattern makes it possible to do a set of things when a loader item is registered. A good example of middleware in-action can be seen in the script loader.

// Register script
plugin_name()->scripts()->add( 'test', [
        'handle'      => 'test',
        'src'         => 'path/to/script/src',
        'name'        => 'test',
        'description' => 'The description',
        'middlewares' => [
          'Underpin_Scripts\Factories\Enqueue_Admin_Script'
        ]
] );

// Enqueue script
$script = underpin()->scripts()->get('test')->enqueue();

The above middlewares array would automatically cause the test script to be enqueued on the admin page. Multiple middlewares can be added in the array, and each one would run right after the item is added to the registry.

The middlewares array uses Underpin::make_class to create the class instances. This means that you can pass either:

a string that references an instance of Script_Middleware (see example above).
An array of arguments to construct an instance of Script_Middleware on-the-fly (see example below).

underpin()->scripts()->add( 'test', [
	'handle'      => 'test',
	'src'         => 'path/to/script/src',
	'name'        => 'test',
	'description' => 'The description',
	'middlewares' => [
		'Underpin_Rest_Middleware\Factories\Rest_Middleware', // Will localize script params.
		'Underpin_Scripts\Factories\Enqueue_Script',          // Will enqueue the script on the front end all the time.
         new Observer( 'custom_middleware', [                 // A custom middleware action
             'update'   => function ( $class, $accumulator ) {
                 // Do an action when this script is set-up.
             },
         ] ),
	],
] );

Using Middleware In Your Loader

The easiest way to use middleware in your loader is with the Middleware trait. Using the shortcode example above again:

class Post_Type_Shortcode extends \Underpin\Abstracts\Shortcode {
    use \Underpin\Traits\With_Middleware;
    
	public function __construct( $post_type ) {
		$this->shortcode = $post_type . '_is_the_best';

		$this->post_type = $post_type;
	}

	public function shortcode_actions() {
		echo $this->post_type . ' is the best post type';
	}
}

You could then register much like before, only now you can provide middleware actions.

add_action( 'init', function() {
	$post_types    = get_post_types( [], 'objects' );

	foreach ( $post_types as $post_type ) {
         $this->shortcodes()->add( $post_type->name . '_shortcode', [
             'class'       => 'Flare_WP\Shortcodes\Post_Type_Shortcode',
             'args'        => [ $post_type ],
             'middlewares' => [/* Add middleware references here. */]
         ] );
     }
} );

Template System Trait

This plugin also includes a templating system. This system clearly separates HTML markup from business logic, and provides ways to do things like set default params for values, and declare if a template should be public or private. Any time a class needs to output HTML on a screen, this trait can be used.

Example: Expand Hello World Shortcode into a Template

Let's take the registered Hello_World class above, and modify it so that it uses the template loader trait to get some actual HTML output, and a user name.


Namespace Underpin\Shortcodes;


class Hello_World extends \Underpin\Abstracts\Shortcode {
	use \Underpin\Traits\Templates;

	public $shortcode = 'hello_world';

	public function shortcode_actions() {
		return 'Hello world!';
	}

	public function get_templates() {
		// TODO: Implement get_templates() method.
	}

	protected function get_template_group() {
		// TODO: Implement get_template_group() method.
	}

	protected function get_template_root_path() {
		// TODO: Implement get_template_root_path() method.
	}
}

The Template loader needs some fundamental information before it can be used futher. Let's fill those out a bit.


class Hello_World extends \Underpin\Abstracts\Shortcode {
	use \Underpin\Traits\Templates;

	public $shortcode = 'hello_world';

	public function shortcode_actions() {
		
		$params = [];
		
		if(is_user_logged_in()){
			$params['name'] = wp_get_current_user()->user_nicename;
		}
		return $this->get_template( 'index', $params );
	}

	public function get_templates() {
		return [
			'index' => 'public',
		];
	}

	protected function get_template_group() {
		return 'hello-world';
	}

	protected function get_template_root_path() {
		underpin()->template_dir();
	}
}

get_templates returns an array of templates that this class supports, as well as each template's visibility. This makes it possible for a plugin to create a template that can be overwritten by a theme by settin the template to public.

get_template_group determines the subdirectory name to look for the templates, and get_template_root_path determines the path to the template directory root.

Finally, get_template actually calls the template method, and passes the instance of the object into the included file. It also passes an array of paramaters.

So based on this, we would need to add a new PHP file: /path/to/directory/root/hello-world/index.php

And that file would look something like:

<?php

if ( ! isset( $template ) || ! $template instanceof Hello_World ) {
	return;
}

?>

<h1>Hello <?= $template->get_param( 'name', 'stranger' ) ?>!</h1>

get_param provides a second argument to provide a fallback value should the specified param not be set, or be invalid. In this case, if a name wasn't provided, the template will automatically replace it with stranger.

Nesting Templates

Since the template loader passes the instance into the template, it's possible to load sub-templates inside of the template. A WordPress loop may look something like this:

<?php

if ( ! isset( $template ) || ! $template instanceof The_Loop ) {
	return;
}

?>

<div>
<?php if( $template->query->has_posts() ): while( $template->query->has_posts() ) :$template->query->the_post() ?>
	<?= $template->get_template( 'post' ) ?>
<?php endwhile; ?>
<?php else: ?>
	<?= $template->get_template( 'no-posts' ); ?>
<?php endif; ?>
</div>

Where post.php and no-posts.php are separate PHP files in the same directory, and registered to The_Loop under get_templates.

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