Simple dependency injection example in WordPress plugin/theme code

01-plugin.php

<?php

/**
 * Plugin bootsrap file.
 *
 * @author    Motivast motivast.com
 * @copyright 2018 - present, Motivast
 *
 * @license   https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt GPL-2.0-or-later
 *
 * @link      https://gist.github.com/kierzniak/acf82c20ed454b10da9c174d1c6a2ec3
 */

require_once plugin_dir_path( __FILE__ ) . 'class-init.php';

$init = new Motivast\Init();

$init->load_dependencies();
$init->run();

02-class-init.php

<?php

namespace Motivast;

/**
 * The core plugin class.
 *
 * This is used to load dependecies and run plugin.
 *
 * @author    Motivast motivast.com
 * @copyright 2018 - present, Motivast
 *
 * @license   https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt GPL-2.0-or-later
 *
 * @link      https://gist.github.com/kierzniak/acf82c20ed454b10da9c174d1c6a2ec3
 */
class Init {

	/**
	 * The object responsible for displaying shortcode
	 *
	 * @var Shortcode
	 */
	private $shortcode;

	/**
	 * The object responsible for displaying widget
	 *
	 * @var Widget
	 */
	private $widget;

	/**
	 * Load the required dependencies.
	 *
	 * @return void
	 */
	public function load_dependencies() {

		require_once plugin_dir_path( __FILE__ ) . 'class-repository.php';
		require_once plugin_dir_path( __FILE__ ) . 'class-shortcode.php';
		require_once plugin_dir_path( __FILE__ ) . 'class-widget.php';

		$repository = new Repository();

		$this->shortcode  = new Shortcode( $repository );
		$this->widget     = new Widget( $repository );
	}

	/**
	 * Run the loader to execute all of the hooks with WordPress.
	 *
	 * @return void
	 */
	public function run() {

		$this->shortcode->run();
		$this->widget->run();
	}
}

03-class-repository.php

<?php

namespace Motivast;

/**
 * The class responsible for retrieving data.
 *
 * @author    Motivast motivast.com
 * @copyright 2018 - present, Motivast
 *
 * @license   https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt GPL-2.0-or-later
 *
 * @link      https://gist.github.com/kierzniak/acf82c20ed454b10da9c174d1c6a2ec3
 */
class Repository {

	/**
	 * Get the data.
	 *
	 * @return array
	 */
	private function get_data() {

		// Get and return data
		return array();
	}
}

04-class-shortcode.php

<?php

namespace Motivast;

/**
 * The class responsible for displaying shortcode.
 *
 * @author    Motivast motivast.com
 * @copyright 2018 - present, Motivast
 *
 * @license   https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt GPL-2.0-or-later
 *
 * @link      https://gist.github.com/kierzniak/acf82c20ed454b10da9c174d1c6a2ec3
 */
class Shortcode {

	/**
	 * The object responsible for retrieving data
	 *
	 * @var Repository
	 */
	private $repository;

	/**
	 * Class constructor
	 */
	public function __constructor( $repository ) {

		$this->repository = $repository;
	}

	/**
	 * Display shortcode
	 *
	 * @return string
	 */
	public function display() {

		// This is how you will get data from repository
		$data = $this->repository->get_data();
	}

	/**
	 * Register the filters and actions related to shortcode.
	 *
	 * @return void
	 */
	public function run() {

		// Add hooks to register and display shortcode
		add_shortcode( 'shortcode', array( $this, 'display' ) );
	}
}

05-class-widget.php

<?php

namespace Motivast;

/**
 * The class responsible for displaying widget.
 *
 * @author    Motivast motivast.com
 * @copyright 2018 - present, Motivast
 *
 * @license   https://www.gnu.org/licenses/old-licenses/gpl-2.0.txt GPL-2.0-or-later
 *
 * @link      https://gist.github.com/kierzniak/acf82c20ed454b10da9c174d1c6a2ec3
 */
class Widget {

	/**
	 * The object responsible for retrieving data
	 *
	 * @var Repository
	 */
	private $repository;

	/**
	 * Class constructor
	 */
	public function __constructor( $repository ) {

		$this->repository = $repository;
	}

	/**
	 * Display widget
	 *
	 * @return string
	 */
	public function display() {

		// This is how you will get data from repository
		$data = $this->repository->get_data();
	}

	/**
	 * Register the filters and actions related to widget.
	 *
	 * @return void
	 */
	public function run() {

		// Add hooks to register and display widget
	}
}