Add initial files.

.editorconfig

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+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+indent_style = tab

.gitattributes

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+# Exclude from release archives.
+
+/.editorconfig        export-ignore
+/.git                 export-ignore
+/.gitattributes       export-ignore
+/.gitignore           export-ignore
+/changelog.md         export-ignore
+/composer.lock        export-ignore
+/contributing.md      export-ignore
+
+# Handle line endings.
+# See https://help.github.com/articles/dealing-with-line-endings/
+
+* text eol=lf

.gitignore

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+composer.lock
+/vendor

readme.md

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+# WPTRT Autoload
+
+A PSR-4 autoloader for WordPress themes.  Primarily, this repository exists for theme authors who want to use autoloading but aren't yet on something such as Composer.
+
+Any classes loaded via this autoloader must follow the [PSR-4: Autoloading](https://www.php-fig.org/psr/psr-4/) standard for naming their namespaces, classes, and directories.
+
+## Usage
+
+Here's a real-world example of loading the [WPTRT Customize Pro](https://github.com/WPTRT/wptrt-customize-pro) package:
+
+```php
+// Include the Loader class.
+require_once( get_theme_file_path( 'path/to/wptrt-autoload/src/Loader.php' ) );
+
+// Create a new instance of the Loader class.
+$themeslug_loader = new \WPTRT\Autoload\Loader();
+
+// Add (one or multiple) namespaces and their paths.
+$themeslug_loader->add( 'WPTRT\\CustomizePro\\', get_theme_file_path( 'path/to/wptrt-customize-pro/src' ) );
+
+// Register all loaders.
+$themeslug_loader->register();
+```
+
+### Loader::add() method
+
+Primarily, theme authors would utilize the `add()` method to add a loader.  You can call `add()` multiple times to register multiple loaders.
+
+```php
+$themeslug_loader->add( $prefix, $path, $prepend = false );
+```
+
+* `$prefix` - This should be the namespace of the project.  Make sure to escape backslashes like `\\` instead of a single `\`.
+* `$path` - This should be the absolute path to the source code of where the classes are housed.
+* `$prepend` - Whether to prepend or append a particular loader to the autoload queue.  `false` by default.

src/Loader.php

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+<?php
+/**
+ * Autoloader Class.
+ *
+ * A basic PSR-4 autoloader for theme developers.
+ *
+ * @author    WPTRT <themes@wordpress.org>
+ * @copyright 2019 WPTRT
+ * @license   https://www.gnu.org/licenses/gpl-2.0.html GPL-2.0-or-later
+ * @link      https://github.com/WPTRT/wptrt-autoload
+ */
+
+namespace WPTRT\Autoload;
+
+class Loader {
+
+	/**
+	 * Array of loaders.
+	 *
+	 * @since  1.0.0
+	 * @access protected
+	 * @var    array
+	 */
+	protected $loaders = [];
+
+	/**
+	 * Adds a new prefix and path to load.
+	 *
+	 * @since  1.0.0
+	 * @access public
+	 * @param  string  $prefix   Namespace prefix.
+	 * @param  string  $path     Absolute path where to look for classes.
+	 * @param  bool    $prepend  Whether to prepend the autoloader to the queue.
+	 * @return void
+	 */
+	public function add( $prefix, $path, $prepend = false ) {
+
+		$this->loaders[ $prefix ] = [
+			'prefix'  => $prefix,
+			'path'    => $path,
+			'prepend' => $prepend
+		];
+	}
+
+	/**
+	 * Removes a loader by prefix.
+	 *
+	 * @since  1.0.0
+	 * @access public
+	 * @param  string  $prefix   Namespace prefix.
+	 * @return void
+	 */
+	public function remove( $prefix ) {
+
+		if ( $this->has( $prefix ) ) {
+			unset( $this->loaders[ $prefix ] );
+		}
+	}
+
+	/**
+	 * Checks if a loader is already added.
+	 *
+	 * @since  1.0.0
+	 * @access public
+	 * @param  string  $prefix   Namespace prefix.
+	 * @return bool
+	 */
+	public function has( $prefix ) {
+		return isset( $this->loaders[ $prefix ] );
+	}
+
+	/**
+	 * Registers all loaders.
+	 *
+	 * @since  1.0.0
+	 * @access public
+	 * @return void
+	 */
+	public function register() {
+
+		foreach ( $this->loaders as $loader ) {
+
+			spl_autoload_register( function( $class ) use ( $loader ) {
+
+				$this->load( $class, $loader['prefix'], $loader['path'] );
+
+			}, true, $loader->prepend );
+		}
+	}
+
+	/**
+	 * Loads a class if it's within the given namespace.
+	 *
+	 * @since  1.0.0
+	 * @access public
+	 * @param  string  $class    Fully-qualified class name.
+	 * @param  string  $prefix   Namespace prefix.
+	 * @param  string  $path     Absolute path where to look for classes.
+	 * @return void
+	 */
+	protected function load( $class, $prefix, $path ) {
+
+		// Bail if the class is not in our namespace.
+		if ( 0 !== strpos( $class, $prefix ) ) {
+			return;
+		}
+
+		// Remove the prefix from the class name.
+		$class = str_replace( $prefix, '', $class );
+
+		// Build the filename.
+		$file = realpath( $path );
+		$file = $file . DIRECTORY_SEPARATOR . str_replace( '\\', DIRECTORY_SEPARATOR, $class ) . '.php';
+
+		// If the file exists for the class name, load it.
+		if ( file_exists( $file ) ) {
+			include( $file );
+		}
+	}
+}