Add feature for GTM script

CHANGELOG.md

@@ -4,7 +4,9 @@ This library adheres to [Semantic Versioning](https://semver.org/) and [Keep a C
 
 ## Unreleased
 
-Nothing yet.
+### Added
+
+- `GTM_Script` feature.
 
 ## 2.1.0
 

docs/feature.md

@@ -14,6 +14,7 @@ interface Feature {
 
 - [Conditional_Feature](https://github.com/alleyinteractive/wp-type-extensions/blob/main/src/alley/wp/features/class-conditional-feature.php): Boot a feature only when a condition is met.
 - [Group](https://github.com/alleyinteractive/wp-type-extensions/blob/main/src/alley/wp/features/class-group.php): Group related features.
+- [GTM_Script](https://github.com/alleyinteractive/wp-type-extensions/blob/main/src/alley/wp/features/class-gtm-script.php): Add the standard Google Tag Manager script and data layer.
 - [Lazy_Feature](https://github.com/alleyinteractive/wp-type-extensions/blob/main/src/alley/wp/features/class-lazy-feature.php): Instantiate a feature only when called upon.
 - [Quick_Feature](https://github.com/alleyinteractive/wp-type-extensions/blob/main/src/alley/wp/features/class-quick-feature.php): Make a callable a feature.
 - [Template_Feature](https://github.com/alleyinteractive/wp-type-extensions/blob/main/src/alley/wp/features/class-template-feature.php): Boot a feature only when templates load.

src/alley/wp/features/class-gtm-script.php

@@ -0,0 +1,79 @@
+<?php
+/**
+ * GTM_Script class file
+ *
+ * @package wp-type-extensions
+ */
+
+namespace Alley\WP\Features;
+
+use Alley\WP\Types\Feature;
+use JsonSerializable;
+use stdClass;
+
+/**
+ * Google Tag Manager script placement.
+ */
+final class GTM_Script implements Feature {
+	/**
+	 * Constructor.
+	 *
+	 * @param string                          $tag_id     GTM tag ID.
+	 * @param array|stdClass|JsonSerializable $data_layer Initial data layer data.
+	 */
+	public function __construct(
+		private readonly string $tag_id,
+		private readonly array|stdClass|JsonSerializable $data_layer,
+	) {}
+
+	/**
+	 * Boot the feature.
+	 */
+	public function boot(): void {
+		add_action( 'wp_head', [ $this, 'render_head' ] );
+		add_action( 'wp_body_open', [ $this, 'render_body' ] );
+	}
+
+	/**
+	 * Render the GTM tag in the document body.
+	 */
+	public function render_head(): void {
+		$data  = $this->data_layer instanceof JsonSerializable ? $this->data_layer : (object) $this->data_layer;
+		$flags = WP_DEBUG ? JSON_PRETTY_PRINT : 0;
+
+		printf(
+			<<<'HTML'
+<script>
+window.dataLayer = window.dataLayer || [];
+window.dataLayer.push(%s);
+</script>
+
+<!-- Google Tag Manager -->
+<script>(function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':
+new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],
+j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src=
+'https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);
+})(window,document,'script','dataLayer',%s);</script>
+<!-- End Google Tag Manager -->
+HTML,
+			wp_json_encode( $data, $flags ),
+			wp_json_encode( $this->tag_id, $flags ),
+		);
+	}
+
+	/**
+	 * Render the GTM tag in the document body.
+	 */
+	public function render_body(): void {
+		printf(
+			<<<'HTML'
+<!-- Google Tag Manager (noscript) -->
+<noscript>
+	<iframe src="%s" height="0" width="0" style="display:none;visibility:hidden"></iframe>
+</noscript>
+<!-- End Google Tag Manager (noscript) -->
+HTML,
+			esc_url( "https://www.googletagmanager.com/ns.html?id={$this->tag_id}" ),
+		);
+	}
+}