Post Thumbnails: Add helper functions for attachment captions.

This adds three new functions for getting/displaying attachment captions:

* `wp_get_attachment_caption` - Retrieves a caption for a specific attachment.
* `get_the_post_thumbnail_caption()` - Returns the post thumbnail caption.
* `the_post_thumbnail_caption()` - Displays the post thumbnail caption.

These are helpful for displaying a caption associated with an image directly
in a template, rather than using the caption shortcode.

This also introduces two new filters:

* `wp_get_attachment_caption` - Filters the value of `wp_get_attachment_caption()`.
* `the_post_thumbnail_caption` - Filters the display of the post thumbnail caption.

`the_post_thumbnail_caption()` is automatically filtered by `wptexturize()`,
`convert_smilies()`, and `convert_chars()` in `wp-includes/default-filters.php`.

Props flixos90, joemcgill.
Fixes #12235.
Built from https://develop.svn.wordpress.org/trunk@37915


git-svn-id: http://core.svn.wordpress.org/trunk@37856 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/default-filters.php

@@ -143,6 +143,10 @@ add_filter( 'the_excerpt',     'wpautop'          );
 add_filter( 'the_excerpt',     'shortcode_unautop');
 add_filter( 'get_the_excerpt', 'wp_trim_excerpt'  );
 
+add_filter( 'the_post_thumbnail_caption', 'wptexturize'     );
+add_filter( 'the_post_thumbnail_caption', 'convert_smilies' );
+add_filter( 'the_post_thumbnail_caption', 'convert_chars'   );
+
 add_filter( 'comment_text', 'wptexturize'            );
 add_filter( 'comment_text', 'convert_chars'          );
 add_filter( 'comment_text', 'make_clickable',      9 );

wp-includes/post-thumbnail-template.php

@@ -210,3 +210,44 @@ function the_post_thumbnail_url( $size = 'post-thumbnail' ) {
 		echo esc_url( $url );
 	}
 }
+
+/**
+ * Returns the post thumbnail caption.
+ *
+ * @since 4.6.0
+ *
+ * @param int|WP_Post $post Optional. Post ID or WP_Post object. Default is global `$post`.
+ * @return string Post thumbnail caption.
+ */
+function get_the_post_thumbnail_caption( $post = null ) {
+	$post_thumbnail_id = get_post_thumbnail_id( $post );
+	if ( ! $post_thumbnail_id ) {
+		return '';
+	}
+
+	$caption = wp_get_attachment_caption( $post_thumbnail_id );
+
+	if ( ! $caption ) {
+		$caption = '';
+	}
+
+	return $caption;
+}
+
+/**
+ * Displays the post thumbnail caption.
+ *
+ * @since 4.6.0
+ *
+ * @param int|WP_Post $post Optional. Post ID or WP_Post object. Default is global `$post`.
+ */
+function the_post_thumbnail_caption( $post = null ) {
+	/**
+	 * Filters the displayed post thumbnail caption.
+	 *
+	 * @since 4.6.0
+	 *
+	 * @param string $caption Caption for the given attachment.
+	 */
+	echo apply_filters( 'the_post_thumbnail_caption', get_the_post_thumbnail_caption( $post ) );
+}

wp-includes/post.php

@@ -4936,6 +4936,37 @@ function wp_get_attachment_url( $post_id = 0 ) {
 	return $url;
 }
 
+/**
+ * Retrieves the caption for an attachment.
+ *
+ * @since 4.6.0
+ *
+ * @param int $post_id Optional. Attachment ID. Default 0.
+ * @return string|false False on failure. Attachment caption on success.
+ */
+function wp_get_attachment_caption( $post_id = 0 ) {
+	$post_id = (int) $post_id;
+	if ( ! $post = get_post( $post_id ) ) {
+		return false;
+	}
+
+	if ( 'attachment' !== $post->post_type ) {
+		return false;
+	}
+
+	$caption = $post->post_excerpt;
+
+	/**
+	 * Filters the attachment caption.
+	 *
+	 * @since 4.6.0
+	 *
+	 * @param string $caption Caption for the given attachment.
+	 * @param int    $post_id Attachment ID.
+	 */
+	return apply_filters( 'wp_get_attachment_caption', $caption, $post->ID );
+}
+
 /**
  * Retrieve thumbnail for an attachment.
  *

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.6-alpha-37914';
+$wp_version = '4.6-alpha-37915';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.