WordPress函数：get_adjacent_post 获取相邻的（上一篇下一篇）文章

函数原型：

get_adjacent_post( bool $in_same_term = false, int[]|string $excluded_terms = '', bool $previous = true, string $taxonomy = 'category' ): WP_Post|null|string

获取当前文章的相邻文章，上一篇或下一篇。

参数说明：

$in_same_term，是否要求同一分类。

$exclude_terms，需要排除的分类。

$previous，true 前一篇文章，false 后一篇文章。

$taxonomy，分类，默认：'category'，与 $in_same_term 配合使用。

函数源码：

function get_adjacent_post( $in_same_term = false, $excluded_terms = '', $previous = true, $taxonomy = 'category' ) {
	global $wpdb;

	$post = get_post();

	if ( ! $post || ! taxonomy_exists( $taxonomy ) ) {
		return null;
	}

	$current_post_date = $post->post_date;

	$join     = '';
	$where    = '';
	$adjacent = $previous ? 'previous' : 'next';

	if ( ! empty( $excluded_terms ) && ! is_array( $excluded_terms ) ) {
		// Back-compat, $excluded_terms used to be $excluded_categories with IDs separated by " and ".
		if ( false !== strpos( $excluded_terms, ' and ' ) ) {
			_deprecated_argument(
				__FUNCTION__,
				'3.3.0',
				sprintf(
					/* translators: %s: The word 'and'. */
					__( 'Use commas instead of %s to separate excluded terms.' ),
					"'and'"
				)
			);
			$excluded_terms = explode( ' and ', $excluded_terms );
		} else {
			$excluded_terms = explode( ',', $excluded_terms );
		}

		$excluded_terms = array_map( 'intval', $excluded_terms );
	}

	$excluded_terms = apply_filters( "get_{$adjacent}_post_excluded_terms", $excluded_terms );

	if ( $in_same_term || ! empty( $excluded_terms ) ) {
		if ( $in_same_term ) {
			$join  .= " INNER JOIN $wpdb->term_relationships AS tr ON p.ID = tr.object_id INNER JOIN $wpdb->term_taxonomy AS tt ON tr.term_taxonomy_id = tt.term_taxonomy_id";
			$where .= $wpdb->prepare( 'AND tt.taxonomy = %s', $taxonomy );

			if ( ! is_object_in_taxonomy( $post->post_type, $taxonomy ) ) {
				return '';
			}
			$term_array = wp_get_object_terms( $post->ID, $taxonomy, array( 'fields' => 'ids' ) );

			// Remove any exclusions from the term array to include.
			$term_array = array_diff( $term_array, (array) $excluded_terms );
			$term_array = array_map( 'intval', $term_array );

			if ( ! $term_array || is_wp_error( $term_array ) ) {
				return '';
			}

			$where .= ' AND tt.term_id IN (' . implode( ',', $term_array ) . ')';
		}

		if ( ! empty( $excluded_terms ) ) {
			$where .= " AND p.ID NOT IN ( SELECT tr.object_id FROM $wpdb->term_relationships tr LEFT JOIN $wpdb->term_taxonomy tt ON (tr.term_taxonomy_id = tt.term_taxonomy_id) WHERE tt.term_id IN (" . implode( ',', array_map( 'intval', $excluded_terms ) ) . ') )';
		}
	}

	// 'post_status' clause depends on the current user.
	if ( is_user_logged_in() ) {
		$user_id = get_current_user_id();

		$post_type_object = get_post_type_object( $post->post_type );
		if ( empty( $post_type_object ) ) {
			$post_type_cap    = $post->post_type;
			$read_private_cap = 'read_private_' . $post_type_cap . 's';
		} else {
			$read_private_cap = $post_type_object->cap->read_private_posts;
		}

		$private_states = get_post_stati( array( 'private' => true ) );
		$where         .= " AND ( p.post_status = 'publish'";
		foreach ( $private_states as $state ) {
			if ( current_user_can( $read_private_cap ) ) {
				$where .= $wpdb->prepare( ' OR p.post_status = %s', $state );
			} else {
				$where .= $wpdb->prepare( ' OR (p.post_author = %d AND p.post_status = %s)', $user_id, $state );
			}
		}
		$where .= ' )';
	} else {
		$where .= " AND p.post_status = 'publish'";
	}

	$op    = $previous ? '<' : '>';
	$order = $previous ? 'DESC' : 'ASC';

	$join = apply_filters( "get_{$adjacent}_post_join", $join, $in_same_term, $excluded_terms, $taxonomy, $post );

	$where = apply_filters( "get_{$adjacent}_post_where", $wpdb->prepare( "WHERE p.post_date $op %s AND p.post_type = %s $where", $current_post_date, $post->post_type ), $in_same_term, $excluded_terms, $taxonomy, $post );

	$sort = apply_filters( "get_{$adjacent}_post_sort", "ORDER BY p.post_date $order LIMIT 1", $post, $order );

	$query        = "SELECT p.ID FROM $wpdb->posts AS p $join $where $sort";
	$key          = md5( $query );
	$last_changed = wp_cache_get_last_changed( 'posts' );
	if ( $in_same_term || ! empty( $excluded_terms ) ) {
		$last_changed .= wp_cache_get_last_changed( 'terms' );
	}
	$cache_key = "adjacent_post:$key:$last_changed";

	$result = wp_cache_get( $cache_key, 'posts' );
	if ( false !== $result ) {
		if ( $result ) {
			$result = get_post( $result );
		}
		return $result;
	}

	$result = $wpdb->get_var( $query );
	if ( null === $result ) {
		$result = '';
	}

	wp_cache_set( $cache_key, $result, 'posts' );

	if ( $result ) {
		$result = get_post( $result );
	}

	return $result;
}

包含钩子：

apply_filters( "get_{$adjacent}_post_excluded_terms", int[]|string $excluded_terms )

apply_filters( "get_{$adjacent}_post_join", string $join, bool $in_same_term, int[]|string $excluded_terms, string $taxonomy, WP_Post $post )

apply_filters( "get_{$adjacent}_post_sort", string $order_by, WP_Post $post, string $order )

apply_filters( "get_{$adjacent}_post_where", string $where, bool $in_same_term, int[]|string $excluded_terms, string $taxonomy, WP_Post $post )

使用举例：

 <?php $prev_post = get_adjacent_post( true, '', true, 'taxonomy_slug' ); ?>
 <?php if ( is_a( $prev_post, 'WP_Post' ) ) { ?>
	<a href="<?php echo get_permalink( $prev_post->ID ); ?>"><?php echo get_the_title( $prev_post->ID ); ?></a>
 <?php } ?>

获取相同分类下的前一篇文章。