Expose preview_link through the REST API and use within client

editor/store/selectors.js

@@ -24,7 +24,6 @@ import createSelector from 'rememo';
  */
 import { serialize, getBlockType, getBlockTypes, hasBlockSupport } from '@wordpress/blocks';
 import { __ } from '@wordpress/i18n';
-import { addQueryArgs } from '@wordpress/url';
 import { moment } from '@wordpress/date';
 
 /***
@@ -354,12 +353,7 @@ export function getEditedPostExcerpt( state ) {
  * @return {string} Preview URL.
  */
 export function getEditedPostPreviewLink( state ) {
-	const link = state.currentPost.link;
-	if ( ! link ) {
-		return null;
-	}
-
-	return addQueryArgs( link, { preview: 'true' } );
+	return getCurrentPost( state ).preview_link || null;
 }
 
 /**

editor/store/test/selectors.js

@@ -1065,14 +1065,14 @@ describe( 'selectors', () => {
 			expect( getEditedPostPreviewLink( state ) ).toBeNull();
 		} );
 
-		it( 'should return the correct url adding a preview parameter to the query string', () => {
+		it( 'should return the correct url when the post object has a preview_link', () => {
 			const state = {
 				currentPost: {
-					link: 'https://andalouses.com/beach',
+					preview_link: 'https://andalouses.com/?p=1&preview=true',
 				},
 			};
 
-			expect( getEditedPostPreviewLink( state ) ).toBe( 'https://andalouses.com/beach?preview=true' );
+			expect( getEditedPostPreviewLink( state ) ).toBe( 'https://andalouses.com/?p=1&preview=true' );
 		} );
 	} );
 

lib/rest-api.php

@@ -424,6 +424,45 @@ function gutenberg_register_rest_api_post_revisions() {
 }
 add_action( 'rest_api_init', 'gutenberg_register_rest_api_post_revisions' );
 
+/**
+ * Get the preview link for the post object.
+ *
+ * @see https://github.com/WordPress/gutenberg/issues/4555
+ *
+ * @param WP_Post $post Post object.
+ * @return string
+ */
+function gutenberg_get_post_preview_link( $post ) {
+	return get_preview_post_link( $post['id'] );
+}
+
+/**
+ * Adds the 'preview_link' attribute to the REST API response of a post.
+ *
+ * @see https://github.com/WordPress/gutenberg/issues/4555
+ */
+function gutenberg_register_rest_api_post_preview_link() {
+	foreach ( get_post_types( array( 'show_in_rest' => true ), 'names' ) as $post_type ) {
+		if ( ! is_post_type_viewable( $post_type ) ) {
+			continue;
+		}
+		register_rest_field( $post_type,
+			'preview_link',
+			array(
+				'get_callback' => 'gutenberg_get_post_preview_link',
+				'schema'       => array(
+					'description' => __( 'Preview link for the post.', 'gutenberg' ),
+					'type'        => 'string',
+					'format'      => 'uri',
+					'context'     => array( 'edit' ),
+					'readonly'    => true,
+				),
+			)
+		);
+	}
+}
+add_action( 'rest_api_init', 'gutenberg_register_rest_api_post_preview_link' );
+
 /**
  * Ensure that the wp-json index contains the 'theme-supports' setting as
  * part of its site info elements.

phpunit/class-gutenberg-rest-api-test.php

@@ -445,4 +445,19 @@ public function test_get_pages_unbounded_per_page_unauthorized() {
 		$data = $response->get_data();
 		$this->assertEquals( 'rest_forbidden_per_page', $data['code'] );
 	}
+
+	public function test_get_page_edit_context_includes_preview() {
+		wp_set_current_user( $this->editor );
+		$page_id = $this->factory->post->create( array(
+			'post_type'   => 'page',
+			'post_status' => 'draft',
+		) );
+		$page    = get_post( $page_id );
+		$request = new WP_REST_Request( 'GET', '/wp/v2/pages/' . $page_id );
+		$request->set_param( 'context', 'edit' );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertEquals( 200, $response->get_status() );
+		$data = $response->get_data();
+		$this->assertEquals( get_preview_post_link( $page ), $data['preview_link'] );
+	}
 }