From 2575d239c586a45c86935dd0b07e11e2b1ca00ca Mon Sep 17 00:00:00 2001
From: Pascal Birchler <pascal.birchler@gmail.com>
Date: Fri, 4 Jan 2019 10:40:24 +0100
Subject: [PATCH] Add filter for preview interstitial markup (#12463)

---
 .../developers/filters/editor-filters.md           | 14 ++++++++++++++
 .../src/components/post-preview-button/index.js    |  8 ++++++++
 2 files changed, 22 insertions(+)

diff --git a/docs/designers-developers/developers/filters/editor-filters.md b/docs/designers-developers/developers/filters/editor-filters.md
index 1289b8241936ba..a205d583103b02 100644
--- a/docs/designers-developers/developers/filters/editor-filters.md
+++ b/docs/designers-developers/developers/filters/editor-filters.md
@@ -16,3 +16,17 @@ var withImageSize = function( size, mediaId, postId ) {
 wp.hooks.addFilter( 'editor.PostFeaturedImage.imageSize', 'my-plugin/with-image-size', withImageSize );
 ```
 
+### `editor.PostPreview.interstitialMarkup`
+
+Filters the interstitial message shown when generating previews.
+
+_Example:_
+
+```js
+var customPreviewMessage = function() {
+    return '<b>Post preview is being generated!</b>';
+};
+
+wp.hooks.addFilter( 'editor.PostPreview.interstitialMarkup', 'my-plugin/custom-preview-message', customPreviewMessage );
+```
+
diff --git a/packages/editor/src/components/post-preview-button/index.js b/packages/editor/src/components/post-preview-button/index.js
index b735ebcd5324ed..265daa1a7db3fe 100644
--- a/packages/editor/src/components/post-preview-button/index.js
+++ b/packages/editor/src/components/post-preview-button/index.js
@@ -12,6 +12,7 @@ import { __, _x } from '@wordpress/i18n';
 import { withSelect, withDispatch } from '@wordpress/data';
 import { DotTip } from '@wordpress/nux';
 import { ifCondition, compose } from '@wordpress/compose';
+import { applyFilters } from '@wordpress/hooks';
 
 function writeInterstitialMessage( targetDocument ) {
 	let markup = renderToString(
@@ -79,6 +80,13 @@ function writeInterstitialMessage( targetDocument ) {
 		</style>
 	`;
 
+	/**
+	 * Filters the interstitial message shown when generating previews.
+	 *
+	 * @param {String} markup The preview interstitial markup.
+	 */
+	markup = applyFilters( 'editor.PostPreview.interstitialMarkup', markup );
+
 	targetDocument.write( markup );
 	targetDocument.title = __( 'Generating preview…' );
 	targetDocument.close();