Adding initial metabox support.

Basic metabox support please see docs/metabox.md for details.

.eslintignore

@@ -2,3 +2,4 @@ build
 coverage
 vendor
 node_modules
+/assets/js

assets/js/metabox.js

@@ -0,0 +1,46 @@
+( function() {
+	var observer;
+
+	if ( ! window.MutationObserver || ! document.getElementById( 'post' ) || ! window.parent ) {
+		return;
+	}
+
+	var previousWidth, previousHeight;
+
+	function sendResize() {
+		var form = document.getElementById( 'post' );
+		var location = form.dataset.location;
+		var newWidth = form.scrollWidth;
+		var newHeight = form.scrollHeight;
+
+		// Exit early if height has not been impacted.
+		if ( newWidth === previousWidth && newHeight === previousHeight ) {
+			return;
+		}
+
+		window.parent.postMessage( {
+			action: 'resize',
+			source: 'metabox',
+			location: location,
+			width: newWidth,
+			height: newHeight
+		}, '*' );
+
+		previousWidth = newWidth;
+		previousHeight = newHeight;
+	}
+
+	observer = new MutationObserver( sendResize );
+	observer.observe( document.getElementById( 'post' ), {
+		attributes: true,
+		attributeOldValue: true,
+		characterData: true,
+		characterDataOldValue: true,
+		childList: true,
+		subtree: true
+	} );
+
+	window.addEventListener( 'load', sendResize, true );
+
+	sendResize();
+} )();

docs/metabox.md

@@ -0,0 +1,130 @@
+# Metaboxes
+
+This is a brief document detailing how metabox support works in Gutenberg. With
+the superior developer and user experience of blocks however, especially once,
+block templates are available, **converting PHP metaboxes to blocks is highly
+encouraged!**
+
+## Breakdown
+
+Each metabox area is rendered by a React component containing an iframe.
+Each iframe will render a partial page containing only metaboxes for that area.
+Metabox data is collected and used for conditional rendering.
+
+### Metabox Data Collection
+
+On each Gutenberg page load, the global state of post.php is mimicked, this is
+hooked in as far back as `plugins_loaded`.
+
+See `lib/register.php gutenberg_trick_plugins_into_registering_metaboxes()`
+
+This will register two new actions, one that fakes the global post state, and
+one that collects the metabox data to determine if an area is empty.
+
+gutenberg_set_post_state() is hooked in early on admin_head to fake the post
+state. This is necessary for ACF to work, no other metabox frameworks seem to
+have this problem. ACF will grab the `$post->post_type` to determine whether a
+box should be registered. Later in `admin_head` ACF will register the metaboxes.
+
+Hooked in later on admin_head is gutenberg_collect_metabox_data(), this will
+run through the functions and hooks that post.php runs to register metaboxes;
+namely `add_meta_boxes, add_meta_boxes_{$post->post_type}`, and `do_meta_boxes`.
+
+A copy of the global $wp_meta_boxes is made then filtered through
+`apply_filters( 'filter_gutenberg_metaboxes', $_metaboxes_copy );`, which will
+strip out any core metaboxes along with standard custom taxonomy metaboxes.
+
+Then each location for this particular type of metabox is checked for whether it
+is active. If it is not empty a value of true is stored, if it is empty a value
+of false is stored. This metabox location data is then dispatched by the editor
+Redux store in `INITIALIZE_METABOX_STATE`.
+
+Ideally, this could be done at instantiation of the editor, and help simplify,
+this flow. However, it is not possible to know the metabox state before
+`admin_enqueue_scripts` where we are calling `createEditorInstance()`. This will
+have to do.
+
+### Redux and React Metabox Management.
+
+*The Redux store by default will hold all metaboxes as inactive*. When
+`INITIALIZE_METABOX_STATE` comes in, the store will update any active metabox
+areas by setting the `isActive` flag to `true`. Once this happens React will
+check for the new props sent in by Redux on the Metabox component. If that
+Metabox is now active, instead of rendering null, a MetaboxIframe component will
+be rendered. The Metabox component is the container component that mediates
+between the MetaboxIframe and the Redux Store. *If no metaboxes are active,
+nothing happens. This will be the default behavior, as all core metaboxes have
+been stripped.*
+
+#### MetaboxIframe Component
+
+When the component renders it will store a ref to the iframe, the component will
+set up a listener for post messages for resizing. assets/js/metabox.js is
+loaded inside the iframe and will send up postMessages for resizing, which the
+MetaboxIframe Component will use to manage its state. A mutation observer will
+also be created when the iframe loads. The observer will detect whether, any
+DOM changes have happened in the iframe, input and change event listeners will
+also be attached to check for changes.
+
+The change detection will store the current form's `FormData`, then whenever a
+change is detected the current form data will be checked vs, the original form
+data. This serves as a way to see if the metabox state is dirty. When the
+metabox state has been detected to have changed, a Redux action
+`METABOX_STATE_CHANGED` is dispatched, updating the store setting the isDirty
+flag to `true`. If the state ever returns back to the original form data,
+`METABOX_STATE_CHANGED` is dispatched again to set the isDirty flag to `false`.
+A selector `isMetaboxStateDirty()` is used to help check whether the post can be
+updated. It checks each metabox for whether it is dirty, and if there is at
+least one dirty metabox, it will return true. This dirty detection does not
+impact creating new posts, as the content will have to change before metaboxes
+can trigger the overall dirty state.
+
+When the post is updated, only metaboxes that are active and dirty, will be
+submitted. This removes any unnecessary requests being made. No extra revisions,
+are created either by the metabox submissions. A redux action will trigger on
+`REQUEST_POST_UPDATE` for any dirty metabox. See editor/effects.js. The
+`REQUEST_METABOX_UPDATE` action will set that metabox's state to isUpdating,
+the isUpdating prop will be sent into the MetaboxIframe and cause a form
+submission. After loading, the original change detection process is fired again
+to handle the new state. Double buffering the iframes here will improve the
+user experience quite a bit, as there currently is a flicker.
+
+### Iframe serving a partial page.
+
+Each iframe will point to an individual source. These are partial pages being
+served by post.php. Why this approach? By using post.php directly, we don't have
+to worry as much about getting the global state 100% correct for each and every
+use case of a metabox, especially when it comes to saving. Essentially, when
+post.php loads it will set up all of its state correctly, and when it hits the
+three `do_action( 'do_meta_boxes' )` hooks it will trigger our partial page.
+
+`gutenberg_metabox_partial_page()` is used to render the metaboxes for a context
+then exit the execution thread early. A `metabox` request parameter is used to
+trigger this early exit. The metabox request parameter should match one of
+`'advanced'`, `'normal'`, or `'side'`. This value will determine which metabox
+area is served. So an example url would look like:
+
+`mysite.com/wp-admin/post.php?post=1&action=edit&metabox=$location`
+
+This url is automatically passed into React via a _wpMetaboxUrl global variable.
+The partial page is very similar to post.php and pretty much imitates it and
+after rendering the metaboxes via do_meta_boxes() it exits early and does some
+hook clean up. There are two extra files that are enqueued; both with a handle
+metabox-gutenberg. One is the js file from assets/js/metabox.js, which resizes
+the iframe. The stylesheet is generated by webpack from
+editor/metaboxes/metabox-iframe.scss and built
+into editor/build/metabox-iframe.css
+
+These styles make use of some of the SASS variables, so that as the Gutenberg
+UI updates so will the Metaboxes.
+
+The partial page mimics the post.php post form, so when it is submitted it will
+normally fire all of the necessary hooks and actions, and have the proper global
+state to correctly fire any PHP metabox mumbo jumbo without needing to modify
+any existing code. On successful submission the page will be reloaded back to
+the same partial page with updated data.
+
+## Wrap Up.
+
+There are some other details I am probably forgetting but this is a pretty good
+overview.

editor/actions.js

@@ -317,6 +317,72 @@ export function removeNotice( id ) {
 	};
 }
 
+// Metabox related actions.
+/**
+ * Returns an action object used to check the state of metaboxes at a location.
+ *
+ * This should only be fired once to initialize meta box state. If a metabox
+ * area is empty, this will set the store state to indicate that React should
+ * not render the meta box area.
+ *
+ * Example: metaboxes = { side: true, normal: false }
+ * This indicates that the sidebar has a metabox but the normal area does not.
+ *
+ * @param {Object} metaboxes Whether metabox locations are active.
+ *
+ * @return {Object} Action object
+ */
+export function initializeMetaboxState( metaboxes ) {
+	return {
+		type: 'INITIALIZE_METABOX_STATE',
+		metaboxes,
+	};
+}
+
+/**
+ * Returns an action object used to signify that a metabox finished reloading.
+ *
+ * @param {String} location Location of metabox: 'normal', 'sidebar'.
+ *
+ * @return {Object} Action object
+ */
+export function handleMetaboxReload( location ) {
+	return {
+		type: 'HANDLE_METABOX_RELOAD',
+		location,
+	};
+}
+
+/**
+ * Returns an action object used to request metabox update.
+ *
+ * @param {String} location Location of metabox: 'normal', 'sidebar'.
+ *
+ * @return {Object}     Action object
+ */
+export function requestMetaboxUpdate( location ) {
+	return {
+		type: 'REQUEST_METABOX_UPDATE',
+		location,
+	};
+}
+
+/**
+ * Returns an action object used to set metabox state changed.
+ *
+ * @param {String} location Location of metabox: 'normal', 'sidebar'.
+ * @param {Boolean}     hasChanged Whether the metabox has changed.
+ *
+ * @return {Object}     Action object
+ */
+export function metaboxStateChanged( location, hasChanged ) {
+	return {
+		type: 'METABOX_STATE_CHANGED',
+		location,
+		hasChanged,
+	};
+}
+
 export const createSuccessNotice = partial( createNotice, 'success' );
 export const createInfoNotice = partial( createNotice, 'info' );
 export const createErrorNotice = partial( createNotice, 'error' );

editor/effects.js

@@ -25,10 +25,12 @@ import {
 	removeNotice,
 	savePost,
 	editPost,
+	requestMetaboxUpdate,
 } from './actions';
 import {
 	getCurrentPost,
 	getCurrentPostType,
+	getDirtyMetaboxes,
 	getEditedPostContent,
 	getPostEdits,
 	isCurrentPostPublished,
@@ -86,7 +88,7 @@ export default {
 	},
 	REQUEST_POST_UPDATE_SUCCESS( action, store ) {
 		const { previousPost, post } = action;
-		const { dispatch } = store;
+		const { dispatch, getState } = store;
 
 		const publishStatus = [ 'publish', 'private', 'future' ];
 		const isPublished = publishStatus.indexOf( previousPost.status ) !== -1;
@@ -113,6 +115,10 @@ export default {
 			) );
 		}
 
+		// Update dirty metaboxes.
+		const metaboxes = getDirtyMetaboxes( getState() );
+		metaboxes.map( metabox => dispatch( requestMetaboxUpdate( metabox ) ) );
+
 		if ( get( window.history.state, 'id' ) !== post.id ) {
 			window.history.replaceState(
 				{ id: post.id },

editor/index.js

@@ -41,6 +41,15 @@ const DEFAULT_SETTINGS = {
 	maxWidth: 608,
 };
 
+/**
+ * Sadly we probably can not add this data directly into editor settings.
+ *
+ * ACF and other metaboxes need admin_head to fire for metabox registry.
+ * admin_head fires after admin_enqueue_scripts which is where we create our
+ * editor instance. If a cleaner solution can be imagined, please change
+ * this, and try to get this data to load directly into the editor settings.
+ */
+
 // Configure moment globally
 moment.locale( dateSettings.l10n.locale );
 if ( dateSettings.timezone.string ) {
@@ -63,6 +72,7 @@ if ( dateSettings.timezone.string ) {
  * @param {String}  id       Unique identifier for editor instance
  * @param {Object}  post     API entity for post to edit
  * @param {?Object} settings Editor settings object
+ * @return {Object} The Redux store of the editor.
  */
 export function createEditorInstance( id, post, settings ) {
 	const store = createReduxStore();
@@ -149,4 +159,6 @@ export function createEditorInstance( id, post, settings ) {
 	);
 
 	render( createEditorElement( <Layout /> ), target );
+
+	return store;
 }

editor/layout/index.js

@@ -21,6 +21,7 @@ import UnsavedChangesWarning from '../unsaved-changes-warning';
 import DocumentTitle from '../document-title';
 import AutosaveMonitor from '../autosave-monitor';
 import { removeNotice } from '../actions';
+import Metabox from '../metaboxes';
 import {
 	getEditorMode,
 	isEditorSidebarOpened,
@@ -32,8 +33,8 @@ function Layout( { mode, isSidebarOpened, notices, ...props } ) {
 		'is-sidebar-opened': isSidebarOpened,
 	} );
 
-	return (
-		<div className={ className }>
+	return [
+		<div key="editor" className={ className }>
 			<DocumentTitle />
 			<NoticeList onRemove={ props.removeNotice } notices={ notices } />
 			<UnsavedChangesWarning />
@@ -44,8 +45,9 @@ function Layout( { mode, isSidebarOpened, notices, ...props } ) {
 				{ mode === 'visual' && <VisualEditor /> }
 			</div>
 			{ isSidebarOpened && <Sidebar /> }
-		</div>
-	);
+		</div>,
+		<Metabox key="metaboxes" location="normal" isSidebarOpened={ isSidebarOpened } />,
+	];
 }
 
 export default connect(