Add editorRef prop to DraftEditor

Summary:
We're gonna use it to remove some findDOMNode usage. This lets you access the actual contentEditable.

In the future, we might be able to remove the `.editor` and `.editorContainer` instance fields, and make them both refs.

Reviewed By: claudiopro

Differential Revision: D19555598

fbshipit-source-id: a86be5370a9fd173b3bb062ecc50cabdf9823b75

src/component/base/DraftEditor.react.js

@@ -285,6 +285,16 @@ class DraftEditor extends React.Component<DraftEditorProps, State> {
     };
   }
 
+  _handleEditorContainerRef: (HTMLElement | null) => void = (
+    node: HTMLElement | null,
+  ): void => {
+    this.editorContainer = node;
+    // Instead of having a direct ref on the child, we'll grab it here.
+    // This is safe as long as the rendered structure is static (which it is).
+    // This lets the child support ref={props.editorRef} without merging refs.
+    this.editor = node !== null ? (node: any).firstChild : null;
+  };
+
   _showPlaceholder(): boolean {
     return (
       !!this.props.placeholder &&
@@ -367,7 +377,8 @@ class DraftEditor extends React.Component<DraftEditorProps, State> {
         {this._renderPlaceholder()}
         <div
           className={cx('DraftEditor/editorContainer')}
-          ref={ref => (this.editorContainer = ref)}>
+          ref={this._handleEditorContainerRef}>
+          {/* Note: _handleEditorContainerRef assumes this div won't move: */}
           <div
             aria-activedescendant={
               readOnly ? null : this.props.ariaActiveDescendantID
@@ -415,7 +426,7 @@ class DraftEditor extends React.Component<DraftEditorProps, State> {
             onMouseUp={this._onMouseUp}
             onPaste={this._onPaste}
             onSelect={this._onSelect}
-            ref={ref => (this.editor = ref)}
+            ref={this.props.editorRef}
             role={readOnly ? null : ariaRole}
             spellCheck={allowSpellCheck && this.props.spellCheck}
             style={contentStyle}

src/component/base/DraftEditorProps.js

@@ -64,6 +64,13 @@ export type DraftEditorProps = {
   // Function that returns a cx map corresponding to block-level styles.
   blockStyleFn: (block: BlockNodeRecord) => string,
 
+  // If supplied, a ref which will be passed to the contenteditable.
+  // Currently, only object refs are supported.
+  editorRef?: ?(
+    | {|current: null | HTMLElement|}
+    | ((HTMLElement | null) => void)
+  ),
+
   // A function that accepts a synthetic key event and returns
   // the matching DraftEditorCommand constant, or a custom string,
   // or null if no command should be invoked.