store composer state when typing in new composer

this doesn't use the MessageComposerStore on purpose so that
both the new and old composer don't overwrite each others state,
as the format is different.

src/components/views/rooms/SendMessageComposer.js

@@ -223,6 +223,7 @@ export default class SendMessageComposer extends React.Component {
         this.model.reset([]);
         this._editorRef.clearUndoHistory();
         this._editorRef.focus();
+        this._clearStoredEditorState();
     }
 
     componentWillUnmount() {
@@ -231,11 +232,37 @@ export default class SendMessageComposer extends React.Component {
 
     componentWillMount() {
         const partCreator = new CommandPartCreator(this.props.room, this.context.matrixClient);
-        this.model = new EditorModel([], partCreator);
+        const parts = this._restoreStoredEditorState(partCreator) || [];
+        this.model = new EditorModel(parts, partCreator);
         this.dispatcherRef = dis.register(this.onAction);
         this.sendHistoryManager = new SendHistoryManager(this.props.room.roomId, 'mx_slate_composer_history_');
     }
 
+    get _editorStateKey() {
+        return `cider_editor_state_${this.props.room.roomId}`;
+    }
+
+    _clearStoredEditorState() {
+        localStorage.removeItem(this._editorStateKey);
+    }
+
+    _restoreStoredEditorState(partCreator) {
+        const json = localStorage.getItem(this._editorStateKey);
+        if (json) {
+            const serializedParts = JSON.parse(json);
+            const parts = serializedParts.map(p => partCreator.deserializePart(p));
+            return parts;
+        }
+    }
+
+    _saveStoredEditorState = () => {
+        if (this.model.isEmpty) {
+            this._clearStoredEditorState();
+        } else {
+            localStorage.setItem(this._editorStateKey, JSON.stringify(this.model.serializeParts()));
+        }
+    }
+
     onAction = (payload) => {
         switch (payload.action) {
             case 'reply_to_event':
@@ -284,6 +311,7 @@ export default class SendMessageComposer extends React.Component {
                     room={this.props.room}
                     label={this.props.placeholder}
                     placeholder={this.props.placeholder}
+                    onChange={this._saveStoredEditorState}
                 />
             </div>
         );