onUpdate prop can be passed to Playground.

docs/api.md

@@ -85,3 +85,19 @@ Annotation map for the docClass. They key is the prop to annotate, the value is
   scope={{React}}
 />
 ```
+
+### `onUpdate`
+*PropTypes.func*
+
+A function which gets called whenever the code updates after the initial render. The callback is fired with a single hash argument which looks like:
+
+```js
+{
+  code,
+  evalError
+}
+```
+
+`code` is the new code text.
+
+`evalError` will either be `null` (if there was no error) or the error object caught during attempted compilation and execution of the new code.

src/components/es6-preview.jsx

@@ -84,7 +84,8 @@ class EsPreview extends Component {
 
   static propTypes = {
     code: PropTypes.string.isRequired,
-    scope: PropTypes.object.isRequired
+    scope: PropTypes.object.isRequired,
+    onError: PropTypes.func.isRequired
   };
 
   _compileCode = () => {
@@ -152,7 +153,9 @@ class EsPreview extends Component {
         }
       }
       render(<Comp />, mountNode);
+      this.props.onError(null);
     } catch (err) {
+      this.props.onError(err);
       this._setTimeout(() => {
         render(
           <div className="playgroundError">{err.toString()}</div>,

src/components/playground.jsx

@@ -30,7 +30,8 @@ class ReactPlayground extends Component {
     es6Console: PropTypes.bool,
     context: PropTypes.object,
     initiallyExpanded: PropTypes.bool,
-    previewComponent: PropTypes.node
+    previewComponent: PropTypes.node,
+    onUpdate: PropTypes.func
   };
 
   state = {
@@ -46,13 +47,28 @@ class ReactPlayground extends Component {
     });
   };
 
+  componentDidUpdate = (prevProps, prevState) => {
+    if (this.state.code !== prevState.code && this.props.onUpdate) {
+      this.props.onUpdate({
+        code: this.state.code,
+        evalError: this.previewError
+      });
+    }
+  };
+
   _handleCodeChange = (code) => {
     this.setState({
       code,
       external: false
     });
   };
 
+  _handlePreviewError = (error) => {
+    // error may be an error object, or null (if there was no problem).
+    // this callback is expected to run before componentDidUpdate.
+    this.previewError = error;
+  };
+
   _toggleCode = () => {
     this.setState({
       expandedCode: !this.state.expandedCode
@@ -107,13 +123,15 @@ class ReactPlayground extends Component {
               <EsPreview
                 code={code}
                 scope={scope}
+                onError={this._handlePreviewError}
               /> :
               <Preview
                 context={context}
                 code={code}
                 scope={scope}
                 noRender={noRender}
                 previewComponent={previewComponent}
+                onError={this._handlePreviewError}
               />
           }
         </div>

src/components/preview.jsx

@@ -16,6 +16,7 @@ class Preview extends Component {
     scope: PropTypes.object.isRequired,
     previewComponent: PropTypes.node,
     noRender: PropTypes.bool,
+    onError: PropTypes.func.isRequired,
     context: PropTypes.object
   };
 
@@ -90,10 +91,12 @@ class Preview extends Component {
       }
       /* eslint-enable no-eval, max-len */
       clearTimeout(this.timeoutID);
+      this.props.onError(null);
       this.setState({ error: null });
     } catch (err) {
       const error = err.toString();
       clearTimeout(this.timeoutID); //eslint-disable-line no-undef
+      this.props.onError(err);
       this.timeoutID = setTimeout(() => {
         this.setState({ error });
       }, 500);