adds specific onCancel event in Dialog (fixes #221)

docs/pages/dialogs.md

@@ -96,6 +96,7 @@ class Demo extends React.Component {
 
 | Element   | Prop         | Type      | Effect       | Remarks      |
 |:----------|:-------------|:----------|:-------------|:-------------|
+| Dialog    | onCancel     | Function  | Defines the handler for the cancel event. (When the user press the "Escape" key)  | Optional. By default, the default behavior (closing the dialog) is prevented. |
 | Dialog    | open         | Boolean   | Set the open state of the dialog  | Optional |
 | DialogTitle | component  | String, Element, Function | Specify the custom component to use to render the element | Optional. Default 'h4' |
 | DialogActions | fullWidth | Boolean  | Apply the full-width effect to all children of dialog actions  | Optional |

src/Dialog/Dialog.js

@@ -2,13 +2,21 @@ import React, { PropTypes } from 'react';
 import { findDOMNode } from 'react-dom';
 import classNames from 'classnames';
 
+const prevent = (event) => event.preventDefault();
+
 class Dialog extends React.Component {
     static propTypes = {
         className: PropTypes.string,
+        onCancel: PropTypes.func,
         open: PropTypes.bool
     };
 
+    static defaultProps = {
+        onCancel: prevent
+    };
+
     componentDidMount() {
+        this.refs.dialog.addEventListener('cancel', prevent);
         if(this.props.open) {
             findDOMNode(this).showModal();
         }
@@ -32,6 +40,10 @@ class Dialog extends React.Component {
         }
     }
 
+    componentWillUnmount() {
+        this.refs.dialog.removeEventListener('cancel', prevent);
+    }
+
     render() {
         const { className, children, open, ...otherProps } = this.props;