DragDropContextProvider don't recreate child context (#859)

* DragDropContextProvider don't recreate child context

* .

packages/react-dnd/src/DragDropContextProvider.js

@@ -29,32 +29,40 @@ export default class DragDropContextProvider extends Component {
 		window: PropTypes.object,
 	}
 
-	constructor(props, context) {
-		super(props, context)
-		this.backend = unpackBackendForEs5Users(props.backend)
-	}
+  constructor(props, context) {
+    super(props, context)
 
-	getChildContext() {
-		/**
+    /**
      * This property determines which window global to use for creating the DragDropManager.
      * If a window has been injected explicitly via props, that is used first. If it is available
      * as a context value, then use that, otherwise use the browser global.
      */
-		const getWindow = () => {
-			if (this.props && this.props.window) {
-				return this.props.window
-			} else if (this.context && this.context.window) {
-				return this.context.window
-			} else if (typeof window !== 'undefined') {
-				return window
-			}
-			return undefined
-		}
-
-		return createChildContext(this.backend, { window: getWindow() })
-	}
+    const getWindow = () => {
+      if (props && props.window) {
+        return props.window
+      } else if (context && context.window) {
+        return context.window
+      } else if (typeof window !== 'undefined') {
+        return window
+      }
+      return undefined
+    };
 
-	render() {
-		return Children.only(this.props.children)
-	}
+    this.backend = unpackBackendForEs5Users(props.backend)
+    this.childContext = createChildContext(this.backend, { window: getWindow() })
+  }
+
+  componentWillReceiveProps(nextProps) {
+    if (nextProps.backend !== this.props.backend || nextProps.window !== this.props.window) {
+      throw new Error('DragDropContextProvider backend and window props must not change.')
+    }
+  }
+
+  getChildContext() {
+    return this.childContext
+  }
+
+  render() {
+    return Children.only(this.props.children)
+  }
 }