Optionally disable wrappedInstance ref

Stateless components cannot use refs, so this adds the withRef option
(enabled by default for compatibility) which enables or disables passing
the wrappedInstance ref to the wrapped component. Addresses issue digidem#30.
Inspired by reduxjs/react-redux@2d3d0be

README.md

@@ -50,6 +50,8 @@ or as an [ES7 class decorator](https://github.com/wycats/javascript-decorators)
     -   `$0.containerStyle`   (optional, default `defaultContainerStyle`)
     -   `$0.className`   (optional, default `null`)
     -   `$0.elementResize`   (optional, default `false`)
+    -   `options.withRef` **[boolean](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Boolean)=** Set true to enable accessing the wrapped instance with
+        [getWrappedInstance()](https://github.com/digidem/react-dimensions#getwrappedinstance). Disable this when wrapping stateless components, as they cannot use `refs`. (optional, default `true`)
 
 **Examples**
 

index.jsx

@@ -83,6 +83,7 @@ module.exports = function Dimensions ({
     containerStyle = defaultContainerStyle,
     className = null,
     elementResize = false
+    withRef = true
   } = {}) {
   return (ComposedComponent) => {
     return class DimensionsHOC extends React.Component {
@@ -147,22 +148,26 @@ module.exports = function Dimensions ({
        * @return {object} The rendered React component
        **/
       getWrappedInstance () {
-        this.refs.wrappedInstance
+        if (!withRef) {
+          console.warn('You need to enable the `withRef` option to access the wrapped instance')
+        }
+        return this.refs.wrappedInstance
       }
 
       render () {
         const {containerWidth, containerHeight} = this.state
         if (!containerWidth && !containerHeight) {
           console.warn('Wrapper div has no height or width, try overriding style with `containerStyle` option')
         }
+        const ref = withRef ? 'wrappedInstance' : null
         return (
           <div className={className} style={containerStyle} ref='container'>
             {(containerWidth || containerHeight) &&
               <ComposedComponent
                 {...this.state}
                 {...this.props}
                 updateDimensions={this.updateDimensions}
-                ref='wrappedInstance'
+                ref={ref}
               />
             }
           </div>