踩坑场景

在做业务的时候，有些模块是可以拖动的，恰好这些模块需要从根组件App的context上拿属性，同时App也是作为拖动上下文，被@DragDropContext(HTML5Backend)装饰，当时年少无知，无脑写下了以下代码

const boxSource = {
  canDrag(props, monitor) {
    ...
  },
  beginDrag(props) {
   ...
  },
  endDrag(props, monitor) {
 ...
  },
};
@DragSource('box', boxSource, (connect, monitor) => ({
  connectDragSource: connect.dragSource(),
  isDragging: monitor.isDragging(),
}))
export default class Box extends Component {
 static contextTypes = {
  value: PropTypes.number
};
  static propTypes = {
    ...
  }
  render() {
    const { isDragging, connectDragSource, src } = this.props;
    const { value } = this.context;
    return (
      connectDragSource(
        ...
      )
    );
  }
}

美滋滋啊，美滋滋啊，so ez，会用react-dnd了，赶紧将代码跑起来，结果傻眼了，居然报这个错误

Invariant Violation: Could not find the drag and drop manager in the context of Box. Make sure to wrap the top-level component of your app with DragDropContext. Read more: http://react-dnd.github.io/react-dnd/docs-troubleshooting.html#could-not-find-the-drag-and-drop-manager-in-the-context

提示我们在拖拽组件Box的context上找不到react-dnd需要的drag and drop manager，懵了，让我想想是咋回事，是不是最后给

  static contextTypes = {
    value: PropTypes.number
  }

给覆盖了原来的Box.contextTypes呀？
不过这也简单，不让他覆盖就好了嘛，于是我写下了如下的代码

Box.contextTypes = Object.assign(Box.contextTypes,{
  value: PropTypes.number
});

真好，报错消失了，大功告成！等等,this.context.value怎么是undefined，拿不到了？我明明在contextTypes里声明了呀，不行，还是得去看一看源码。

React-dnd源码

查看DragSource的源码，可以看到DragSource就是一个普通装饰器包装函数

function DragSource(type, spec, collect, options = {}) {
...
return function decorateSource(DecoratedComponent) {
    return decorateHandler({
      connectBackend: (backend, sourceId) => backend.connectDragSource(sourceId),
      containerDisplayName: 'DragSource',
      createHandler: createSource,
      registerHandler: registerSource,
      createMonitor: createSourceMonitor,
      createConnector: createSourceConnector,
      DecoratedComponent,
      getType,
      collect,
      options,
    });
  };
}

那我们继续去看一看 decorateHandler这个函数呗

export default function decorateHandler({
  DecoratedComponent,
  createHandler,
  createMonitor,
  createConnector,
  registerHandler,
  containerDisplayName,
  getType,
  collect,
  options,
}) {
...
 class DragDropContainer extends Component {
  ...
    static contextTypes = {
      dragDropManager: PropTypes.object.isRequired,
    }
  ...
    render() {
      return (
        <DecoratedComponent
          {...this.props}
          {...this.state}
          ref={this.handleChildRef}
          />
        );
      }
    }

    return hoistStatics(DragDropContainer, DecoratedComponent);
}

嗯， decorateHandler就是一个HOC生成函数嘛，hoistStatics就是hoist-non-react-statics这个库，做过HOC的童鞋一定不陌生，他就是将WrappedComponent的静态方法和静态属性提到HOC上，面，避免WrappedComponent的静态属性和静态方法丢失了，看似挺合理，嗯嗯。等等！这不就用WrappedComponent的contextTypes将HOC的contextTypes给覆盖了么？这也很合理的解释了为啥会报错了。

解决步骤

知道了其中的原来，那我们就让HOC和WrappedComponent各自保留一份contextTypes好了，首先我们需要用另一个变量来保留对WrappedComponent的引用，因为被@DragSource装饰后，WrappedComponent的变量名就会被HOC覆盖了，然后我们再对WrappedComponent加上contextTypes就好了，代码如下：

class Box extends Component {
  static propTypes = {
    connectDragSource: PropTypes.func.isRequired,
    ...
  }
  render() {
    const { isDragging, connectDragSource, src } = this.props;
    const { value } = this.context;
   ...
    return (
      connectDragSource(
       ...
      )
    );
  }
}

const Temp = Box;
const Box1 = DragSource('box', boxSource, (connect, monitor) => ({
  connectDragSource: connect.dragSource(),
  isDragging: monitor.isDragging(),
}))(Box);
Temp.contextTypes = {
  value: PropTypes.number,
}
export default Box1;

大功告成，我们再来跑一跑。
哇，又报错了，囧，说

Invariant Violation: App.getChildContext(): childContextTypes must be defined in order to use getChildContext().

好，那我们来看看根组件咋回事，我写的根组件如下

@DragDropContext(HTML5Backend)
class App extends React.Component {
  constructor(props) {
    super(props);

  }
  static childContextTypes = {
    value:PropTypes.number,
 }
  getChildContext(){
    return {
      value:1
    }
  }

  render() {
    return (
      <Box />
    )
  }
}

让我们看看DragDropContext源码

export default function DragDropContext(backendOrModule) {
 ...
 return function decorateContext(DecoratedComponent) {
  ...
   class DragDropContextContainer extends Component {
      getChildContext() {
        return childContext;
      }

      render() {
        return (
          <DecoratedComponent
            {...this.props}
            ref={(child) => { this.child = child; }}
          />
        );
      }
    }

    return hoistStatics(DragDropContextContainer, DecoratedComponent);
  };
}

得，又是HOC的问题，但是有点不同，就是contextTypes一定要准确设置在需要的组件上，但是childContextTypes只要放在上层组件就可以了，所以我做了如下修改：

• 删去class App 中的

 static childContextType = {
    value: PropTypes.number
  }

• 加上一下代码

App.childContextTypes = Object.assign(App.childContextTypes,{
  value: PropTypes.number
});

这次总该行了吧，心累啊。嗯？还是拿不到this.context.value，想起来了！，虽然hoist-non-react-statics将静态属性拿了出来，但是原型方法不会拿出来啊，所以WrappedComponent的getChildContext就没用了，所以我们需要也将他拿出来，于是，加上一下代码

const temp = {...App.prototype.getChildContext()};
App.prototype.getChildContext = () => ({...temp, value:1})

这次总算拿到正确的结果了，开心