react-dnd中context丟失解決方法

steinns發表於2017-08-19
ReactContext

踩坑場景

在做業務的時候，有些模組是可以拖動的，恰好這些模組需要從根元件App的context上拿屬性，同時App也是作為拖動上下文，被@DragDropContext(HTML5Backend)裝飾，當時年少無知，無腦寫下了以下程式碼

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，會用react-dnd了，趕緊將程式碼跑起來，結果傻眼了，居然報這個錯誤

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

提示我們在拖拽元件Box的context上找不到react-dnd需要的drag and drop manager，懵了，讓我想想是咋回事，是不是最後給

  static contextTypes = {
    value: PropTypes.number
  }

給覆蓋了原來的Box.contextTypes呀？
不過這也簡單，不讓他覆蓋就好了嘛，於是我寫下了如下的程式碼

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

真好，報錯消失了，大功告成！等等,this.context.value怎麼是undefined，拿不到了？我明明在contextTypes裡宣告瞭呀，不行，還是得去看一看原始碼。

React-dnd原始碼

檢視DragSource的原始碼，可以看到DragSource就是一個普通裝飾器包裝函式

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,
    });
  };
}

那我們繼續去看一看 decorateHandler這個函式唄

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就是一個HOC生成函式嘛，hoistStatics就是hoist-non-react-statics這個庫，做過HOC的童鞋一定不陌生，他就是將WrappedComponent的靜態方法和靜態屬性提到HOC上，面，避免WrappedComponent的靜態屬性和靜態方法丟失了，看似挺合理，嗯嗯。等等！這不就用WrappedComponent的contextTypes將HOC的contextTypes給覆蓋了麼？這也很合理的解釋了為啥會報錯了。

解決步驟

知道了其中的原來，那我們就讓HOC和WrappedComponent各自保留一份contextTypes好了，首先我們需要用另一個變數來保留對WrappedComponent的引用，因為被@DragSource裝飾後，WrappedComponent的變數名就會被HOC覆蓋了，然後我們再對WrappedComponent加上contextTypes就好了，程式碼如下：

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，我們再來跑一跑。
哇，又報錯了，囧，說

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

好，那我們來看看根元件咋回事，我寫的根元件如下

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

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

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

讓我們看看DragDropContext原始碼

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的問題，但是有點不同，就是contextTypes一定要準確設定在需要的元件上，但是childContextTypes只要放在上層元件就可以了，所以我做了如下修改：

  • 刪去class App 中的
 static childContextType = {
    value: PropTypes.number
  }
  • 加上一下程式碼
App.childContextTypes = Object.assign(App.childContextTypes,{
  value: PropTypes.number
});

這次總該行了吧，心累啊。嗯？還是拿不到this.context.value，想起來了！，雖然hoist-non-react-statics將靜態屬性拿了出來，但是原型方法不會拿出來啊，所以WrappedComponent的getChildContext就沒用了，所以我們需要也將他拿出來，於是，加上一下程式碼

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

這次總算拿到正確的結果了，開心

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