content: import React from 'react';
  import warning from 'warning';
  
  var { object } = React.PropTypes;
  
  var TransitionHook = {
  
    contextTypes: {
      router: object.isRequired
    },
  
    componentDidMount() {
      warning(
        typeof this.routerWillLeave === 'function',
        'Components that mixin TransitionHook should have a routerWillLeave method, check %s',
        this.constructor.displayName || this.constructor.name
      );
  
      if (this.routerWillLeave)
        this.context.router.addTransitionHook(this.routerWillLeave);
    },
  
    componentWillUnmount() {
      if (this.routerWillLeave)
        this.context.router.removeTransitionHook(this.routerWillLeave);
    }
  
  };
  
  export default TransitionHook;
  