代码改改：

class AnimateDemo extends React.Component{

    constructor(props) {
        super(props);
        this.state = {
            current: 0
        };
    }

    componentDidMount() {
        this.interval = setInterval(this.tick.bind(this), INTERVAL);//就是这部分出错了，因为this在interval之内变了，需要专门绑定一下
    }

    componentWillUnmount() {
        clearInterval(this.interval);
    }

    tick() {
        this.setState({current: this.state.current + 1});
    }

    render() {
        let children = [];
        let pos = 0;
        let colors = ['red', 'gray', 'blue'];
        for (let i = this.state.current; i < this.state.current + colors.length; i++) {
            let style = {
                left: pos * 128,
                background: colors[i % colors.length]
            };
            pos++;
            children.push(<p key={i} className="animateItem" style={style}>{i}</p>);
        }
        return (
            <CSSTransitionGroup
                className="animateExample"
                transitionEnterTimeout={250}
                transitionLeaveTimeout={250}
                transitionName="example">
                {children}
            </CSSTransitionGroup>
        );
    }
}