Method handleSubmit

src/components/Navbar/LoginForm.js:15–19

()

  }

  handleSubmit () {
    let username = this.state.username
    if (!username) return alert('用户名为空')
    this.props.login({ username })
  }

  render () {
  /* 由于 ES6 中 React 不会自动绑定this，直接 onSubmit={this.handleSubmit} 会报错