React-router4 第五篇 Preventing Transitions 防止转换

文档地址：https://reacttraining.com/react-router/web/example/preventing-transitions

大概意思就是说：我在这个页面上写东西呢？不小心点到了其他路由上，我们可以做一个提示，来一个弹窗，提示你真的要切换走吗?然后选择确定和取消，

关键代码就是

<Prompt
    when={isBlocking}  // when 值为true时启用防止转换
    message={(location, a,b) => {    // 验证出，这个属性会传入2个参数。。。
          console.log(location) // 是一个对象，内含 hash key pathname search state
          console.log(a)  // PUSH  ??不能明白这个字符串是啥
          console.log(b)  // undefind  
          return `Are you sure you want to go to ${location.pathname}`
      }}
    />

import React from 'react'
import ReactDOM from 'react-dom'
import {
  BrowserRouter as Router,
  Route,
  Link,
  Prompt
} from 'react-router-dom'

const PreventingTransitionsExample = () => (
  <Router>
    <div>
      <ul>
        <li><Link to="/">Form</Link></li>
        <li><Link to="/one">One</Link></li>
        <li><Link to="/two">Two</Link></li>
      </ul>
      <Route path="/" exact component={Form}/>
      <Route path="/one" render={() => <h3>One</h3>}/>
      <Route path="/two" render={() => <h3>Two</h3>}/>
    </div>
  </Router>
)

class Form extends React.Component {
  state = {
    isBlocking: false
  }

  render() {
    const { isBlocking } = this.state

    return (
      <form
        onSubmit={event => {
          event.preventDefault()
          event.target.reset()
          this.setState({
            isBlocking: false
          })
        }}
      >
        <Prompt
          when={isBlocking}
          message={location => (
            `Are you sure you want to go to ${location.pathname}`  // ES6的模板字符串
          )}
        />

        <p>
          Blocking? {isBlocking ? 'Yes, click a link or the back button' : 'Nope'}
        </p>

        <p>
          <input
            size="50"
            placeholder="type something to block transitions"
            onChange={event => {
              this.setState({
                isBlocking: event.target.value.length > 0
              })
            }}
          />
        </p>

        <p>
          <button>Submit to stop blocking</button>
        </p>
      </form>
    )
  }
}
ReactDOM.render(
	<PreventingTransitionsExample />,
	document.getElementById("app")
)