text: const Readable = require('stream').Readable
  
  // 底层数据
  const dataSource = ['a']
  
  const readable = Readable()
  readable._read = function () {
    const data = dataSource.shift()
    if (data) {
      this.push('a')
      process.nextTick(() => {
        this.push('b')
      })
    } else {
      this.push(null)
    }
  }
  
  readable.on('data', data => console.log('PRINT', data))
  
  