// If the user pushes more data while we're writing to dest then we'll end up
// in ondata again. However, we only want to increase awaitDrain once because
// dest will only emit one 'drain' event for the multiple writes.
// => Introduce a guard on increasing awaitDrain.
var increasedAwaitDrain = false;
src.on('data', ondata);
function ondata(chunk) {
  debug('ondata');
  increasedAwaitDrain = false;
  var ret = dest.write(chunk);
  if (false === ret && !increasedAwaitDrain) {
    if (((state.pipesCount === 1 && state.pipes === dest) ||
        (state.pipesCount > 1 && state.pipes.indexOf(dest) !== -1)) &&
      !cleanedUp) {
      debug('false write response, pause', src._readableState.awaitDrain);
      src._readableState.awaitDrain++;
      increasedAwaitDrain = true;
    }
    src.pause();
  }
}

increasedAwaitDrain = false;
var ret = dest.write(chunk);
if (false === ret && !increasedAwaitDrain) {}

const Readable = require('stream').Readable;
let letter = 'ABCDEFG'.split('');
let index = 0;
const rs = new Readable({
  read(size) {
    this.push(letter[index++] || null);
  }
});
rs.on('data', chunk => {
  console.log(chunk.toString());
});
// 输出
// A
// B
// C
// ...

function Readable(options) {
  this._read = options.read; // 将参数传递的read函数赋值到this._read
}
// 使用者通过调用read方法获取数据
Readable.prototype.read = function (size) {
  var state = this._readableState;
  // 模拟锁，一次_read如果没有返回（this.push），后续read不会继续调用_read读取数据
  if (!state.reading) {
    state.reading = true;
    state.sync = true; // sync用于在push方法中指示_read内部是否同步调用了push
    this._read(size);
    state.sync = false;   
  }
  // _read内部如果是同步调用push，数据会放入缓冲区
  // _read内部如果是异步调用push且缓冲区没有内容，数据可能emit data返回
  // 尝试从缓冲区（state.buffer）中获取大小为size的数据，如果获取成功则触发data事件
  if (ret)
    this.emit('data', ret);
  return ret;
};
// 在this._read执行过程中通过this.push输出数据
Readable.prototype.push = function (chunk, encoding) {
  var state = this._readableState;
  // 本次_read获取到数据，打开锁
  state.reading = false;
  // 流动模式 & 缓冲区没有数据 & 非同步返回，则直接触发data事件
  if (state.flowing && state.length === 0 && !state.sync) {
    stream.emit('data', chunk);
    stream.read(0); // 触发下一次读取，_read异步push的话还是会到这里，类似flow中的保持流出于流动
  }
  else {
    // 将数据放入缓冲区
    state.length += chunk.length;
    state.buffer.push(chunk);
  }
};
// 暂停流动
Readable.prototype.pause = function() {
  if (this._readableState.flowing !== false) {
    this._readableState.flowing = false;
    this.emit('pause');
  }
  return this;
};
function flow(stream) {
  const state = stream._readableState;
  while (state.flowing && stream.read() !== null);
}

et count = 0;
src.on('data', chunk => {
  let ret = dest.write(chunk);
  if (!ret) {
    count++;
    src.pause();
  }
});