Beyond Promise - stop & regain

[pengVc]

受到 Promise.stop 灵感启发, 既然能 "停止" 为何不来试试 "恢复", 以及 "恢复" 时可拿到停止的返回值.
作为技术实现上探讨, 在业务应用上, 可各自脑洞.

停止/恢复 方法:

• Promise.stop()
• Promise.regain()

停止示例:

  .then(() => Promise.stop())

恢复示例:

  // 用法较为特别, Promise.regain 包装上 onResolved 回调
  .then(Promise.regain((res) => { 
    // here promise is regained
  }));

如下为实现:

(() => {

  const then = Promise.prototype.then;
  const STOP_SIG = {};
  const REGAIN_SIG = "regain";

  Promise.stop = () => Object.create(STOP_SIG);

  Promise.regain = (onResolved) => {
    return function regain(res) {
      return "function" === typeof onResolved ? onResolved(res) : res;
    }
  };

  Promise.prototype.then = function (onResolved, onRejected) {

    return then.call(this, (val) => {

      const willResume = _isResumeSignal(onResolved);
      const shouldStop = _isStopValue(val);

      if (shouldStop && !willResume) {
        return val;
      }

      // retrive latest stop value
      if (willResume && _isStopValue(val)) {
        // prepare gc
        let newValue = val.stopedValue;
        val.stopedValue = null;
        val = newValue;
      }

      const nextValue = "function" === typeof onResolved ? onResolved(val) : val;
      // First Stop
      if (!shouldStop && _isStopValue(nextValue)) {
        // cache First stop value
        nextValue.stopedValue = val;
      }
      return nextValue;

    }, onRejected);

  };

  function _isStopValue(val) {
    return Object.getPrototypeOf(val || "") === STOP_SIG;
  }

  function _isResumeSignal(onResulved) {
    return "function" === typeof onResulved && onResulved.name === REGAIN_SIG;
  }

})();

完整示例:

var iPr = Promise

  .resolve("hello")

  .then((res) => {

    let val = "wow !";

    console.log(`then I receive [${ res }], and return [${ val }]`);

    return val;
  })

  // stop promise:
  .then((res) => Promise.stop())

  // never get call:
  .then((res) => console.log("then II: ", res))

;

// sometime later...
setTimeout(() => {
  iPr
    .then(Promise.regain((res) => {
      
      // give me stopped values
      console.log(`promise is recovered, \nand get stopped values: [${ res }]`);

    }));
}, 1e3);

further reading: #xieranmaya/blog#5