如何在 JavaScript 类中编写生成器？

Question

通常，我会这样写代码：

//definition
exports.getReply = function * (msg){
    //...
    return reply; 
}
//usage
var msg = yield getReply ('hello');

但是如何在 es6 类中编写和使用生成器呢？我试过这个：

class Reply{
    *getReply (msg){
        //...
        return reply; 
    }
     *otherFun(){
        this.getReply();  //`this` seem to have no access to `getReply`
    }
}
var Reply = new Reply();
Reply.getReply();   //out of class,how can I get access to `getReply`?

我也试过了：

 class Reply{
      getReply(){
         return function*(msg){
            //...
            return reply;
         }
      }
    }

所有这两种方法似乎都是错误的答案。那么如何在类中正确编写生成器函数呢？

Answer 1

编辑：添加更多示例。
您的class 定义（几乎）是正确的。错误出现在实例化var Reply = new Reply(); 中。这试图重新定义分配给类名的变量。 generator 函数也应该是 yield 的东西。我详细阐述了一些 OP 代码来展示工作示例。

class Reply {
  //added for test purpose
  constructor(...args) {
    this.args = args;
  }
  * getReply(msg) {
      for (let arg in this.args) {
        let reply = msg + this.args[arg];
        //generator should yield something
        yield reply;
      }
      //next call returns (yields) {done:true,value:undefined}
  }
  * otherFun() {
      yield this.getReply('Nice to meet you '); //yields Generator object
      yield this.getReply('See you '); //Yes, this can access 
      //next call yields {done:true, value:undefined}
  }
  * evenMore() {
      yield* this.getReply('I miss you '); //yields generator result(s)
      yield* this.getReply('I miss you even more ');
  }
}
//now test what we have
const reply = new Reply('Peter', 'James', 'John');
//let and var here are interchangeable because of Global scope
var r = reply.getReply('Hello ');
var msg = r.next(); //{done:false,value:"..."}
while (!msg.done) {
  console.log(msg.value);
  msg = r.next();
}
var other = reply.otherFun();
var o = other.next(); //{done:false,value:Generator}
while (!o.done) {
  let gen = o.value;
  msg = gen.next();
  while (!msg.done) {
    console.log(msg.value);
    msg = gen.next();
  }
  o = other.next();
}
var more = reply.evenMore();
msg = more.next();
while (!msg.done) {
  console.log(msg.value);
  msg = more.next();
}
//update of 1/12/2019
//more examples
for (let r of reply.getReply('for ')) {
  console.log(r);
}
for (let r of reply.evenMore()) {
  console.log(r);
}
//note that the following doesn't work because of lack of star (*) inside the generator function
for (let r of reply.otherFun()) {
  console.log(r);
}

2019 年 1 月 12 日更新

正如@BugBuddy for..of 所建议的那样，循环看起来更好（但并非在所有情况下都有效）。查看 sn-p 中的更新行。

Answer 2

TL;来自 Google 的困惑访问者的 DR：

在 Javascript 中，如何在类中编写生成器函数？

class A {
    * values() {
        yield "a value";
        yield* [1, 2, 3, 4, 5];
    }
}

在语法上是正确的。有用。不客气，现在被解雇了。

Answer 3

生成器是带有.next() 的函数，用于获取yield'ed 值，或者您可以yield 生成器函数让它知道它在遇到yield 语句时不需要“等待”一个要调用的.next (reply.getReply().next(fn))

您的第二段代码几乎是正确的：

class Reply{
    *getReply (msg){
        //...
        return reply; 
    }
     *otherFun(){
        this.getReply();  //`this` seem to have no access to `getReply`
    }
}
var Reply = new Reply();
Reply.getReply();   //out of class,how can I get access to `getReply`?

首先，在 ES6 中工作时请使用const 或let，并且只对类使用大写变体。 您正试图用var Reply = 语句覆盖class Reply 语句，这是不可能的，因为Identifier 'Reply' 已经声明。

您正在寻找的答案如下：
就像您在第一个示例中所做的那样，您应该 yield 生成器函数，所以您的可能应该是这样的：

class Reply{
    *getReply (msg){
        // yield something here, otherwise you should use a normal function
        return reply;
    }
     *otherFun(){
        const reply = yield this.getReply();  // yield the generator so it does what it needs to and doesn't wait for the .next() to be called on it
        return `${reply} within class ${this.constructor.name}`;
    }
}
const reply = new Reply();
const answer = yield reply.getReply('foo'); 
// getReply is a generator function, so it needs a `yield` or `.next()` to run beyond the first `yield` in the function