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在ES6生成器中,如何将值从第一个产量传递到下一个产量?

yield generator ecmascript-6 javascript

que1326 · 技术社区 · 6 年前

我搜索了真实的ES6发电机案例,但没有找到正确的解释。我想先了解一下许诺脚本

假设我们有2个rest API: getUser(获取用户) 和 getUserComments

我们不想创建线性代码结构,避免嵌套然后回调功能。所以

定义模拟API

function getUser () {
    return new Promise(function(resolve, reject) {
       return setTimeout(function() { resolve({id: 1, label: 'John'}); }, 200);
    });
}

function getUserComments(userId) {
    return new Promise(function(resolve, reject) {
      return setTimeout(function() {resolve({comments: 'comments', userId: userId}); }, 200);
    });
}

定义生成器

function* generator () {
  yield getUser();
  yield getUserComments(userId); // userId represents data from first request / first yield how should I pass it ?!
}

var g = generator();
var first  = g.next(1);
var second = g.next(2);

如何从第一个yield(yield getUser())获取响应并将其传递给下一个yield(yield getUserComments(userId))?

4 回复 | 直到 6 年前

dfsq 6 年前

您可以将生成的结果分配给变量:

next()方法还接受一个值,该值可用于修改生成器的内部状态。传递给next()的值将被视为暂停生成器的最后一个yield表达式的结果。( 从…起 MDN, Advanced generators )

然后,因为您正在处理承诺,所以使用async/await将其展开为value是有意义的。

尝试以下操作(我稍微更正了setTimeout参数):

async function getUser() {
  return new Promise(function(resolve, reject) {
    setTimeout(resolve, 0, {
      id: 1,
      label: 'John'
    })
  })
};

async function getUserComments(user) {
  return new Promise(function(resolve, reject) {
    setTimeout(resolve, 0, {
      ...user,
      comments: 'comments',
    }, 'userComments');
  });
}

function* generator () {
  const user = yield getUser();
  yield getUserComments(user);
}

(async function () {
  const g = generator();
  const user = await g.next().value
  const comments = await g.next(user).value

  console.log(comments)
})()

Me.Name 6 年前

在某些情况下,必须等待或锁定承诺( then ). 目标是防止完全链接还是仅在实现代码中链接?否则,生成器本身可以进行链接,这样调用代码就不必:

function* generator () {
    const u = getUser(), c = u.then(userId => getUserComments(userId));
  yield *[u,c];
}

例子:

function getUser () {
    return new Promise(function(resolve, reject) {
       setTimeout(function() { resolve({id: 1, label: 'John'}); }, 200);
    });
}

function getUserComments(userId) {
    return new Promise(function(resolve, reject) {
      setTimeout(function() {resolve({comments: 'comments', userId: userId}); }, 200);
    });
}


function* generator () {
	const u = getUser(), c = u.then(userId => getUserComments(userId));
  yield *[u,c];
}


(async function(){
	var g = generator();
  var first  = await g.next().value;
  var second = await g.next().value;
  console.log(first, second);
})();

对于调用代码,先解决/等待哪个承诺并不重要。当然,如果等待后续步骤,则必须完成之前步骤的链。例如。:

function getUser () {
    return new Promise(function(resolve, reject) {
       setTimeout(function() { resolve({id: 1, label: 'John'}); }, 200);
    });
}

function getUserComments(userId) {
    return new Promise(function(resolve, reject) {
      setTimeout(function() {resolve({comments: 'comments', userId: userId}); }, 200);
    });
}


function* generator () {
	const u = getUser(), c = u.then(userId => getUserComments(userId));
  yield *[u,c];
}


(async function(){
	var g = generator();
  var prom  = g.next().value;  
  var second = await g.next().value;
  
  console.log(await prom, second);
})();

Fathy 6 年前

你可以打电话 next 使用参数并使用 yield . 例子:

function* generator () {
  var userId = yield getUser();
  yield getUserComments(userId); // userId represents data from first request / first yield how should I pass it ?!
}

var g = generator();
var first  = g.next();
var userId = getUserId(first);
var second = g.next(userId);

Prashanth KR 6 年前

我试着得到你要求的结果。没有 .then() ,我无法得到 getUser()

function getUser () {
    return new Promise(function(resolve, reject) {
       return setTimeout(function() { resolve({id: 1, label: 'John'}); }, 200);
    });
}

function getUserComments(userId) {
    console.log(userId)
    return new Promise(function(resolve, reject) {
      return setTimeout(function() {resolve({comments: 'comments', userId: userId}); }, 200);
    });
}

function* generator () {
  var userId = yield getUser();
  yield getUserComments(userId); // userId represents data from first request / first yield how should I pass it ?!
}

var g = generator();
   
(async function(){
  var first = await g.next();
  var userId = await first.value.then((res)=>{ return res.id });
  g.next(userId);
})();