蓝鸟承诺加入行为 [英] Bluebird Promises Join behaviour
问题描述
如果我使用Node.js执行以下代码
var Promise = require('bluebird');
Promise.join(
函数A(){console.log(A);},
函数B(){console.log(B); }
).done(
function done(){console.log(done);}
);
控制台将记录
B
完成
但我希望
A
B
或
B
A
如果在功能A中设置断点,则永远不会达到。为什么它处理B而不是A?
Promise.join
将 promises 作为其所有参数,但是它的最后一个参数,这是一个函数。
承诺.join(Promise.delay(100),request(http://...com),function(_,res){
//已经过了100毫秒并且请求已经返回。
});
您正在为它提供两个功能,因此它执行以下操作:
- 承诺超过
函数A(){...}
- 基本上返回对它的承诺 - 完成后(立即)执行最后一个参数,
函数B(){...}
记录它。
查看文档:
Promise.join(Promise | Thenable | value promises ...,Function handler) - >承诺
用于协调多个并发离散承诺。虽然.all()适用于处理动态大小的统一承诺列表,但是当你有一些固定数量的离散承诺要同时协调时,Promise.join更容易使用(并且性能更高),例如: / p>
var Promise = require(bluebird);
var join = Promise.join;
join(getPictures(),getComments (),getTweets(),
function(pictures,comments ,推文){
console.log(总计: + pictures.length + comments.length + tweets.length);
});
更新:
JSRishe提出了另一种巧妙的方法来解决这种模式在这个答案中看起来像:
Promise.delay(100).return(request(http://...com).then(function(res){
//已经过了100毫秒,请求已返回
});
这是有效的,因为请求已经在延迟返回之前发生,因为在同一范围内调用该函数。
If I execute the following code with Node.js
var Promise = require('bluebird');
Promise.join(
function A() { console.log("A"); },
function B() { console.log("B"); }
).done(
function done() { console.log("done");}
);
The console will log
B
done
However I would expect
A
B
done
or
B
A
done
If it set a break point in function A it is never reached. Why is it that it processes B but not A?
Promise.join
takes promises as all its arguments but its last one, which is a function.
Promise.join(Promise.delay(100), request("http://...com"), function(_, res){
// 100 ms have passed and the request has returned.
});
You're feeding it two functions so it does the following:
- Make a promise over
function A() { ... }
- basically returning a promise over it - When it's done (immediately) execute the last argument,
function B() {... }
logging it.
See the docs:
Promise.join(Promise|Thenable|value promises..., Function handler) -> Promise
For coordinating multiple concurrent discrete promises. While .all() is good for handling a dynamically sized list of uniform promises, Promise.join is much easier (and more performant) to use when you have a fixed amount of discrete promises that you want to coordinate concurrently, for example:
var Promise = require("bluebird");
var join = Promise.join;
join(getPictures(), getComments(), getTweets(),
function(pictures, comments, tweets) {
console.log("in total: " + pictures.length + comments.length + tweets.length);
});
Update:
JSRishe came up with another clever way to solve this sort of pattern in this answer which looks something like:
Promise.delay(100).return(request("http://...com").then(function(res){
// 100 ms have passed and the request has returned
});
This works because the request already happens before the delay returns since the function is called in the same scope.
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