使用bluebird和coffeescript的简单promise示例可以工作一半时间 [英] Simple promise example with bluebird and coffeescript works half the time

问题描述

所以我基本上是在尝试使用Promise写一些简单的代码，而在理解为什么这种特殊代码每隔一段时间就能工作的时候，我有些麻烦。

So I'm basically attempting to write some simple code using promises and I'm having a bit of trouble understanding why this particular code works every other time.

Promise = require('bluebird')
mkdirp = Promise.promisify(require('mkdirp'))
rm = Promise.promisify(require('rimraf'))

console.log "Preparing build directory"
rm('build')
  .then(mkdirp('build'))

这将在第一次运行中成功完成，但第二次运行将失败，依此类推。

This will complete successfully the first run, but the second will fail, and so on.

这是步骤：

┌[adam@bigboi] [/dev/pts/5] [master ⚡] 
└[~/Projects/bummyjab]> time coffee index.coffee ~/Dropbox/Articles/*.md
Preparing build directory
coffee index.coffee ~/Dropbox/Articles/*.md  0.25s user 0.02s system 100% cpu 0.267 total
┌[adam@bigboi] [/dev/pts/5] [master ⚡] 
└[~/Projects/bummyjab]> stat build
  File: ‘build’
  Size: 4096       Blocks: 8          IO Block: 4096   directory
Device: 804h/2052d Inode: 17172395    Links: 2
Access: (0775/drwxrwxr-x)  Uid: ( 1000/    adam)   Gid: ( 1000/    adam)
Access: 2015-06-25 22:07:49.061331341 -0400
Modify: 2015-06-25 22:07:49.061331341 -0400
Change: 2015-06-25 22:07:49.061331341 -0400
 Birth: -
┌[adam@bigboi] [/dev/pts/5] [master ⚡] 
└[~/Projects/bummyjab]> time coffee index.coffee ~/Dropbox/Articles/*.md
Preparing build directory
Unhandled rejection Error: EEXIST: file already exists, mkdir '/home/adam/Projects/bummyjab/build'
  at Error (native)

coffee index.coffee ~/Dropbox/Articles/*.md  0.20s user 0.03s system 100% cpu 0.235 total

不幸的是，我对此的Google技能还没有给出发生这种情况的原因。

Unfortunately, my google skills for this haven't come up with a reason why this is happening.

谢谢

推荐答案

如果您要控制顺序，以便 mkdirp（'build '）仅在 rm（'build'）完成之后发生，然后您需要将函数引用传递给。 then（）像这样：

If you're trying to control the order so that mkdirp('build') happens only after rm('build') has completed, then you need to pass a function reference to .then() like this:

rm('build').then(function () {
    return mkdirp('build');
});

或者，您可以使用 .bind（）：

rm('build').then(mkdirp.bind(null, 'build'));

您正在执行的是 mkdirp（）立即将返回值传递给 .then（），直到承诺被解决，它才等着执行。

What you were doing is executing mkdirp() immediately and passing that return value to .then() which doesn't wait to execute it until the promise is resolved.

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