May 2019
Intermediate to advanced
464 pages
9h 22m
Japanese
Content preview from Node.jsデザインパターン 第2版
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3.3
非同期パターン（
async
ライブラリ編）
67
同時実行数の制限に必要な処理を
TaskQueue
オブジェクトに委譲（
delegate
）したからです。それに
より、
spiderLinks()
が行うべき仕事はタスクの定義だけになりました。タスクのコールバックの処
理内容は次のようになります。
●
ページのリンクごとに
spider()
を呼び出す
●
ページのすべてのリンクの処理が完了した場合、
callback()
を呼び出す
●
各タスクの完了時に
done()
を呼び出す。これにより、
downloadQueue
は次のタスクを処理で
きる
もう一度ウェブスパイダーを実行してみてください。今度は同時に
2
つの
URL
しかダウンロードされ
ないはずです。
3.3
非同期パターン（
async
ライブラリ編）
この章で紹介してきた制御フローのパターンをあらためて見直してみてください。これらのコードを
使って汎用的なライブラリを構築でき
そうだと思いませんか。たとえば、並行処理パターンの節で実
装したコードを、タスクのリストを受け取って処理完了時にコールバックを呼び出すような
API
でラッ
プできます。そのような
API
でラップすることで、制御フローをより宣言的に記述できます。ここで
紹介する
async
ライブラリ（
https://npmjs.org/package/async
）もそのような目的で作られています。
async
は、非同期処理を簡単に扱うためのライブラリで、
Node
のみならず
JavaScript
全般で利用され
ているライブラ
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.