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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94
4
章
ES2015
以降の機能を使った非同期パターン
ここでもジェネレータと
co
がすべての面倒な仕事を引き受けてくれるので、普通にループの中で同
期
API
を呼び出すのと同じ要領でプログラミングが可能になります。
そしてすべてのエントリポイントとなるコードは次のようになります。
co(function* () {
try {
yield spider(process.argv[2], 1);
console.log('Download complete');
} catch(err) {
console.log(err);
}
});
ようやくここで
co
が登場しました。こうやってエントリポイントとなるジェネレータを
co()
に渡し
てやることで、以降のすべての
yield
文に渡したジェネレータが再帰的に展開されます。それにより、
ジェネレータ内部では
co
に依存しない形でのプログラミングが可能です。
では、完成したウェブスパイダーを実行してみ
てください。
4.2.4
並行処理
ジェネレータは逐次処理には向いていますが、残念ながら並行処理に利用することはできません。
ここで取りうる方法としては、コールバックもしくはプロミスベースの関数とジェネレータの併用が考
えられます。
ただし
co
に限って言えば、プロミス（およびサンクもしくはジェネレータ）の配列を
yield
できるの
で、その機能を使って簡単に並行処理を実現することが可能です。
ではウェブスパイダーのバージョン
3
を再実装してみましょう。変更点は
spiderLi
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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.