進歩的ウェブアプリの紹介

この翻訳は不完全です。英語から この記事を翻訳 してください。

この記事では進歩的ウェブアプリ (Progressive Web Apps) (PWA) を紹介し、普通のウェブアプリよりもどのような利点があるのかを説明します。

進歩的ウェブアプリとは

PWA は数々の特定の技術と、ウェブアプリとネイティブアプリの機能の利点を併せ持った標準パターンを使用して開発されたウェブアプリです。

例えば、ウェブアプリはより見つけやすいものです。 — アクセスするのは、アプリケーションをインストールするよりはるかに簡単で手早く、リンクを通してウェブアプリを共有することもできます。

一方、ネイティブアプリはオペレーティングシステムとよりよく調和するので、ユーザーにもっと継ぎ目のない使い勝手を提供します。ネイティブアプリをインストールするとオフラインで動作し、ユーザーはブラウザーを使ってアプリに移動するのではなく、ホーム画面のアイコンから好きなアプリをタップするだけで簡単にアクセスできます。

PWAは私達にネイティブアプリと同様の強みを持つウェブアプリを作成する能力を私達に授けます。

それらは真新しいコンセプトではありません — それらのアイデアは過去に何度かWebプラットフォームにおいて複数のやり方で改訂されてきました。 プログレッシブエンハンスメントとレスポンシブデザインは我々がモバイルフレンドリーなウェブサイトを作成するのを可能にしています。オフラインでの動作やアプリのインストールはFirefox OSのエコシステムで数年前から可能でした。

しかしながら、PWAはウェブをすばらしいものにしている既存の機能を排除することなく、これらすべてとそれ以上のものを提供します。

何がアプリを PWA にするのか

As we hinted at above, PWAs are not created with a single technology. They represent a new philosophy for building web apps, involving some specific patterns, APIs, and other features. It's not that obvious if a web app is a PWA or not from first glance. An app could be considered a PWA when it meets certain requirements, or implements a set of given features: works offline, is installable, is easy to synchronize, can send push notifications, etc.

In addition, there are tools to measure the completeness of an app in percentages. (Lighthouse is currently the most popular one.) By implementing various technological advantages, we can make an app more progressive, thus ending up with a higher Lighthouse score. But this is only a rough indicator.

There are some key principles a web app should try to observe to be identified as a PWA. It should be:

  • Discoverable, so the contents can be found through search engines.
  • Installable, so it's available on the device's home screen.
  • Linkable, so you can share it by simply sending a URL. 
  • Network independent, so it works offline or with a poor network connection.
  • Progressive, so it's still usable on a basic level on older browsers, but fully-functional on the latest ones.
  • Re-engageable, so it's able to send notifications whenever there's new content available.
  • Responsive, so it's usable on any device with a screen and a browser — mobile phones, tablets, laptops, TVs, fridges, etc.
  • Safe, so the connection between you and the app is secured against any third parties trying to get access to your sensitive data.

Is it worth doing all that?

Absolutely! With a relatively small amount of effort required to implement the core PWA features, the benefits are huge. For example:

  • A decrease in loading times after the app has been installed, thanks to caching with Service Workers, along with saving precious bandwidth and time.
  • The ability to update only the content that has changed when an app update is available. In contrast, with a native app, even the slightest modification can force the user to download the entire application again.
  • A look and feel that is more integrated with the native platform — app icons on the homescreen, apps that run fullscreen, etc.
  • Re-engaging with users via system notifications and push messages, leading to more engaged users and better conversion rates.

There are many success stories of companies trying the PWA route, opting for an enhanced website experience rather than a native app, and seeing significant measurable benefits as a result. The website PWA Stats shares many case studies which indiciate these benefits.

The best known success story is probably Flipkart Lite — India's largest e-commerce site rebuilt as a progressive web app in 2015, which resulted in 70% increase in conversions. The AliExpress PWA has also seen much better results than the web or native app, with a 104% increase in conversion rates for new users. Given their profit increase, and the relatively low amount of work required for the conversion to PWAs, the advantage is clear.

Early stage emerging startups like couponmoto have also started using progressive web apps to drive more consumer engagement, showing that they can help small as well as big companies to (re-)engage users more effectively.

You can check the list at pwa.rocks for more examples. Particularly worth mentioning is the hnpwa.com page — this lists an example implementation of the Hacker News website (instead of the usual TodoMVC app), in which you can see the use of various front-end frameworks.

You can even generate PWAs online using the PWABuilder website.

For service worker- and push- specific information, be sure to check The Service Worker Cookbook, a collection of recipes using service workers in modern sites.

It's well worth trying out a PWA approach, so you can see for yourself if it works for your app.

ブラウザーの対応

As mentioned before, PWAs don't depend on a single API, but rather using various technologies to achieve the goal of delivering the best web experience possible.

The key ingredient required for PWAs is service worker support. Thankfully service workers are now supported on all major browsers on desktop and mobile.

Other features such as Web App Manifest, Push, Notifications, and Add to Home Screen functionality have wide support too. Currently Safari has limited support for Web App Manifest and Add to Home Screen and no support for web push notifications. However, other major browsers support all these features.

Some of these APIs are experimental, with the documentation still in draft, but seeing success stories like those of Flipkart and AliExpress should convince you to try and implement some of the PWA features in your web app already.

Above all you should follow the progressive enhancement rule — use the technologies that provide such enhancements only where they are supported, but still offer the basic functionality of your app if it isn't. This way everybody will be able to use it, but those with modern browsers will benefit from PWA features even more.

アプリケーションの例

In this series of articles we will examine the source code of a super simple website that lists information about games submitted to the A-Frame category in the js13kGames 2017 competition. You don't have to think about what the actual content on the website is — the main point is to learn how to use PWA features in your own projects.

You can find the online version at mdn.github.io/pwa-examples/js13kpwa (also see the source code), which we will be carefully explaining in the next few articles.

Now, let's move to the second part of this series, where we’ll be looking at the structure of our example app.

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