Progressive Web Apps (PWA) are a type of webpage or website with numerous features that were previously reserved for native apps. It can therefore also be described as a symbiosis of a responsive website and a traditional native app. Progressive web apps can be created like a web page using HTML5, CSS3, and JavaScript. Besides, so-called service workers use offline functionalities through optimized caching. The HTTPS protocol is required for communication between the web client and the webserver.
More and more people are using their smartphones to access the Internet. The share of Internet users via smartphone already affected about 69% of all Internet users in 2014. Until now, the problem for website operators was that both a responsive or mobile website and a native app had to be developed if they wanted to compete on the market. A PWA is designed to eliminate the need for duplication of development since it can be accessed via URL like a web page and at the same time provides offline functionality.
One part of the progressive web apps are developed using the web techniques HTML, CSS and JavaScript. The other part is derived from the concept of Progressive Enhancement, which was introduced in 2003. This is a method for developing Web pages that require that the basic functionality of a web application be operational in any browser. Novel features should only be used to improve the user experience if they are supported by the browser. This is necessary because progressive web apps use service worker techniques that only work in modern browsers. Progressive web apps are responsive and HTTPS-transmitted web applications that use the capabilities of browsers for progressive improvement following the principle of progressive enhancement, ensuring a reliable, motivating, and native user experience through offline functionality through service workers, installation using a web app manifest, and push notifications.
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How Progressive Web Application (PWA) works
A progressive web app is launched by entering the URL of the web server in the browser and sending the first request. The web server accepts the request and passes it to the PWA, which acts as a web application here. This generates or loads the HTML source code of a web page, which is sent from the web server back to the user’s browser (HTTPS response).
The user sees a website that is adapted to his device due to the responsive design. Although the Progressive Web App was retrieved from a URL, the user can drag an icon onto the smartphone’s screen or receive push notifications and use the page offline. The technology, based on progressive enhancement, is designed to provide users with the best possible user experience, depending on the device used.
Technically, this is implemented with HTML5, CSS3, JavaScript, Service Worker and optionally a framework such as Angular or a library like a polymer. Progressive Web Apps only allows access to the native file systems.
Service Worker
A service worker is a JavaScript that runs a web browser in the background. It provides essential features for progressive web apps, such as caching for offline use. Once accessed online, content can be viewed the next time you visit the site without an Internet connection (offline operation). Push notifications known from native apps are also possible with service workers. Service workers are programmed, registered in the JavaScript of the page and installed. Service workers require HTTPS, which is why every Progressive Web app runs HTTPS. Numerous frameworks, such as Angular with the Mobile Toolkit, provide service workers so that you don’t have to develop them yourself.
App Shell
App Shell is the minimum HTML, CSS, and Javascript required to run the user interface of the progressive app correctly. The first shell loading should be extremely fast and cached immediately. This means that the shell files are loaded over the network once and later stored on a local repository. Any further opening of the app should then be loaded from the cache by Shell very quickly.
The architecture of the application shell separates the core of the application infrastructure and the user interface from the data. User interface and infrastructure are cached locally by using Service Worker, each other loading of the app loads only the required data instead of all data.
The purpose of an application shell is similar to uploading an app package to the App Store when developing a native app.
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