深入编程：HTML5应用程序实战

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"《Programming HTML5 Applications》是Zachary Kessin撰写的一本关于开发HTML5应用的英文书籍，旨在深入探讨如何利用HTML5构建强大的Web应用程序。本书覆盖了JavaScript的重要特性，测试方法以及HTML5的LocalStorage等功能。" 在这本书中，作者首先介绍了Web作为应用程序平台的概念，阐述了开发Web应用程序的重要性以及如何通过HTML5为Web应用添加更多功能。Web应用已经从简单的静态页面发展为提供丰富交互和复杂功能的应用，HTML5作为新一代的Web标准，为实现这一转变提供了关键支持。第二章深入讨论JavaScript的核心力量。JavaScript在Web开发中的胜利源于其非阻塞I/O和回调函数，使得异步编程成为可能。此外，书中还介绍了lambda函数的威力、闭包的概念、函数式编程的运用，以及如何通过原型扩展对象。JavaScript的原型机制使得动态扩展对象变得简单，而currying（柯里化）和将对象作为参数等高级技巧进一步增强了其灵活性。数组迭代操作的讨论则让开发者能更高效地处理数据集合。第三章关注JavaScript应用程序的测试。书中提到了QUnit，一个流行的JavaScript测试框架，通过一个简单的例子展示了如何使用QUnit进行单元测试。同时，书中还介绍了mocking和stubs，用于模拟对象和方法，以隔离测试环境。Selenium作为一种自动化测试工具，可以跨浏览器运行测试，书中详述了Selenium的命令结构，如何使用Selenium IDE构造测试，以及如何设置自动化测试运行和Selenium RC测试农场。第四章涉及HTML5的LocalStorage特性，这是Web应用本地存储数据的重要手段。通过localStorage和sessionStorage对象，开发者可以在用户浏览器中持久保存数据，即使在页面关闭后也能保留。书中展示了如何使用这些对象来实现应用程序的状态管理和数据交换，这对于构建离线优先或增强型Web应用至关重要。《Programming HTML5 Applications》是一本深入剖析HTML5开发技术的著作，涵盖了从JavaScript编程到Web应用测试，再到HTML5的本地存储等多个重要方面，对希望提升HTML5应用开发技能的读者来说，是一份宝贵的资源。

can be taken, the application will feel slow. From my office in Tel Aviv to a server in

California, the round trip time for an ICMP ping is about 250 ms. Any action on the

server would be extra and slow that down even more. Mobile device communications

can, of course, be even slower.

Adding Power to Web Applications

HTML5 raises the bar for web applications. While it still has to work under security

constraints, it finally provides tools that desktop developers have expected for years:

Local data storage

Up to 5 MB of data, referenced with a key-value system.

Databases

Originally a SQLite-based API, the tide seems to have shifted to IndexedDB, a

NoSQL system that is natively JavaScript.

Files

While applications still can't freely access the file system, for obvious security rea-

sons, they can now work with files the user specifies and are starting to be able to

create files as well.

Taking it offline

When a laptop or phone is in "Airplane Mode", web applications can't communi-

cate with the server. Manifest files help developers get around that by caching files

for later use.

Web Workers

Threads and forks have always been problematic, but JavaScript simply didn't offer

them. Web Workers provide a way to put applications processes into separate

spaces where they can work without blocking other code.

Web Sockets

Hypertext Transfer Protocol (HTTP) has been the foundation of the Web, despite

a few updates over time. Web Sockets transform the request-response approach to

create much more flexible communications systems.

There's much more, of course, from geolocation to audio and video to canvas graphics

to a wide variety of minor new tags, but these provide the foundations for building

industrial-strength applications on HTML5.

Adding Power to Web Applications | 3

few things, and have something that mostly works. Unfortunately, maintaining such

code becomes more and more difficult over time.

With the Ajax revival, developers took a new look at JavaScript. Some of those devel-

opers worked on improving the engines interpreting and running JavaScript code,

leading to substantial speed improvements. Others focused on the language itself, re-

alizing that it had some very nice features and were able to develop best practices like

those outlined in Douglas Crockford's JavaScript: The Good Parts (O'Reilly, 2008).

Beyond the core language, developers built tools that made debugging JavaScript much

easier. Although Venkman, an early debugger had appeared in 1998, the 2006 release

of Firebug became the gold standard of javascript debuggers. It allows the developer

to track Ajax calls, view the state of the DOM and CSS, as well as single stepping code

and much more. Browsers built on WebKit, notably Apple's Safari and Google Chrome,

offer similar functionality built in, and Opera Dragonfly provides support for Opera.

Even developers working in the confined spaces of mobile devices can now get Firebug-

like debugging with weinre, Web Inspector Remote.

The final key component in this massive recent investment in JavaScript was libraries.

Developers still might not understand all the code they were using, but organizing that

code into readily upgradeable and sometimes even interchangeable libraries simplified

code management.

jQuery

If anything can be described as the Gold Standard of JavaScript libraries it would

have to be John Resig's jQuery library which forms a wrapper around the DOM

and other Javascript objects such as the XMLHttpRequest object and makes doing

all sorts of things in JavaScript a lot easier and a lot more fun. In many ways jQuery

is the essential JavaScript library that every JavaScript programmer should know.

To learn jQuery see the jQuery website http://jquery.org, or a number of good

books on the subject such as "Head First jQuery" or the "jQuery Cookbook" both

published by O'Reilly. Many examples in this book are written using jQuery.

ExtJS

Whereas jQuery forms a wrapper around the DOM, ExtJS, from Sencha http://

sencha.com tries to abstract it away as much as possible. ExtJS features a rich

widget set that can live in a web page and provide many of the widgets, such as

trees, grids, forms, buttons and so on that desktop developers are familar with.

The entire system is very well thought out and fits together well and makes devel-

oping many kinds of applications a joy. While ExtJS tends to be heavy in terms of

the size of the library it is a good fit for some kinds of application development.

On nice feature of ExtJS is that many of its objects know how to save their state.

So if a user takes a grid and reorganizes the columns the next time the user views

that grid it is possible to set it up so that this state is saved. How to use the HTML5

localStorage to enhance this will be shown in “Using localStorage in

ExtJS” on page 51.

6 | Chapter 2: The Power of JavaScript

Others

Tools such as GWT allow the programmer to write Java code, which is then com-

piled down to JavaScript and then can be run on the browser.

Non-blocking I/O and Callbacks

The first key to JavaScript, past the language itself, is to understand event-driven pro-

gramming. In the environment where JavaScript runs, operations tend to be asynchro-

nous, which is to say that they are set up in one place and will execute later after some

external event happens.

This can represent a major change from the way I/O happens in traditional languages.

Take Example 2-1 as a typical example of I/O in a traditional language, in this case

PHP. The line $db->getAll($query); will require that the database access the disk, and

therefore will take orders of mangitude more time to run than the rest of the function.

While the program is waiting for the server to execute the query statement is blocked

and the program is doing nothing. In a server side language like PHP, where there can

be many parallel threads or processes of execution, this isn't usually a problem.

Example 2-1. Blocking I/O in PHP

function getFromDatabase()

{

$db = getDatabase();

$query = "SELECT name FROM countries";

$result = $db->getAll($query);

return $result;

}

In JavaScript, however, there is only one thread of execution, so if the function is

blocked nothing else happens and the user interface is frozen. As such, JavaScript must

find a different way to handle I/O (including all network operations). What JavaScript

does is return right away from a method that might be perceived as slow, leaving behind

a function that gets called when the operation (say, downloading new data from the

web server) is done. The function is known as a callback. When making an Ajax call

to the server the Javascript launches the request then goes on to do something else. It

then provides a function that is called when the server call is finished. This function is

called (Hence the name callback) with the data that is returned from the server at the

time when the data is ready.

As an analogy, consider two ways of buying an item at a grocery store. Some stores

leave items behind the counter, so you have to ask a salesperson for it and wait while

she retrieves it. That's like the PHP program just shown. On the other hand, some stores

have a deli counter where you can request an order and get a number. You can go off

Non-blocking I/O and Callbacks | 7

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