构建下一代Web应用：Ajax设计模式与最佳实践详解

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Ajax（Asynchronous JavaScript and XML）是一种创新的技术框架，它极大地改变了现代Web应用程序的开发方式。它的核心理念是利用JavaScript、XML或其替代技术（如JSON）实现客户端与服务器之间的异步通信，从而实现网页无需刷新就能实时更新数据。Ajax的设计模式和最佳实践对于构建下一代Web应用至关重要。首先，Ajax的设计模式涉及以下几个关键方面： 1. **事件驱动**：Ajax应用通常基于事件模型，通过监听用户的交互或定时任务，触发相应的后台请求。这使得应用程序能够响应用户输入而无需等待整个页面刷新。 2. **数据交换**：XMLHttpRequest对象是Ajax的核心，它允许在后台与服务器进行非阻塞的HTTP请求，获取或发送数据。后来，JSON成为更受欢迎的数据交换格式，因为其更简洁且易于解析。 3. **响应处理**：服务器返回的数据通常被解析并以JavaScript对象的形式处理，然后在客户端动态更新DOM，展示新的内容，保持用户体验的一致性。 4. **错误处理**：Ajax请求可能遭遇各种网络问题或服务器错误，因此需要良好的错误处理机制，确保用户得到清晰的反馈，并在必要时提供备用方案。 5. **缓存和优化**：为了提高性能，Ajax通常会利用HTTP缓存，减少不必要的服务器交互。同时，通过压缩数据和优化请求策略，可以进一步提升加载速度。最佳实践包括： 1. **逐步采用**：逐渐引入Ajax功能，而不是一次性大规模改动，以便更好地控制风险并适应用户习惯。 2. **代码组织**：遵循模块化和可重用的原则，将Ajax代码分离到单独的函数或库中，便于维护和调试。 3. **响应式设计**：确保应用程序对不同设备和屏幕尺寸有良好的兼容性，提供一致的用户体验。 4. **安全性和隐私**：注意跨站脚本攻击（XSS）和跨站请求伪造（CSRF）的风险，实施适当的防御措施。 5. **可测试性**：Ajax应用的异步特性增加了测试复杂性，需确保代码可测试，比如使用断言和模拟网络延迟。 6. **文档和教程**：为开发团队提供足够的文档和培训，确保大家了解如何有效地使用Ajax及其相关工具。 Ajax的设计模式和最佳实践为现代Web开发者提供了一种革命性的方法，使他们能够创建更加动态、交互式的用户界面，推动了Web应用从传统的静态页面向更富于活力的动态体验转变。随着JavaScript生态系统的发展，如Node.js和Web Workers等，Ajax将继续影响着Web开发的未来趋势。

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xix

Introduction

You probably picked up this book because of the buzzwords Ajax, REST, and patterns. You will

probably read this introduction and skim through the pages. But I want to stop you from skimming

through the pages, at least for a moment. I want you to read this introduction and then decide

whether you want to buy the book.

Here are the things you need to know about Ajax:

• Ajax is an acronym, and the ramifications of the acronym are immense.

• Ajax is not just about a fat client, JavaScript, XML, or asynchronous behavior, but about

developing the next generation of web applications.

• We are at the beginning of building the next generation of web applications.

You are still reading, and that means I still have your interest, which is a good thing. So now

let me tell you what this book is about:

• Using Ajax is the act of creating a web application that implies using REST, that implies

using HTTP, and that implies using the Internet. The patterns of this book illustrate how

JavaScript can be used to control the XMLHttpRequest object to make XMLHttpRequest calls

that process XML or HTML.

• This book for the server side focuses on using Java and C# .NET. The patterns can be used

with Python or Ruby on Rails. I focus on Java and C# because at the time of this writing I

feel that most developers are using them. In the next edition of this book, I want to extend

the materials to include Python and Ruby on Rails examples, because I happen to be an

avid Python programmer.

• The patterns in this book can be used in other contexts, such as Flex (Flash Ajax). For

example, the Permutations pattern can be used to generate Flex content.

Good, you’re still reading and haven’t closed the book. That means you are still interested

and probably willing to spend a few more moments. Here is what I suggest: finish reading the

Introduction because it includes a road map of the patterns. Skim Chapter 1 to get an idea of

what Ajax does and is. Then skim the patterns and focus on reading the “Motivation” and

“Architecture” sections. And if after that you are still interested, please buy this book because

the remaining sections fill in the details of what the patterns are trying to achieve. If you would

like to experiment with the patterns, I suggest you surf to the site http://www.devspace.com/

ajaxpatterns, which will either have the live patterns or redirect you to where the live patterns are.

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■INTRODUCTION

What’s My Vision of Ajax?

Philosophizing about the vision of Ajax raises the question of what Ajax really is. Some say Ajax

is a client-side–only technology. Some say it is an extension of a server framework. Yet others

say, “Heck, it’s new technology blah and now technology bleh can be ignored.” However,

ignoring REST is like saying if it’s a liquid, it can be drunk by humans. Sure, humans can drink

anything that is a liquid, but the bigger question is, will you survive? Sometimes you will, and

sometimes you won’t! Drinking without questioning is playing Russian roulette. The same can

be said of writing Ajax that ignores REST, ignores XML, ignores JSON, and ignores JavaScript.

Ajax is Ajax because of these new technologies that can be combined in new and interesting ways.

My vision of Ajax goes beyond the technologies and represents a new way of building

applications. In the early days when building web applications, the server was responsible for

generating the content, navigating the content, and controlling the content. The web application

became a technology that depended on the sophistication of the server framework to determine

whether the application would be interactive. Ajax breaks those bonds!

Ajax breaks those bonds because it decouples the client from the server. An Ajax application

still needs a server, but an Ajax application can decide when, where, and how that content will

be delivered. A web application that relies on the server is a tightly coupled web application

that can exist only while the server exists. Any content required by the client is controlled by the

server. With Ajax, content can be focused because pieces of content can be assembled, as illustrated

by the Content Chunking pattern.

Where I become very concerned with respect to Ajax is when individuals like to sell a server

framework that relies on said framework to implement Ajax. If Ajax is about decoupling the

client from the server, why must a server framework be used to implement Ajax? The logic simply

does not make any sense. I can understand the argument that a framework has extensions to

enable Ajax-like architectural designs. But I cannot accept the argument that a sever framework

is necessary to make an Ajax application happen.

The focus of this book is the advantages of Ajax using specific patterns that, among other

techniques, extensively use de-coupling to create architectures that can be maintained and

extended. I personally believe that productivity is a good thing, but in specific situations what

may be more important is the ability to figure out what you did and why you did it.

Book and Pattern Road Map

The book is pattern based, with the exception of Chapters 1 and 2. Following is the road map

for those first two chapters:

• Chapter 1: This chapter is used as an introduction to the book and the topic of Ajax. The

focus of the chapter is to provide the context of Ajax and to compare an Ajax application

to other methodologies (for example, traditional client-server).

• Chapter 2: This chapter introduces the XMLHttpRequest object. When you are writing

Ajax applications, the XMLHttpRequest object is a core technology used to communicate

with an HTTP server. Best practices when using the XMLHttpRequest object are also

illustrated.

Chapter 3 and thereafter present patterns. A hierarchy of the patterns is illustrated in

Figure 1.

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