Xamarin.Forms Preview 2：跨平台C#开发iOS、Android与WP8应用指南

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本篇内容是关于使用Xamarin.Forms Preview Edition 2编写的移动应用开发书籍的介绍。Xamarin.Forms是由Xamarin在2014年5月发布的激动人心的新移动开发平台，它支持iOS、Android和Windows Phone三大平台。Xamarin.Forms的核心理念是让开发者使用C#和XAML（可扩展应用程序标记语言）编写共享的用户界面代码，这种代码可以映射到这三个操作系统上的原生控件，实现了跨平台开发。作为预览版，这本书只有16章，内容尚未全面覆盖，可能包含了一些重要话题的遗漏。预计最终版将在2015年夏季发布，届时将会有更完整的内容。书中由Charles Petzold撰写，他是一位经验丰富的跨平台C#编程专家，专注于iOS、Android和Windows Phone的应用开发。通过使用Xamarin.Forms，开发者能够利用C#的优势，实现一次编写、多次使用的代码库，大大提高了开发效率和维护性。然而，由于是预览版，读者在阅读时需要注意部分章节可能还在开发和完善之中，可能需要后续的正式版来获取完整的教程和技术细节。书中提供的资源包括但不限于：数百种来自行业专家的书籍、电子书和在线资源，免费美国境内运输，多格式电子书适应不同设备阅读，以及定期的优惠活动如打印版与电子版捆绑销售和每周精选折扣。此外，订阅微软Press Store的新闻通讯，可以第一时间了解新书发布、特别优惠和技巧文章。最后，读者可以通过微软Press Guided Tours应用深入了解新技术的深度指导，该应用可在Windows Store上下载。这将为学习者提供一个系统的学习路径和实践机会，帮助他们更好地掌握Xamarin.Forms开发技术。这本书对于希望进入跨平台移动应用开发领域的专业人士来说是一个宝贵的学习资源，但需要配合正式版本以获取最全面的知识体系。

Introduction xii

Over the months that the first and second Preview Editions were in progress, I have benefited from

valuable feedback, corrections, and edits from several people. This book wouldn’t exist without the col-

laboration of Bryan Costanich at Xamarin and Devon Musgrave at Microsoft Press. Both Bryan and

Craig Dunn at Xamarin read some of my drafts of early chapters and managed to persuade me to take

a somewhat different approach to the material. Later on, Craig kept me on track and reviewed the

chapters in this second Preview Edition, and John Meade did the copyediting. For the first Preview Edi-

tion, Stephane Delcroix at Xamarin and Andy Wigley with Microsoft offered essential technical reads

and persistently prodded me to make the book better. Microsoft’s copyeditor for this second Preview

Edition was John Pierce.

Almost nothing I do these days would be possible without the daily companionship and support of

my wife, Deirdre Sinnott.

Charles Petzold

March 23, 2015

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Chapter 1

How does Xamarin.Forms fit in?

There is much joy in programming. There is joy in analyzing a problem, breaking it down into pieces,

formulating a solution, mapping out a strategy, approaching it from different directions, and crafting

the code. There is very much joy in seeing the program run for the first time, and then more joy in ea-

gerly diving back into the code to make it better and faster.

There is also often joy in hunting down bugs, in ensuring that the program runs smoothly and pre-

dictably. Few occasions are quite as joyful as finally identifying a particularly recalcitrant bug and defin-

itively stamping it out.

There is even joy in realizing that the original approach you took is not quite the best. Many devel-

opers discover that they’ve learned a lot writing a program, including that there’s a better way to struc-

ture the code. Sometimes, a partial or even a total rewrite can result in a much better application. The

process is like standing on one’s own shoulders, and there is much joy in attaining that perspective and

knowledge.

However, not all aspects of programming are quite so joyful. One of the nastier programming jobs

is taking a working program and rewriting it in an entirely different programming language or porting

it to another operating system with an entirely different application programming interface (API).

A job like that can be a real grind. Yet, such a rewrite may very well be necessary: an application

that’s been so popular on the iPhone might be even more popular on Android devices, and there’s

only one way to find out.

But here’s the problem: As you’re going through the original source code and moving it to the new

platform, do you maintain the same program structure so that the two versions exist in parallel? Or do

you try to make improvements and enhancements?

The temptation, of course, is to entirely rethink the application and make the new version better.

But the further the two versions drift apart, the harder they will be to maintain in the future.

For this reason, a sense of dread envelopes the forking of one application into two. With each line

of code you write, you realize that all the future maintenance work, all the future revisions and en-

hancements, have become two jobs rather than one.

This is not a new problem. For over half a century, developers have craved the ability to write a sin-

gle program that runs on multiple machines. This is one of the reasons that high-level languages were

invented in the first place, and this is why the concept of “cross-platform development” continues to

exert such a powerful attraction for programmers.

Chapter 1 How does Xamarin.Forms fit in? 2

Cross-platform mobile development

The personal computer industry has experienced a massive shift in recent years. Desktop computers

still exist, of course, and they remain vital for tasks that require keyboards and large screens: program-

ming, writing, spread-sheeting, data tracking. But much of personal computing now occurs on smaller

devices, particularly for quick information, media consumption, and social networking. Tablets and

smartphones have a fundamentally different user-interaction paradigm based primarily on touch, with

a keyboard that pops up only when necessary.

The mobile landscape

Although the mobile market has the potential for rapid change, currently two major phone and tablet

platforms dominate:

 The Apple family of iPhones and iPads, all of which run the iOS operating system.

 The Android operating system, developed by Google based on the Linux kernel, which runs on

a variety of phones and tablets.

How the world is divided between these two giants depends on how they are measured: there are

more Android devices currently in use, but iPhone and iPad users are more devoted and spend more

time with their devices.

There is also a third mobile development platform, which is not as popular as iOS and Android but

involves a company with a strong history in the personal computer industry:

 Microsoft’s Windows Phone.

In recent years, the Windows Phone has become a more compelling alternative as Microsoft has been

merging the APIs of its platforms. A single API called the Windows Runtime (or WinRT) is now available

for applications running on desktop machines, laptops, tablets, and phones.

For software developers, the optimum strategy is to target more than just one of these platforms.

But that’s not easy. There are four big obstacles:

Problem 1: Different user-interface paradigms

All three platforms incorporate similar ways of presenting the graphical user interface (GUI) and inter-

action with the device through multitouch, but there are many differences in detail. Each platform has

different ways to navigate around applications and pages, different conventions for the presentation of

data, different ways to invoke and display menus, and even different approaches to touch.

Users become accustomed to interacting with applications on a particular platform and expect to

leverage that knowledge with future applications as well. Each platform acquires its own associated

culture, and these cultural conventions then influence developers.

Chapter 1 How does Xamarin.Forms fit in? 3

Problem 2: Different development environments

Programmers today are accustomed to working in a sophisticated integrated development environ-

ment (IDE). Such IDEs exist for all three platforms, but of course they are different:

 For iOS development, Xcode on the Mac.

 For Android development, Android Studio on a variety of platforms.

 For Windows Phone development, Visual Studio on the PC.

Problem 3: Different programming interfaces

All three of these platforms are based on different operating systems with different APIs. In many cases,

the three platforms all implement similar types of user-interface objects but with different names.

For example, all three platforms have something that lets the user toggle a Boolean value:

 On the iPhone, it’s a “view” called UISwitch.

 On Android devices, it’s a “widget” called Switch.

 On Windows Phone, one possibility is a “control” called ToggleSwitchButton from the

Windows Phone Toolkit NuGet package.

Of course, the differences go far beyond the names into the programming interfaces themselves.

Problem 4: Different programming languages

Developers have some flexibility in choosing a programming language for each of these three plat-

forms, but, in general, each platform is very closely associated with a particular programming lan-

guage:

 Objective-C for the iPhone

 Java for Android devices

 C# for Windows Phone

These three languages are cousins because they are all object-oriented descendants of C, but they

have become rather distant cousins.

For these reasons, a company that wants to target multiple platforms might very well employ three

different programmer teams, each team skilled and specialized in a particular language and API.

This language problem is particularly nasty, but it’s the problem that is the most tempting to solve:

If you could use the same programming language for these three platforms, you could at least share

some code between the platforms. This shared code likely wouldn’t be involved with the user interface

because each platform has different APIs, but there might well be application code that doesn’t touch

Chapter 1 How does Xamarin.Forms fit in? 4

the user interface at all.

A single language for these three platforms would certainly be convenient. But what language

would that be?

The C# and .NET solution

A roomful of programmers would come up with a variety of answers to the question just posed, but a

good argument can be made in favor of C#. Unveiled by Microsoft in the year 2000, C# is a fairly new

programming language, at least when compared with Objective-C and Java. At first, C# seemed to be a

rather straightforward, strongly typed, imperative object-oriented language, certainly influenced by

C++ (and Java as well), but with a much cleaner syntax than C++ and none of the historical baggage.

In addition, the first version of C# had language-level support for properties and events, which turn out

to be member types that are particularly suited for programming graphical user interfaces.

But C# has continued to grow and get better over the years. The support of generics, lambda func-

tions, LINQ, and asynchronous operations has successfully elevated C# to be classified as a multipara-

digm programming language. C# code can be traditionally imperative or flavored with declarative or

functional programming paradigms.

Also exciting is the future of C#: on April 3, 2014, C# creator Anders Hejlsberg stood on a stage at

the Microsoft Build 2014 conference and clicked a button that published an open-source version of the

C# compiler, called the .NET Compiler Platform (formerly known by its code name “Roslyn”).

Since its inception, C# has been closely associated with the Microsoft .NET Framework. At the lowest

level, .NET provides an infrastructure for the C# basic data types (int, double, string, and so forth).

But the extensive .NET Framework class library provides support for many common chores encoun-

tered in many different types of programming. These include:

 Math

 Debugging

 Reflection

 Collections

 Globalization

 File I/O

 Networking

 Security

 Threading

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