通过实例学习Kotlin开发Android应用

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"Kotlin for android developers - 学习如何用Kotlin开发Android应用" 这本书是为那些希望使用Kotlin语言进行Android应用开发的开发者准备的。作者Antonio Leiva通过构建一个完整的Android应用程序，让读者在实践中学习Kotlin。与传统的参考书籍不同，本书更注重实践和例子，以便读者能够更好地理解和掌握这门语言。 Kotlin是一种现代的、静态类型的编程语言，它在Android开发中逐渐成为主流，因其简洁的语法和对Java虚拟机（JVM）的良好支持而受到欢迎。书中将对比Kotlin和Java 7，帮助读者了解两者之间的差异，并突出Kotlin的特点，如空安全、类型安全和更少的样板代码，这些特性可以提高开发效率和代码质量。书中涵盖的主要知识点包括： 1. **Kotlin基础**：变量声明、数据类型、控制流（if/else, for, while）、函数和类的定义。 2. **高阶函数和 Lambda 表达式**：理解如何使用函数作为参数传递，以及使用Lambda简化代码。 3. **扩展函数和属性**：Kotlin允许在不修改原有类的情况下为其添加新的功能。 4. **空安全**：Kotlin强制检查空值，以减少因空指针异常而导致的错误。 5. **类型系统**：了解可空和不可空类型的区别，以及智能转换的概念。 6. **构造器和初始化块**：Kotlin中的类构造和对象初始化方式。 7. **协程（Coroutines）**：Kotlin提供的轻量级线程机制，用于处理异步操作。 8. **Anko库**：Kotlin的Android库，简化了Android API的使用。 9. **Android特定的Kotlin特性**：如Anko DSL进行布局编写，和Kotlin Android Extensions简化视图访问。 10. **Android Studio集成**：学习如何在Android Studio中配置Kotlin环境，以及使用Kotlin插件的便利功能。 11. **测试和调试**：Kotlin中的单元测试和集成测试，以及调试技巧。 12. **模块化和依赖注入**：如何利用Kotlin进行项目结构优化和依赖管理。 13. **Jetpack组件的使用**：结合Kotlin，学习如何使用Android Jetpack组件，如LiveData、ViewModel和Room数据库。作者还特别感谢了那些支持他写作的人，包括设计封面的Luis Herrero Jiménez和审阅书籍的Gautier Mechling，他们的努力使得这本书的质量得以保证。此外，作者鼓励读者分享这本书的信息，并欢迎任何对提升书本质量的建议和反馈。如果你有任何问题或想法，可以直接联系作者。通过这本书，开发者不仅能学会Kotlin语言，还能了解到如何将Kotlin的优势应用到实际的Android开发中，从而提升开发效率和应用性能。

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1 Introduction 8

1 fun Fragment.toast(message: CharSequence, duration: Int = Toast.LENGTH_SHORT) {

2 Toast.makeText(getActivity(), message, duration).show()

3 }

We can now do:

1 fragment.toast("Hello world!")

Functional support (Lambdas)

What if, instead of having to implement an anonymous class every time we need to implement

a click listener, we could just define what we want to do? We can indeed. This (and many more

interesting things) is what we get thanks to lambdas:

1 view.setOnClickListener { toast("Hello world!") }

This is only a small selection of what Kotlin can do to simplify your code. Now that you know some

of the many interesting features of the language, you may decide this is not for you. If you continue,

we’ll start with the practice right away in the next chapter.

2 Getting ready

Now that you know some little examples of what you may do with Kotlin, I’m sure you want to start

to put it into practice as soon as possible. Don’t worry, these first chapters will help you configure

your development environment so that you can start writing some code immediately.

2.1 Android Studio

First thing you need is to have Android Studio installed. As you may know, Android Studio is the

official Android IDE, which was publicly presented in 2013 as a preview and finally released in 2014.

Android Studio is implemented as a plugin over IntelliJ IDEA⁶, a Java IDE created by Jetbrains⁷, the

company which is also behind Kotlin. So, as you can see, everything is tightly connected.

The adoption of Android Studio was an important change for Android developers. First, because

we left behind the buggy Eclipse and moved to a software specifically designed for Java developers,

which gives us a perfect interaction with the language. We enjoy awesome features such as a fast and

impressively smart code completion, or really powerful analysing and refactor tools among others.

And second, Gradle⁸ became the official build system for Android, which meant a whole bunch of

new possibilities related to version building and deploy. Two of the most interesting functions are

build systems and flavours, which let you create infinite versions of the app (or even different apps)

in an easy way while using the same code base.

If you are still using Eclipse, I’m afraid you need to switch to Android Studio if you want to follow

this book. The Kotlin team is creating a plugin for Eclipse, but it will be always far behind the one

for Android Studio, and the integration won’t be so perfect. You will also discover what you are

missing really soon as you start using it.

I’m not covering the use of Android Studio or Gradle because this is not the focus of the book, but

if you haven’t used these tools before, don’t panic. I’m sure you’ll be able to follow the book and

learn the basics in the meanwhile.

Download Android Studio from the official page⁹ if you don’t have it already.

⁶https://www.jetbrains.com/idea

⁷https://www.jetbrains.com

⁸https://gradle.org/

⁹https://developer.android.com/sdk/index.html

3 Creating a new project

If you are already used to Android Studio and Gradle, this chapter will be quite easy. I don’t want to

give many details nor screens, because UI changes from time to time and these lines won’t be useful

anymore.

Our app is consisting on a simple weather app, such as the one used in Google’s Beginners Course

in Udacity¹⁰. We’ll be probably paying attention to different things, but the idea of the app will be

the same, because it includes many of the things you will find in a typical app. If your Android level

is low I recommend this course, it’s really easy to follow.

3.1 Create the project in Android Studio

First of all, open Android Studio and choose Create new Project. It will ask for a name, you can

call it whatever you want: WeatherApp for instance. Then you need to set a Company Domain. As

you are not releasing the app, this field is not very important either, but if you own a domain, you

can use that one. Also choose the location where you want to save the project.

In the next step, you’ll be asked about the minimum API version. We’ll select API 15, because one

of the libraries we’ll be using needs API 15 as minimum. You’ll be targeting most Android users

anyway. Don’t choose any other platform rather than Phone and Tablet for now.

Finally, we are required to choose an activity template to start with. We can choose Add no Activity

and start from scratch (that would be the best idea when starting a Kotlin project), but we’ll rather

choose Empty Activity because I’ll show you later an interesting feature from the Kotlin plugin.

Don’t worry much about the name of the activities, layouts, etc. that you will find in next screen.

We’ll change them later if we need to. Press Finish and let Android Studio do its work.

3.2 Configure Gradle

The Kotlin plugin includes a tool which does the Gradle configuration for us. But I prefer to keep

control of what I’m writing in my Gradle files, otherwise they can get messy rather easily. Anyway,

it’s a good idea to know how things work before using the automatic tools, so we’ll be doing it

manually this time.

First, you need to modify the parent build.gradle so that it looks like this:

¹⁰https://www.udacity.com/course/android-development-for-beginners--ud837

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通过实例学习Kotlin开发Android应用

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