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Android Programming with

Kotlin for Beginners

Build Android apps starting from zero programming

experience with the new Kotlin programming language

John Horton

BIRMINGHAM - MUMBAI

Android Programming with Kotlin for Beginners

Copyright © 2019 Packt Publishing

All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, without the prior written permi ssion of the publisher, except in the case of brief quotations embedded in critical articles or reviews. Every effort has been made in the preparation of this book to ensure the accuracy of the information presented. However, the information contained in this book i s sold without warranty, either express or implied. Neither the author, nor Packt Publishing or i ts dealers and distributors, will be held liable for any damages caused or alleged to have been cause d directly or indirectly by this book. Packt Publishing has endeavored to provide trademark information about a ll of the companies and products mentioned in this book by the appropriate use of capitals. Howe ver, Packt Publishing cannot guarantee the accuracy of this information.

Commissioning Editor:

Pavan Ramchandani

Acquisition Editor: Larissa Pinto

Content Development Editor:

Smit Carvalho

Technical Editor: Surabhi Kulkarni

Copy Editor:

Project Coordinator: Kinjal Bari

Proofreader:

Indexers:

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Graphics:

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Production Coordinator:

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First published: April 2019

Production reference: 1260419

Published by Packt Publishing Ltd.

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ISBN 978-1-78961-540-1

www.packtpub.com I think that everybody has an app inside of them, and all you need do is work hard enough to get it out of you.

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Contributors

About the author

John Horton is a programming and gaming enthusiast based in the UK. He has a passion for writing apps, games, books, and blog articles. He is the fou nder of Game

Code School.

About the reviewers

Ashok Kumar S has been working in the mobile development domain for about six years. In his early days, he was a JavaScript and Node develop er. With his strong web development skills, he mastered web and mobile development. H e is a developers. He also contributes heavily to open source to improve his e- karma. He has written books on WearOS programming and the Firebase toolchain. Ashok has also reviewed books on mobile and web development, namely

Mastering Junit5

Android Programming for Beginners

, and

Developing Enterprise

Applications Using JavaScript

Mitchell Wong Ho was born in Johannesburg, South Africa, where he completed his national diploma in electrical engineering. Mitchell's software deve lopment career started on embedded systems and then moved to Microsoft desktop/s erver applications. Mitchell has been programming in Java since 2000 on J2ME, JEE, desktop, and Android applications, and has more recently been advocating Kotlin for A ndroid.

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If you're interested in becoming an author for Packt, please visit authors.packtpub. com and apply today. We have worked with thousands of developers and tech professionals, just like you, to help them share their insight with the global tech we are recruiting an author for, or submit your own idea. [ i ]

Table of Contents

Preface

xv

Chapter 1:

Getting Started with Android and Kotlin

1

Why use Kotlin and Android?

2

The Android API

5

Kotlin is object-oriented

6

Run that by me again - what, exactly, is Android?

7

Android Studio

8

Setting up Android Studio

9

Final step - for now

16

What makes an Android app?

17

Android resources

17

The structure of Android's code

18

Packages

18

Classes

19

Functions

19

Possible extra step 1

24

Possible extra step 2

25

Deploying the app so far

25
Running and debugging the app on an Android emulator 26

Running the app on a real device

29

Table of Contents

[ ii ]

Chapter 2:

Filtering the logcat output

35

Code folding (hiding) in Android Studio

36

The package declaration

37

Importing classes

37

The class declaration

38

Functions inside the class

38

A summary of the Kotlin code so far

39

UI layout elements

41

UI text elements

42

Adding a button via the visual designer

43

Editing the button's attributes

45

Examining the XML code for the new button

48

Adding a button by editing the XML code

49

Giving the buttons unique id attributes

51

Positioning the two buttons in the layout

52

Leaving comments in our Kotlin code

56

Coding messages to the user and the developer

57

Adding message code to the onCreate function

58

Examining the output

59

Writing our own Kotlin functions

60

Examining the output

62
Exploring Android Studio and the Project Structure 65

A quick guided tour of Android Studio

66

Project Explorer and project anatomy

68

The Empty Activity project

68

Exploring the Empty Activity project

70

The manifests folder

71

The java folder

74

The res folder

76

The res/drawable folder

76

The res/layout folder

77

The res/mipmap folder

78

The res/values folder

79

Table of Contents

[ iii ]

Exploring the Basic Activity project

85

The extra functions in MainActivity.kt

88

Exploring the Android emulator

89

The emulator control panel

90

Using the emulator as a real device

92

Accessing the app drawer

93

Viewing active apps and switching between apps

94

Summary

95

Getting Started with Layouts and Material Design

97

Material design

98

Exploring Android UI design

98

Layouts

99

Creating the Exploring Layouts project

99

Adding a LinearLayout to the project

101

Preparing your workspace

102

Examining the generated XML 103

Adding a TextView to the UI

103

Adding a multi-line TextView to the UI

106

Wiring up the UI with the Kotlin code (part 2)

116

Building a precise UI with ConstraintLayout

116

Adding a CalenderView

116

Resizing a view in a ConstraintLayout

117

Using the Component Tree window

118

Adding constraints manually

120

Adding and constraining more UI elements

121

Making the text clickable

125

Laying out data with TableLayout

126

Adding a TableRow to TableLayout

126
Using the Component Tree when the visual designer won't do 127

Organizing the table columns

128

Linking back to the main menu

130

Table of Contents

[ iv ]

Chapter 5:

Sizing using dp

134

Sizing fonts using sp

134

Determining size with wrap or match

135

Using padding and margin

137

Using the layout_weight property

138

Using gravity

139

Setting the view with Kotlin code

142

Adding image resources

142

Creating the content for the cards

143

Adding CardView to our layout

147

Themes and material design

152

Using the Android Studio theme designer

153

Creating a tablet emulator

156

Frequently asked question

159

Summary

159

Chapter 6:

The Android Lifecycle

161

The life and times of an Android app

162

How Android interacts with our apps

162

How we handle the lifecycle phases

165

The lifecycle demo app

168

Coding the lifecycle demo app

168

Running the lifecycle demo app

171

Examining the lifecycle demo app output

172

The structure of Kotlin code - revisited

175

Summary

176

Chapter 7:

Kotlin Variables, Operators, and Expressions

177

Learning the jargon

178

More on code comments

179

Variables

181

Types of variables

182

Declaring and initializing variables

184

Saving keystrokes with type inference

186

Table of Contents

[ v ]

Operators and expressions

188

The assignment operator

188

The addition operator

188

The subtraction operator

189

The division operator

189

The multiplication operator

189

The increment operator

190

The decrement operator

190

Chapter 8:

Kotlin Decisions and Loops

195

Making decisions in Kotlin

196

Indenting code for clarity

196

More Kotlin operators

197
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