Android recipes

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BOOKS FOR PROFESSIONALS BY PROFESSIONALS®

Android Recipes, Fourth Edition offers more than 100 down-to-earth code recipes,

and guides you step-by-step through a wide range of useful topics using complete

and real-world working code examples. This book is updated to include the Android 5.0

SDK - including the new Android Wear and TV SDKs - as well as earlier releases.

Instead of abstract descriptions of complex concepts, in Android Recipes, you’ll find live

code examples. When you start a new project you can consider copying and pasting

the code and configuration files from this book and then modifying them for your own

customization needs.

Crammed with insightful instruction and helpful examples, this fourth edition of

Android Recipes is your guide to writing apps for one of today’s hottest mobile platforms.

It offers pragmatic advice that will help you get the job done quickly and well. This can

save you a great deal of work over creating a project from scratch!

• Code for Android smartphones, tablets and now wearables and TV apps

• Use external libraries to save time and effort

• Boost app performance by using the Android NDK and RenderScript

• Design apps for performance, responsiveness, and seamlessness

• Send data between devices and other external hardware

• Persist application data and share it between applications

• Capture and play back various device media items

• Communicate with web services

• Get the most out of your user interface

Companion

eBook

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SOURCE CODE ONLINE

FOURTH

EDITION Smith

Android

Recipes

Dave Smith

A Problem-Solution Approach

Recipes Android

Android

FOURTH EDITION

Hundreds of problems and solutions of code recipes

using Android 5.0

9 781484 204764

54999

ISBN 978-1-4842-0476-4

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For your convenience Apress has placed some of the front

matter material after the index. Please use the Bookmarks

and Contents at a Glance links to access them.

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iii

Contents at a Glance

About the Author ��xxi

About the Technical Reviewer ��xxiii

Acknowledgments��xxv

Introduction��xxvii

■Chapter 1: Layouts and Views �� 1

■Chapter 2: User Interaction Recipes�� 89

■Chapter 3: Communications and Networking�� 199

■Chapter 4: Interacting with Device Hardware and Media�� 289

■Chapter 5: Persisting Data�� 391

■Chapter 6: Interacting with the System�� 471

■Chapter 7: Graphics and Drawing�� 613

■Chapter 8: Working with Android NDK and RenderScript�� 689

Index�� 737

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xxvii

Introduction

Welcome to the fourth edition of Android Recipes!

If you are reading this book, you probably don’t need to be told of the immense opportunity

that mobile devices represent for software developers and users. In recent years, Android

has become one of the top mobile platforms for device users. This means that you, as a

developer, must know how to harness Android so you can stay connected to this market

and the potential that it offers. But any new platform brings with it uncertainty about best

practices and solutions to common needs and problems.

What we aim to do with Android Recipes is give you the tools to write applications for the

Android platform through direct examples targeted at the specific problems you are trying

to solve. This book is not a deep dive into the Android SDK, NDK, or any of the other tools.

We don’t weigh you down with all the details and theory behind the curtain. That’s not to

say that those details aren’t interesting or important. You should take the time to learn them,

as they may save you from making future mistakes. However, more often than not, they are

simply a distraction when you are just looking for a solution to an immediate problem.

This book is not meant to teach you Java programming or even the building blocks of an

Android application. You won’t find many basic recipes in this book (such as how to display

text with TextView, for instance), as we feel these are tasks easily remembered once learned.

Instead, we set out to address tasks that developers, once comfortable with Android, need

to do often but find too complex to accomplish with a few lines of code.

Treat Android Recipes as a reference to consult, a resource-filled cookbook that you can

always open to find the pragmatic advice you need to get the job done quickly and well.

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xxviii Introduction

What Will You Find in the Book?

We dive into using the Android SDK to solve real problems. You will learn tricks for effectively

creating a user interface that runs well across device boundaries. You will become a master

at incorporating the collection of hardware (radios, sensors, and cameras) that makes

mobile devices unique platforms. We’ll even discuss how to make the system work for you

by integrating with the services and applications provided by Google and various device

manufacturers.

Performance matters if you want your applications to succeed. Most of the time, this isn’t

a problem because the Android runtime engines get progressively better at compiling

bytecode into the device’s native code. However, you might need to leverage the Android

NDK to boost performance. Chapter 8 offers you an introduction to the NDK and integrating

native code into your application using Java Native Interface (JNI) bindings.

The NDK is a complex technology, which can also reduce your application’s portability.

Also, while good at increasing performance, the NDK doesn’t address multicore processing

very well for heavy workloads. Fortunately, Google has eliminated this tedium and simplified

the execute-on-multiple-cores task while achieving portability by introducing RenderScript.

Chapter 8 introduces you to RenderScript and shows you how to use its compute engine

(and automatically leverage CPU cores) to process images.

Keep a Level Eye on the Target

Throughout the book, you will see that we have marked most recipes with the minimum API

level that is required to support them. Most of the recipes in this book are marked API Level 1,

meaning that the code used can be run in applications targeting any version of Android

since 1.0. However, where necessary, we use APIs introduced in later versions. Pay close

attention to the API level marking of each recipe to ensure that you are not using code that

doesn’t match up with the version of Android your application is targeted to support.

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Chapter 1

Layouts and Views

The Android platform is designed to operate on a variety of device types, screen sizes,

and screen resolutions. To assist developers in meeting this challenge, Android provides a

rich toolkit of user interface (UI) components to utilize and customize to the needs of their

specific applications. Android also relies heavily on an extensible XML framework and set

resource qualifiers to create liquid layouts that can adapt to these environmental changes.

In this chapter, we take a look at some practical ways to shape this framework to fit your

specific development needs.

1-1. Styling Common Components

Problem

You want to create a consistent look and feel for your application across all the versions of

Android your users may be running, while reducing the amount of code required to maintain

those customizations.

Solution

(API Level 1)

You can abstract common attributes that define the look and feel of your application views

into XML styles. Styles are collections of view attribute customizations, such as text size

or background color, that should be applied to multiple views throughout the application.

Abstracting these attributes into a style allows the common elements to be defined in a

single location, making the code easier to update and maintain.

Android also supports grouping multiple styles together in a global element called a theme.

Themes apply to an entire context (such as an activity or application), and define styles that

should apply to all the views within that context. Every activity launch in your application has

a theme applied to it, even if you don’t define one. In such cases, the default system theme

is applied instead.

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2 CHAPTER 1: Layouts and Views

How It Works

To explore the styles concept, let’s create an activity layout that looks like Figure 1-1.

Figure 1-1. Styled widgets

As you can see, this view has some elements that we want to customize to look different

than they normally do with the styling from the default system theme applied. One option

would be to define all the attributes for all the views directly in our activity layout. If we were

to do so, it would look like Listing 1-1.

Listing 1-1. res/layout/activity_styled.xml

<?xml version="1.0" encoding="utf-8"?>

<TableLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:layout_width="match_parent"

android:layout_height="match_parent"

android:padding="8dp">

<TextView

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:textSize="22sp"

android:textStyle="bold"

android:text="Select One"/>

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CHAPTER 1: Layouts and Views 3

<RadioGroup

android:layout_width="match_parent"

android:layout_height="wrap_content"

android:orientation="horizontal">

<RadioButton

android:layout_width="0dp"

android:layout_height="wrap_content"

android:layout_weight="1"

android:minHeight="@dimen/buttonHeight"

android:button="@null"

android:background="@drawable/background_radio"

android:gravity="center"

android:text="One"/>

<RadioButton

android:layout_width="0dp"

android:layout_height="wrap_content"

android:layout_weight="1"

android:minHeight="@dimen/buttonHeight"

android:button="@null"

android:background="@drawable/background_radio"

android:gravity="center"

android:text="Two"/>

<RadioButton

android:layout_width="0dp"

android:layout_height="wrap_content"

android:layout_weight="1"

android:minHeight="@dimen/buttonHeight"

android:button="@null"

android:background="@drawable/background_radio"

android:gravity="center"

android:text="Three"/>

</RadioGroup>

<TextView

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:textSize="22sp"

android:textStyle="bold"

android:text="Select All"/>

<CheckBox

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:minHeight="@dimen/buttonHeight"

android:minWidth="@dimen/checkboxWidth"

android:button="@null"

android:gravity="center"

android:textStyle="italic"

android:textColor="@color/text_checkbox"

android:text="One"/>

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4 CHAPTER 1: Layouts and Views

<CheckBox

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:minHeight="@dimen/buttonHeight"

android:minWidth="@dimen/checkboxWidth"

android:button="@null"

android:gravity="center"

android:textStyle="italic"

android:textColor="@color/text_checkbox"

android:text="Two"/>

<CheckBox

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:minHeight="@dimen/buttonHeight"

android:minWidth="@dimen/checkboxWidth"

android:button="@null"

android:gravity="center"

android:textStyle="italic"

android:textColor="@color/text_checkbox"

android:text="Three"/>

</TableRow>

<Button

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:minWidth="@dimen/buttonWidth"

android:background="@drawable/background_button"

android:textColor="@color/accentPink"

android:text="@android:string/ok"/>

<Button

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:minWidth="@dimen/buttonWidth"

android:background="@drawable/background_button"

android:textColor="@color/accentPink"

android:text="@android:string/cancel"/>

</TableRow>

</TableLayout>

To add emphasis, we’ve highlighted the attributes in each view that are common to other

views of the same type. These are the attributes that make the buttons, text headings, and

checkable elements all look the same. There’s a lot of duplication to make this happen, and

we can clean it up with a style.

First, we need to create a new resource file, and define each attribute group with a <style>

tag. Listing 1-2 shows the completed abstractions.

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CHAPTER 1: Layouts and Views 5

Listing 1-2. res/values/styles.xml

</style>

<item name="android:minWidth">@dimen/buttonWidth</item>

<item name="android:background">@drawable/background_button</item>

<item name="android:textColor">@color/accentPink</item>

</style>

<item name="android:minHeight">@dimen/buttonHeight</item>

<item name="android:background">@drawable/background_radio</item>

<item name="android:gravity">center</item>

</style>

<item name="android:minHeight">@dimen/buttonHeight</item>

<item name="android:minWidth">@dimen/checkboxWidth</item>

<item name="android:gravity">center</item>

<item name="android:textStyle">italic</item>

<item name="android:textColor">@color/text_checkbox</item>

</style>

</resources>

A <style> groups together the common attributes we need to apply to each view type.

Views can accept only a single style definition, so all the attributes for that view must be

collected in one group. Styles do support inheritance, however, which allows us to cascade

our definitions of each style before they are applied to the view.

Notice how each style also declares a parent. This is the base framework style that we

should inherit from. Parent styles are not required, but because of the single style rule on

each view, overwriting the default with your custom version replaces the theme’s default.

If you don’t inherit from a base parent, you will be forced to define all the attributes that

view needs. Extending a widget’s style from the framework’s base ensures that we are

responsible only for adding the attributes we want to customize beyond the default theme’s

look and feel.

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6 CHAPTER 1: Layouts and Views

EXPLICIT VS. IMPLICIT PARENTING

Style inheritance takes one of two forms. A style can explicitly declare its parent, as we’ve seen before:

NewStyle is an extension of BaseStyle, and includes all the attributes defined in the parent. Styles also

support an implicit parenting syntax as follows:

In the same way, BaseStyle.Extended inherits its attributes from BaseStyle. The functionality of this

version is identical to the explicit example, just in a more compact convention. The two forms should never be

mixed, and doing so doesn’t allow for multiple parents on a single style. When this is done, the explicit parent

always wins anyway, and the readability of the code is reduced.

We can apply the new styles to our original layout file, and the cleaner result is shown in

Listing 1-3.

Listing 1-3. res/layout/activity_styled.xml

<?xml version="1.0" encoding="utf-8"?>

<TableLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:layout_width="match_parent"

android:layout_height="match_parent"

android:padding="8dp">

<TextView

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:textAppearance="@style/LabelText"

android:text="Select One"/>

<RadioGroup

android:layout_width="match_parent"

android:layout_height="wrap_content"

android:orientation="horizontal">

<RadioButton

style="@style/FormRadioButton"

android:layout_width="0dp"

android:layout_height="wrap_content"

android:layout_weight="1"

android:text="One"/>

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CHAPTER 1: Layouts and Views 7

<RadioButton

style="@style/FormRadioButton"

android:layout_width="0dp"

android:layout_height="wrap_content"

android:layout_weight="1"

android:text="Two"/>

<RadioButton

style="@style/FormRadioButton"

android:layout_width="0dp"

android:layout_height="wrap_content"

android:layout_weight="1"

android:text="Three"/>

</RadioGroup>

<TextView

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:textAppearance="@style/LabelText"

android:text="Select All"/>

<CheckBox

style="@style/FormCheckBox"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="One"/>

<CheckBox

style="@style/FormCheckBox"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="Two"/>

<CheckBox

style="@style/FormCheckBox"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="Three"/>

</TableRow>

<Button

style="@style/FormButton"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="@android:string/ok"/>

<Button

style="@style/FormButton"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="@android:string/cancel"/>

</TableRow>

</TableLayout>

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8 CHAPTER 1: Layouts and Views

By applying a style attribute to each view, we can remove the explicit attribute references

that were duplicated in favor of a single reference on each element. The one exception to

this behavior is our TextView headings, which accept a special android:textAppearance

attribute. This attribute takes a style reference, and applies only to text-formatting attributes

(size, style, color, and so forth). When used, a TextView still allows a separate style attribute

to be applied concurrently. In this way, it is the one supported instance in the framework of

multiple styles on a single view.

Themes

A theme in Android is a type of appearance style that is applicable to an entire application or

activity. There are two choices when applying a theme: use a system theme or create a custom

one. In either case, a theme is applied in the AndroidManifest.xml file, as shown in Listing 1-4.

Listing 1-4. AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>

<manifest xmlns:android="http://schemas.android.com/apk/res/android"

...>

<!—Apply to the application tag for a global theme -->

<application android:theme="APPLICATION_THEME_NAME"

...>

<!—Apply to the activity tag for an individual theme -->

<activity android:name=".Activity"

android:theme="ACTIVITY_THEME_NAME"

...>

<intent-filter>

...

</intent-filter>

</activity>

</application>

</manifest>

System Themes

The styles.xml and themes.xml files packaged with the Android framework include a few

options for themes with some useful custom properties. Referencing R.style in the SDK

documentation will provide the full list, but here are a few useful examples:

 Theme.Light: Variation on the standard theme that uses an inverse

color scheme for the background and user elements. This is the default

recommended base theme for applications prior to Android 3.0.

 Theme.NoTitleBar.Fullscreen: Removes the title bar and status bar, filling

the entire screen (minus any onscreen controls that may be present).

 Theme.Dialog: A useful theme to make an activity look like a dialog box.

 Theme.Holo.Light: (API Level 11) Theme that uses an inverse color

scheme and that has an action bar by default. This is the default

recommended base theme for applications on Android 3.0.

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CHAPTER 1: Layouts and Views 9

 Theme.Holo.Light.DarkActionBar: (API Level 14) Theme with an

inverse color scheme but a dark solid action bar. This is the default

recommended base theme for applications on Android 4.0.

 Theme.Material.Light: (API Level 21) Theme with a simplified color

scheme governed by a small palette of primary colors. This theme also

supports tinting of the standard widgets using the supplied primary

colors. This is the default recommended base theme for applications on

Android 5.0.

Note When using the AppCompat Library, other versions for each of these themes should be used

instead (for example, Theme.AppCompat.Light.DarkActionBar).

Listing 1-5 is an example of a system theme applied to the entire application by setting the

android:theme attribute in the AndroidManifest.xml file.

Listing 1-5. Manifest with Theme Set on Application

<?xml version="1.0" encoding="utf-8"?>

<manifest xmlns:android="http://schemas.android.com/apk/res/android"

...>

<!—Apply to the application tag for a global theme -->

<application android:theme="Theme.Material.Light"

...>

...

</application>

</manifest>

Custom Themes

Sometimes the provided system choices aren’t enough. After all, some of the customizable

elements in the window are not even addressed in the system options. Defining a custom

theme to do the job is simple.

If there is not one already, create a styles.xml file in the res/values path of the project.

Remember, themes are just styles applied on a wider scale, so they are defined in the same

place. Theme aspects related to window customization can be found in the R.attr reference

of the SDK, but here are the most common items:

 android:windowNoTitle: Governs whether to remove the default title bar;

set to true to remove the title bar.

 android:windowFullscreen: Governs whether to remove the system

status bar; set to true to remove the status bar and fill the entire screen.

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10 CHAPTER 1: Layouts and Views

 android:windowBackground: Color or drawable resource to apply as a

background.

 android:windowContentOverlay: Drawable placed over the window

content foreground. By default, this is a shadow below the status bar.

Set to any resource to use in place of the default status bar shadow, or

null (@null in XML) to remove it.

In addition, the Material themes accept a series of color attributes that are used to tint the

application interface widgets:

 android:colorPrimary: Used to tint primary interface elements, like the

action bar and the scrolling edge glow effects. Also affects the recent

tasks title bar color.

 android:colorPrimaryDark: Tints the system controls, such as the

status bar background.

 android:colorAccent: Default color applied to controls that are focused

or activated.

 android:colorControlNormal: Override color for controls that are not

focused or activated.

 android:colorControlActivated: Override color for focused and

activated controls. Takes place of the accent color if both are defined.

 android:colorControlHighlight: Override color for controls that are

being pressed.

Listing 1-6 is an example of a styles.xml file that creates a custom theme in order to supply

brand-specific colors for the application interface.

Listing 1-6. res/values/styles.xml

<?xml version="1.0" encoding="utf-8"?>

<item name="colorPrimary">@color/primaryBlue</item>

<item name="colorPrimaryDark">@color/primaryDarkBlue</item>

<item name="colorAccent">@color/accentPink</item>

<item name="colorControlNormal">@color/controlNormalGreen</item>

<item name="colorControlActivated">@color/controlActivatedGreen</item>

</style>

</resources>

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