Apress Pro Android Python with SL4A (2011).pdf

Build Android Apps

with Python

Pro

Android Python

with SL4A

Paul Ferrill

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.

Contents at a Glance

About the Author ....................................................................................................... xi

About the Technical Reviewer .................................................................................. xii

Acknowledgments ................................................................................................... xiii

Preface .................................................................................................................... xiv

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Chapter 1: Introduction ........................................................................................... 1

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Chapter 2: Getting Started ..................................................................................... 27

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Chapter 3: Navigating the Android SDK ................................................................. 57

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Chapter 4: Developing with Eclipse ....................................................................... 83

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Chapter 5: Exploring the Android API .................................................................. 113

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Chapter 6: Background Scripting with Python .................................................... 139

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Chapter 7: Python Scripting Utilities ................................................................... 165

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Chapter 8: Python Dialog Box–based GUIs .......................................................... 195

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Chapter 9: Python GUIs with HTML ...................................................................... 221

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Chapter 10: Packaging and Distributing .............................................................. 249

Index ....................................................................................................................... 273

C H A P T E R 1

Introduction

This book is about writing real-world applications for the Android platform primarily using the Python

language and a little bit of JavaScript. While there is nothing wrong with Java, it really is overkill when all

you need to do is turn on or off a handful of settings on your Android device. The Scripting Layer for

Android (SL4A) project was started to meet that specific need. This book will introduce you to SL4A and

give you the power to automate your Android device in ways you never thought possible.

Why SL4A?

One of the first questions you probably have about this book is, “Why would I want to use SL4A instead

of Java?” There are several answers to that question. One is that not everyone is a fan of Java. The Java

language is too heavyweight for some and is not entirely open source. It also requires the use of an edit /

compile / run design loop that can be tedious for simple applications. An equally legitimate answer is

simply “I want to use X”, where X could be any number of popular languages.

Google provides a comprehensive software development kit (SDK) aimed specifically at Java

developers, and most applications available from the Android market are probably written in Java. I’ll

address the Android SDK in Chapter 3 and use a number of the tools that come with it throughout the

book.

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Note SL4A currently supports Beanshell, JRuby, Lua, Perl, PHP, Python, and Rhino.

SL4A is really targeted at anyone looking for a way to write simple scripts to automate tasks on an

Android device using any of the supported languages, including Java through Beanshell. It provides an

interactive console in which you can type in a line of code and immediately see the result. It even makes

it possible, in many cases, to reuse code you’ve written for a desktop environment. The bottom line is

that SL4A makes it possible both to write code for Android-based devices in languages other than Java

and to do it in a more interactive way.

CHAPTER 1

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INTRODUCTION

The World of Android

Google jumped into the world of mobile operating systems in a big way when it bought Android, Inc. in

2005. It’s really pretty amazing how far it has come in such a short time. The Android community is huge

and has spawned a wide range of conferences, books, and support materials that are easily available

over the Internet.

This is a good point to define a few terms that you’ll see throughout the rest of this book. Android

applications are typically packaged into .apk files. These are really just .zip files containing everything

needed by the application. In fact, if you rename an .apk file to .zip, you can open it with any archive

tool and examine the contents.

Most Android devices come from the manufacturer with the systems files protected to prevent any

inadvertent or malicious manipulation. The Android operating system (OS) is essentially Linux at the

core and provides much of the same functionality you would find on any Linux desktop. There are ways

to unlock the system areas and provide root , or unrestricted, access to the entire filesystem on an

Android device. This process is appropriately called rooting your device, and once complete, the device

is described as rooted . SL4A does not require a rooted device, but will work on one if you have chosen

this path.

Android Application Anatomy

Android is based on the Linux operating system (at the time of writing, version 2.6 of the Linux kernel).

Linux provides all the core plumbing such as device drivers, memory and process management, network

stack, and security. The kernel also adds a layer of abstraction between the hardware and applications.

To use an anatomical analogy, you might think of Linux as the skeleton, muscles, and organs of the

Android body.

The next layer up the Android stack is the Dalvik Virtual Machine (DVM). This piece provides the

core Java language support and most of the functionality of the Java programming language. The DVM is

the brains in which the majority of all processing takes place. Every Android application runs in its own

process space in a private instance of the DVM. The application framework provides all the necessary

components needed by an Android application. From the Google Android documentation:

“Developers have full access to the same framework APIs used by the core applications.

The application architecture is designed to simplify the r euse of components. Any

application can publish its capabilities, and any other application may th en make

use of those capabilities (subject to security constraints enforced by the framework).

This same mechanism allows components to be replaced by the user.

Underlying all applications is a set of services and systems, including:

• A rich and e xtensible set of Views th at can be used to build an ap plication,

including lis ts, grids, tex t boxes, butt ons, and ev en an emb eddable w eb

browser

• Content Pr oviders that enable a pplications to a ccess dat a f rom other

applications (such as Contacts) or to share their own data

• A Resource Manager, providing access to non-code resources such as localized

strings, graphics, and layout files

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