Table of Contents

Asynchronous Android Programming Second Edition

Credits

About the Author

About the Reviewer

www.PacktPub.com

eBooks, discount offers, and more

Why subscribe?

Preface

What this book covers

What you need for this book

Who this book is for

Conventions

Reader feedback

Customer support

Downloading the example code

Errata

Piracy

Questions

1. Asynchronous Programming in Android

Android software stack

Dalvik runtime

ART runtime

Memory sharing and Zygote

Android process model

Process ranks

Process sandboxing

Android thread model

The main thread

The Application Not Responding (ANR) dialog

Maintaining responsiveness

Concurrency in Android

Correctness issues in concurrent programs

Liveness issues in concurrent programs

Thread coordination

Concurrent package constructs

Executor framework

Android primary building blocks

Activity concurrent issues

Manipulating the user interface

Service concurrent issues

Started services issues

Bound services issues

Service in a separate process

Broadcast receiver concurrent issues

Android concurrency constructs

Summary

2. Performing Work with Looper, Handler, and HandlerThread

Understanding Looper

Understanding Handler

Sending work to a Looper

Scheduling work with post

Using Handler to defer work

Leaking implicit references

Leaking explicit references

Updating the UI with Handler

Canceling a pending Runnable

Scheduling work with send

Cancelling pending messages

Composition versus inheritance

Multithreading with Handler and ThreadHandler

Looper message dispatching debugging

Sending messages versus posting runnables

Applications of Handler and HandlerThread

Summary

3. Exploring the AsyncTask

Introducing AsyncTask

Declaring AsyncTask types

Executing AsyncTasks

Providing indeterministic progress feedback

Providing deterministic progress feedback

Canceling an AsyncTask

AsyncTask Execution State

Handling exceptions

Controlling the level of concurrency

Common AsyncTask issues

Fragmentation issues

Memory leaks

Activity lifecycle issues

Handling lifecycle issues with early cancellation

Handling lifecycle issues with retained headless fragments

Applications of AsyncTask

Summary

4. Exploring the Loader

Introducing Loaders

Loader API

Loader

Loader Manager

LoaderManager.LoaderCallbacks

Loader lifecycle

Loading data with Loader

Building responsive apps with AsyncTaskLoader

Building responsive apps with CursorLoader

Combining Loaders

Applications of Loader

Summary

5. Interacting with Services

Introducing Service

Started service

Building responsive apps with IntentService

Handling results

Posting results with PendingIntent

Posting results as system notifications

Applications of IntentService

HTTP uploads with IntentService

Reporting progress

Bound Service

Communicating with a Local Service

Broadcasting results with intents

Detecting unhandled broadcasts

Applications of Services

Summary

6. Scheduling Work with AlarmManager

Introducing AlarmManager

Scheduling alarms with AlarmManager

Setting alarms in recent Android versions

Testing your alarms in Doze Mode

Setting a Window alarm

Debugging AlarmManager alarms

Canceling alarms

Scheduling repeating alarms

Scheduling an alarm clock

Handling alarms

Handling alarms with Activities

Handling alarms with BroadcastReceiver

Working with BroadcastReceiver

Asynchronous work with goAsync

Handling alarms with Services

Staying awake with WakeLocks

Resetting alarms after a system reboot

Applications of AlarmManager

Summary

7. Exploring the JobScheduler API

Introduction to JobScheduler

Setting running criteria

Scheduling a job

Implementing the JobService

Listing pending jobs

Canceling a job

Scheduling a periodic job

Applications of the JobScheduler

Summary

8. Interacting with the Network

Introducing Android HTTP clients

AndroidHttpClient

HttpURLConnection

Performing HTTP requests asynchronously

Retrieving a text response

Interacting with JSON web APIs

Converting Java objects to JSON

Interacting with XML web APIs

Converting Java objects to XML

Converting XML to Java objects

Customizing HTTP timeouts

Communicating securely over SSL sessions

Summary

9. Asynchronous Work on the Native Layer

Introduction to JNI

Android NDK (Native Development Kit)

Calling C functions from Java code

Calling C++ functions from native code

Accessing Java objects from native code

Executing native background work on Java threads

Executing asynchronous work on a native thread

Attaching and detaching native threads from JVM

JNI references explained

Interacting with UI from native threads

Starting the native threads

Stopping the native threads

Handling Java exceptions in the native layer

Interacting with a Java monitor from native code

Wrapping native data objects

Summary

10. Network Interactions with GCM

Introduction to GCM

Setting up and configuring GCM for your application

Registering the GCM Receiver

Setting up a registration service

InstanceID listener

Receiving downstream messages

Receiving messages from topic

Sending upstream messages

GcmListenerService delivery callbacks

Executing tasks with GCM Network Manager

Building a one shot task

Summary

11. Exploring Bus-based Communications

Introduction to bus-based communication

EventBus library

Defining events

Submitting events

Registering sbscribers

Thread mode

Posting sticking events

Removing sticky events

Summary

12. Asynchronous Programing with RxJava

Introduction to RxJava

Cold versus Hot Observable

RxJava setup

Creating Observables

Transforming Observables

Understanding Schedulers

Performing IO operations with Schedulers

Canceling subscriptions

Composing Observables

Monitoring the event stream

Combining Observables

Observing UI Events with RxJava

Working with Subjects

Summary

Index

Asynchronous Android Programming Second Edition

Asynchronous Android Programming Second Edition

Copyright © 2016 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 permission 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 is sold without warranty, either express or implied. Neither the author, nor Packt Publishing, and its dealers and distributors will be held liable for any damages caused or alleged to be caused directly or indirectly by this book.

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First published: July 2016

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Credits

Author

Helder Vasconcelos

Reviewer

Gavin Matthews

Commissioning Editor

Edward Gordon

Acquisition Editor

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Cover Work

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About the Author

Helder Vasconcelos is a Portuguese Software Engineer based on Dublin, Ireland, with more than 10 years of experience designing and developing real-time/multithreaded Java and C++ applications for the telecommunications and aviation industries. Apart from his day-to-day job, he occupies his spare time building native Android applications for Bearstouch Software and other third-party companies.

He graduated with a degree in Electronic and Telecommunications Engineering from the University of Aveiro in January 2006. During his career, he has worked as a Software Engineer for companies such as PT Inovação (Portugal), Airtel ATN (Dublin, Ireland) and Axway (Dublin, Ireland). You can find Hélder on LinkedIn at (https://ie.linkedin.com/in/heldervasc/en) or on his website at (http://hvasconcelos.github.io).

I would like to sincerely thanks all technical reviewers, but especially Gavin. I really appreciate your invaluable feedback and commit that shaped the quality of the book. A special thanks to my awesome wife Tania for encourage me when the lack of motivation was killing my productivity. It would not have been possible without your precious support. Thanks also to my parents and family for their awesome effort in my education. Additionally, I would like to thank my friends, colleagues, clients, and teachers for helping me to shape and improve my skills and perspectives during my career.

About the Reviewer

Gavin Matthews is a veteran software engineer specializing in enterprise scale B2B, MFT and EFSS systems.

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Preface

Whether you are Android beginner developer or an Android seasoned programmer, this book will explore how to achieve efficient and reliable multithreaded Android applications.

We'll look at best asynchronous constructs and techniques, commonly used by Android Developer community, to execute computation intensive or blocking tasks off the main thread, keeping the UI responsive, telling the user how things are going, making sure we finish what we started, using those powerful multicore processors, and doing it all without wasting the battery.

By using the right asynchronous construct, much of the complexity is abstracted from the developer, making the application source code more readable and maintainable and less error prone.

Using step-by-step guidelines and code examples, you will learn how manage interactions between several threads and avoid concurrency and synchronization problems that might occur when two or more threads access a shared resource to complete a background job, to update the UI or retrieve the latest application data.

At the end of this journey you will know how build well-behaved applications with smooth, responsive user-interfaces that delight users with speedy results and data that's always fresh.

What this book covers

Chapter 1, Asynchronous Programming in Android, gives an overview of the Android process and thread model, and describes some of the challenges and benefits of concurrency in general, before discussing issues specific to Android.

Chapter 2, Performing Work with Looper, Handler and HandlerThread details the fundamental and related topics of Handler, HandlerThread, and Looper, and illustrates how they can be used to schedule tasks on the main thread, and to coordinate and communicate work between cooperating background threads.

Chapter 3, Exploring the AsyncTask, covers the most common concurrent construct of programming in Android. We learn how AsyncTask works, how to use it correctly, and how to avoid the common pitfalls that catch out even experienced developers.

Chapter 4, Exploring the Loader, introduces the Loader framework and tackles the important task of loading data asynchronously to keep the user interface responsive and glitch free.

Chapter 5, Interacting with Services, we explored the very powerful Service Android component, putting it to use to execute long-running background tasks with or without a configurable level of concurrency. This component gives us the means to perform background operations beyond the scope of a single Activity lifecycle and to ensure that our work is completed even if the user leaves the application.

Chapter 6, Scheduling Work with AlarmManager, introduces to us a system API that could be used to defer work or create periodic tasks. The scheduled task could wake up the device to complete the work or alert users to new content.

Chapter 7, Exploring the JobScheduler API, covers a job scheduling system API introduced with Android Lollipop that allows us to start background work when a set of device conditions, such as energy or network, are fulfilled.

Chapter 8, Interacting with the Network, we cover in detail HttpUrlConnection Android HTTP client. With the HttpUrlConnection HTTP client, we will create an asynchronous toolkit that is able to fetch JSON documents, XML or text from a remote server.

Chapter 9, Asynchronous Work on the Native layer, introduces the JNI interface, an Java standard interface that will allow us to execute concurrent tasks on native code (C/C++), interact with the Java code from the native layer or update the UI from the native code.

Chapter 10, Network Interactions with GCM, we will learn how to use the Google GCM to efficiently push and pull efficiently realtime messages from your server and how to schedule work with Google Play Services framework.

Chapter 11, Exploring Bus-based Communications, we will introduce to the reader the publish-subscribe messaging pattern and the Event Bus Library, a publish-subscribe implementation that allow us to deliver asynchronous messages between Android application components.

Chapter 12, Asynchronous Programing with RxJava, we will introduce RxJava, a library used to easily compose asynchronous and event-based tasks on Java by using observable data streams.

What you need for this book

To follow along and experiment with the examples, you will need a development computer with a Java 7 (or 8) SE Development Kit and the Android Software Development Kit Version 9 or above (you will need at least Version 21 to try all of the examples).

You will also need Android Studio IDE. The examples have been developed using Google's new Android Studio IDE and use its integrated build system, Gradle.

While you can run the examples using the emulator provided by the Android SDK, it is a poor substitute for the real thing. A physical Android device is a much faster and more pleasurable way to develop and test Android applications!

Many of the examples will work on a device running any version of Android since 2.3, GingerBread. Some examples demonstrate newer APIs and as a result, require a more recent Android version—up to Android 5, Lollipop.

Who this book is for

This book is for Android Developers who want to learn how to build multithreaded and reliable Android applications using high level and advanced asynchronous techniques and concepts.

They want to learn this technology because they want learn how to build efficient applications that are able to interact orderly with internal/external services and frameworks using Android standard constructs and APIs.

No prior knowledge of of concurrent and asynchronous programming is required. This book is also targeted towards Java experts who are new to Android.

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Chapter 1. Asynchronous Programming in Android

Asynchronous programming has become an important topic of discussion in the past few years, especially when using the concurrent processing capabilities available on the most recent mobile hardware.

In recent years, the number of independent processing units (cores) available on the CPU have increased, so to benefit from this new processing power, a new programming model called asynchronous programming has appeared to orchestrate the work between the several independent hardware-processing units available on the device. Asynchronous programming comes to the rescue to solve the problems that could arise from this new processing paradigm.

Android applications, since they mostly run on devices with multiple units of processing, should take advantage of asynchronous programming to scale and improve the application performance when blocking operations, and when CPU-intensive tasks are required.

Android is an open source operating system (OS) based on Linux kernel that was devised in 2003 by Andy Rubin, Nick Sears, Chris White, and Rick Miner, and then acquired by Google in July, 2005.

The Android OS, actually maintained by Google and the Open Handset Alliance, was created to provide an open mobile-device platform for devices with limited resources of computation, memory, and energy.

The platform has been incorporating advanced mobile devices standards, such as NFC and Bluetooth LE, and its scope has grown from a pure smartphone platform to a broader software platform for smart watches, TVs, tablets, and consoles.

The maintainers have been regularly updating the platform with great features and some improvements over minor and major releases since the first release.

The following diagram displays the Android versions over time:

Android software stack

Android software stack (C libraries and Java frameworks), orchestrated by the Android runtime (Dalvik VM, and most recently, ART) was created around the Linux kernel to provide highly interactive user experiences over a well-proven group of technologies.

In each new OS version, a well-defined application interface (API) is provided to the developer in order to create applications around the new features and standards introduced with the release.

The Android application compiled code (bytecode), typically a Java compiled code, runs on a virtual machine based on Dalvik or ART.

Dalvik runtime

The Dalvik VM (DVM) runtime, created by Dan Borstein, was the first runtime for the platform and is a register-based virtual machine that was created to run the Java code efficiently in a constrained runtime with a limited amount of power processing, RAM, and electric power.

Dalvik's creators claim that the DVM is, on an average, around 30% more efficient than the standard Java VM (Oracle). According to Bornstein, it requires 30% less instructions and 35 % less coding units.

Clearly, Google has gone to great lengths to squeeze every drop of performance out of each mobile device to help developers build responsive applications.

The virtual machine, which runs the Java code compiled and transformed to the dex format over the dx tool, runs on a Linux process with its own memory space and file descriptors. It also manages its own group of threads.

In more advanced architectures, an Android application might run a service in a separate process and communicate over the IPC mechanism, but most of the time, it runs on a single self-contained process.

The dex file and application resources are packed in an Android application package (APK) by the AAPT and installed over Google Play in the end user devices.

Note

The application store distribution model has become extremely popular on the mobile platforms since the launch of the Apple iPhone in 2007.

Since Android 2.2 the DVM comes with a trace-based Just-In-Time (JIT) compilation feature that actively optimizes every time the application runs some short segments of frequently used bytecode called traces.

The generated machine code provides significant performance improvements in the application execution and on the time spent on some intensive CPU tasks, and thereafter, decreases the battery power used.

ART runtime

The ART runtime is a new version of the DVM and was introduced to improve the runtime performance and memory consumption. The new runtime was introduced in Android 4.4 KitKat as an experimental runtime, and since the Android 5.0 Lollipop, it has become the main Android runtime.

This new runtime, making use of the ahead-of-time (AOT) compilation, brings new app-performance optimizations on startup time and application execution. The AOT, as opposed to DVM JIT (Just in Time), compiles the dex files during the installation time using the device dex2oat tool. The compiled code generated from the dex2oat tool generates system-dependent code for the target device and removes the delay introduced by the JIT compilation during each application execution.

The AOT compiler also reduces the number of processor cycles used by the application as it removes the time spent by the JIT compiler to convert the code into machine code, and then uses less battery power to run the application.

One of the drawbacks of the AOT compilation is the larger memory footprint in comparison with the JIT used by DVM.

With the new runtime, some improvements were also introduced in the memory allocation and on Garbage Collection (GC), resulting in a more responsive UI and better application experience.

Memory sharing and Zygote

Basically, the platform runs an instance of DVM/ART for each application, but large optimization of the platform is brought about by the way a new DVM instance is created and managed.

A special process called the Zygote (first life cell in an animal's reproduction)—the process that all the Android applications are based on—is launched when an Android device initially boots.

The Zygote starts up a virtual machine, preloads the core libraries, and initializes various shared structures. It then waits for instructions by listening on a socket.

When a new Android application is launched, the Zygote receives a command to create a virtual machine to run the application on. It does this by forking its pre-warmed VM process and creating a new child process that shares some memory portions with the parent, using a technique called copy-on-write (COW).

The COW technique, available on most Unix systems, only allocates new memory on the child process when the process tries to change the memory cloned from the parent process.

This technique has some fantastic benefits, as listed in the following:

First, the virtual machine and core libraries are already loaded into the memory. Not having to read this significant chunk of data from the filesystem to initialize the virtual machine drastically reduces the startup overhead.

Second, the memory in which these core libraries and common structures reside is shared by the Zygote with all other applications, resulting in saving a lot of memory when the user is running multiple apps.

Android process model

Android is a multiuser, multitasking system that can run multiple applications in parallel, where all the applications attempt to acquire CPU time to execute its job.

Each application runs independently on an isolated Linux process cloned from the Zygote process, and by default, all the Android components run within the same process with the same name as the application package specified in Android Application Manifest (AAM).

The Linux kernel will fairly allocate small amounts of CPU time for application execution called CPU time slices. This time-slicing approach means that even a single-processor device can appear to be actively working in more than one application at the same time, when in fact, each application is taking very short turns on the CPU.

Process ranks

The Android operating system tries to maintain the application running for as long as possible, but when the available memory is low, it will try to free resources in the system by killing the processes with lower importance first.

This is when process ranking comes into the picture; the Android processes are ranked in the next five categories from the higher priority to the lower priorities:

Foreground process: This is a process that hosts an activity or service that the user is currently interacting with: a service started in the foreground or service running its life cycle callbacks

Visible process: This is a process that hosts a paused activity or service bounded to a visible activity

Service process: This is a process that hosts a service not bound to a visible activity

Background process: This is a process that hosts a non-visible activity; all background processes are sorted over a Least-Recently-Used (LRU) list, therefore, the most recently used processes are the last killed processes when they have the same rank

Empty process: This is a process used to cache inactive Android components and to improve any component startup time

When the system reaches a point that it needs to release resources, the processes available to be killed will be sorted, taking into account the process rank, last used processes, and components running.

Process sandboxing

The Android application always runs under a unique Linux user ID (UID) assigned to the application during the application installation so that the process runs on a sandboxed environment, which by default, isolates your data and code execution from other apps.

In some cases, a user could explicitly be required to share the UID with another application to have access to their data:

USER    PID  PPID  VSIZE  RSS  PC  NAME

root            319  1    1537236 31324 S zygote

….

u0_a221  5993  319  1731636 41504 S com.whatsapp

u0_a96    3018  319  1640252 29540 S com.dropbox.android

u0_a255  4892  319  1583828 34552 S com.accuweather.android…

The preceding table that results from running the adb shell ps command in the computer with Android SDK Table is a list of Android running processes.

The first column shows the user identifier (UID) assigned at the time of installation, the second column is the process ID (PID), the third column shows the parent process ID (PPID) that for Android applications is the Zygote process, and the last column shows the application package.

From this list, we can assure that the WhatsApp application is running under the user ID u0_a221 with the process ID 5993 and the parent process is the Zygote process with the PID 319.

Android thread model

Within an Android process, there may be many threads of execution. Each thread is a separate sequential flow of control within the overall program—it executes its instructions in order, one after the other, and they also share allocated slices of CPU time managed by the operating system task scheduler.

While the application process is started by the system and prevented from directly interfering with data in the memory address space of other processes, the threads may be started by an application code and can communicate and share data with other threads within the same process. Apart from the shared data that all the threads share in the same process, a thread can use its own memory cache to store its data in its own memory space.

The main thread

When the application process starts, apart from DVM housekeeping threads, the system creates a thread of execution called main. This thread, as the name explains, plays a crucial role in the application lifetime as it is the thread that interacts with the Android UI components, updating the state and their look on the device screen.

Moreover, by default, all the Android application components (Activity, Service, ContentProvider, and BroadcastsReceiver) are also executed over the main thread line of execution. The following image shows the lists of threads running inside an application process with the main thread at the top of the list with a unique thread ID (TID) assigned by the system:

The main thread, also known as UI Thread, is also the thread where your UI event handling occurs, so to keep your application as responsible as possible, you should:

Avoid any kind of long execution task, such as input/output