2023年6月27日发(作者：)

Android

Android, as a system, is a Java-based operating system that runs on the Linux

kernel. The system is very lightweight and full featured. Android applications are

developed using Java and can be ported rather easily to the new platform. If you have

not yet downloaded Java or are unsure about which version you need, I detail the

installation of the development environment in Chapter 2. Other features of Android

include an accelerated 3-D graphics engine based on hardware support, database

support powered by SQLite, and an integrated web browser.

If you are familiar with Java programming or are an OOP developer of any sort,

you are likely used to programmatic user interface UI development—that is, UI

placement which is handled directly within the program code. Android, while

recognizing and allowing for programmatic UI development, also supports the newer,

XML-based UI layout. XML UI layout is a fairly new concept to the average desktop

developer. I will cover both the XML UI layout and the programmatic UI

development in the supporting chapters of this book.

One of the more exciting and compelling features of Android is that, because of

its architecture, third-party applications—including those that are “home grown”—are

executed with the same system priority as those that are bundled with the core system.

This is a major departure from most systems, which give embedded system apps a

greater execution priority than the thread priority available to apps created by

third-party developers. Also, each application is executed within its own thread using

a very lightweight virtual machine.

Aside from the very generous SDK and the well-formed libraries that are

available to us to develop with, the most exciting feature for Android developers is

that we now have access to anything the operating system has access to. In other

words, if you want to create an application that dials the phone, you have access to the

phone’s dialer; if you want to create an application that utilizes the phone’s internal GPS if equipped, you have access to it. The potential for developers to create dynamic

and intriguing applications is now wide open.

On top of all the features that are available from the Android side of the equation,

Google has thrown in some very tantalizing features of its own. Developers of

Android applications will be able to tie their applications into existing Google

offerings such as Google Maps and the omnipresent Google Search. Suppose you

want to write an application that pulls up a Google map of where an incoming call is

emanating from, or you want to be able to store common search results with your

contacts; the doors of possibility have been flung wide open with Android.

Chapter 2 begins your journey to Android development. You will learn the how’s

and why’s of using specific development environments or integrated development

environments IDE, and you will download and install the Java IDE Eclipse.

Application Components

A central feature of Android is that one application can make use of elements of

other applications provided those applications permit it. For example, if your

application needs to display a scrolling list of images and another application has

developed a suitable scroller and made it available to others, you can call upon that

scroller to do the work, rather than develop your own. Your application doesn't

incorporate the code of the other application or link to it. Rather, it simply starts up

that piece of the other application when the need arises.

For this to work, the system must be able to start an application process when

any part of it is needed, and instantiate the Java objects for that part. Therefore, unlike

applications on most other systems, Android applications don't have a single entry

point for everything in the application no main function, for example. Rather, they

have essential components that the system can instantiate and run as needed. There

are four types of components:

Activities An activity presents a visual user interface for one focused endeavor the user can

undertake. For example, an activity might present a list of menu items users can

choose from or it might display photographs along with their captions. A text

messaging application might have one activity that shows a list of contacts to send

messages to, a second activity to write the message to the chosen contact, and other

activities to review old messages or change settings. Though they work together to

form a cohesive user interface, each activity is independent of the others. Each one is

implemented as a subclass of the Activity base class.

An application might consist of just one activity or, like the text messaging

application just mentioned, it may contain several. What the activities are, and how

many there are depends, of course, on the application and its design. Typically, one of

the activities is marked as the first one that should be presented to the user when the

application is launched. Moving from one activity to another is accomplished by

having the current activity start the next one.

Each activity is given a default window to draw in. Typically, the window fills

the screen, but it might be smaller than the screen and float on top of other windows.

An activity can also make use of additional windows — for example, a pop-up dialog

that calls for a user response in the midst of the activity, or a window that presents

users with vital information when they select a particular item on-screen.

The visual content of the window is provided by a hierarchy of views — objects

derived from the base View class. Each view controls a particular rectangular space

within the window. Parent views contain and organize the layout of their children.

Leaf views those at the bottom of the hierarchy draw in the rectangles they control

and respond to user actions directed at that space. Thus, views are where the activity's

interaction with the user takes place.

For example, a view might display a small image and initiate an action when the

user taps that image. Android has a number of ready-made views that you can use —

including buttons, text fields, scroll bars, menu items, check boxes, and more. A view hierarchy is placed within an activity's window by the method. The

content view is the View object at the root of the hierarchy. See the separate User

Interface document for more information on views and the hierarchy.

Services

A service doesn't have a visual user interface, but rather runs in the background

for an indefinite period of time. For example, a service might play background music

as the user attends to other matters, or it might fetch data over the network or

calculate something and provide the result to activities that need it. Each service

extends the Service base class.

A prime example is a media player playing songs from a play list. The player

application would probably have one or more activities that allow the user to choose

songs and start playing them. However, the music playback itself would not be

handled by an activity because users will expect the music to keep playing even after

they leave the player and begin something different. To keep the music going, the

media player activity could start a service to run in the background. The system would

then keep the music playback service running even after the activity that started it

leaves the screen.

It's possible to connect to bind to an ongoing service and start the service if it's

not already running. While connected, you can communicate with the service through

an interface that the service exposes. For the music service, this interface might allow

users to pause, rewind, stop, and restart the playback.

Like activities and the other components, services run in the main thread of the

application process. So that they won't block other components or the user interface,

they often spawn another thread for time-consuming tasks like music playback. See

Processes and Threads, later.

Broadcast receivers

A broadcast receiver is a component that does nothing but receive and react to broadcast announcements. Many broadcasts originate in system code — for example,

announcements that the timezone has changed, that the battery is low, that a picture

has been taken, or that the user changed a language preference. Applications can also

initiate broadcasts — for example, to let other applications know that some data has

been downloaded to the device and is available for them to use.

An application can have any number of broadcast receivers to respond to any

announcements it considers important. All receivers extend the BroadcastReceiver

base class.

Broadcast receivers do not display a user interface. However, they may start an

activity in response to the information they receive, or they may use the

NotificationManager to alert the user. Notifications can get the user's attention in

various ways — flashing the backlight, vibrating the device, playing a sound, and so

on. They typically place a persistent icon in the status bar, which users can open to get

the message.

Content providers

A content provider makes a specific set of the application's data available to

other applications. The data can be stored in the file system, in an SQLite database, or

in any other manner that makes sense. The content provider extends the

ContentProvider base class to implement a standard set of methods that enable other

applications to retrieve and store data of the type it controls. However, applications do

not call these methods directly. Rather they use a ContentResolver object and call its

methods instead. A ContentResolver can talk to any content provider; it cooperates

with the provider to manage any interprocess communication that's involved.

See the separate Content Providers document for more information on using

content providers.

Whenever there's a request that should be handled by a particular component,

Android makes sure that the application process of the component is running, starting it if necessary, and that an appropriate instance of the component is available, creating

the instance if necessary.

Key Skills & Concepts

● Creating new Android projects

● Working with Views

● Using a TextView

● Modifying the file

Creating Your First Android Project in Eclipse

To start your first Android project, open Eclipse. When you open Eclipse for the

first time, it opens to an empty development environment see Figure 5-1, which is

where you want to begin. Your first task is to set up and name the workspace for your

application. Choose File | New | Android Project, which will launch the New Android

Project wizard.

CAUTION Do not select Java Project from the New menu. While Android

applications are written in Java, and you are doing all of your development in Java

projects, this option will create a standard Java application. Selecting Android Project

enables you to create Android-specific you do not see the option for Android Project,

this indicates that the Android plugin for Eclipse was not fully or correctly installed.

Review the procedure in Chapter 3 for installing the Android plugin for Eclipse to

correct this.

The New Android Project wizard creates two things for you

A shell application that ties into the Android SDK, using the file, and ties the

project into the Android Emulator. This allows you to code using all of the Android

libraries and packages, and also lets you debug your applications in the proper

environment. Your first shell files for the new project. These shell files contain some of the

vital application blocks upon which you will be building your programs. In much the

same way as creating a Microsoft application in Visual Studio generates some

Windows-created program code in your files, using the Android Project wizard in

Eclipse generates your initial program files and some Android-created code. In

addition, the New Android Project wizard contains a few options, shown next, that

you must set to initiate your Android project. For the Project Name field, for purposes

of this example, use the title HelloWorldText. This name sufficiently distinguishes

this Hello World project from the others that you will be creating in this the

Contents area, keep the default selections: the Create New Project in Workspace radio

button should be selected and the Use Default Location check box should be checked.

This will allow Eclipse to create your project in your default workspace directory. The

advantage of keeping the default options is that your projects are kept in a central

location, which makes ordering, managing, and finding these projects quite easy. For

example, if you are working in a Unix-based environment, this path points to your

$HOME directory.

If you are working in a Microsoft Windows environment, the workspace path

will be C:/Users//workspace, as shown in the previous illustration.

However, for any number of reasons, you may want to uncheck the Use Default

Location check box and select a different location for your project. One reason you

may want to specify a different location here is simply if you want to choose a

location for this specific project that is separate from other Android projects. For

example, you may want to keep the projects that you create in this book in a different

location from projects that you create in the future on your own. If so, simply override

the Location option to specify your own custom location directory for this project.