基于Android开发的外文文献

1、AndroidAndroid, as a system, is a Java-based operating system that runs on the Linuxkernel. The system is very lightweight and full featured. Android applications aredeveloped using Java and can be ported rather easily to the new platform. If you havenot yet downloaded Java or are unsure about which

2、 version you need, I detail theinstallation of the development environment in Chapter 2. Other features of Androidinclude an accelerated 3-D graphics engine (based on hardware support), databasesupport powered by SQLite, and an integrated web browser.If you are familiar with Java programming or are

3、an OOP developer of any sort,you are likely used to programmatic user interface (UI) developmentthat is, UIplacement which is handled directly within the program code. Android, whilerecognizing and allowing for programmatic UI development, also supports the newer,XML-based UI layout. XML UI layout i

4、s a fairly new concept to the average desktopdeveloper. I will cover both the XML UI layout and the programmatic UI development inthe supporting chapters of this book.Oneof the more exciting and compelling features of Android is that, because of itsarchitecture, third-party applicationsincluding tho

5、sethat are“home grown”are executed with the same system priority as those that arebundled with the core system. This is a major departure from most systems, which giveembedded system apps a greater execution priority than the thread priority available toapps created by third-party developers. Also,

6、each application is executed within itsownthread using a very lightweight virtual machine.Aside from the very generous SDK and the well-formed libraries that are availableto us to develop with, the most exciting feature for Androiddevelopers is that we now have access to anything the operating syste

7、m has access to.In other words, if you want to create an application that dials the phone, you haveaccess to the phones dialer; if you want tocreate an application that utilizes the phones internal GPS (if equipped), you haveaccess to it. The potential for developers to create dynamic and intriguing

8、 applicationsis 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 Androidapplications will be able to tie their applications into existing Google offerings such asGoogle Maps

9、 and the omnipresent Google Search. Suppose you want to write anapplication that pulls up a Google mapof where an incoming call is emanating from, oryou want to be able to store common search results with your contacts; the doors ofpossibility have been flung wide open with Android.Chapter 2 begins

10、your journey to Android development. You will learn the howsand whys of using specific development environments or integrated developmentenvironments (IDE), and you will download and install the Java IDE Eclipse.Application ComponentsA central feature of Android is that one application can make use

11、of elements ofother applications (provided those applications permit it). For example, if yourapplication needs to display a scrolling list of images and another application hasdeveloped a suitable scroller and made it available to others, you can call upon thatscroller to do the work, rather than d

12、evelop your own. Your application doesntincorporate the code of the other application or link to it. Rather, it simply starts up thatpiece of the other application when the need arises.For this to work, the system must be able to start an application process when anypart of it is needed, and instant

13、iate the Java objects for that part. Therefore, unlikeapplications on most other systems, Android applications dont have a single entrypoint for everything in the application (no main() function, for example). Rather, theyhave essential components that the system can instantiate and run as needed. T

14、hereare four types of components:ActivitiesAn activity presents a visual user interface for one focused endeavor the user canundertake. For example, an activity might present alist ofmenu items users can choose from or it might display photographs along with theircaptions. A text messaging applicati

15、on might have one activity that shows a list ofcontacts to send messages to, a second activity to write the message to the chosencontact, and other activities to review old messages or change settings. Though theywork together to form a cohesive user interface, each activity is independent of theoth

16、ers. Each one is implemented as a subclass of the Activity base class.An application might consist of just one activity or, like the text messagingapplication just mentioned, it may contain several. What the activities are, and howmany there are depends, of course, on the application and its design.

17、 Typically, one ofthe activities is marked as the first one that should be presented to the user when theapplication is launched. Moving from one activity to another is accomplished by havingthe current activity start the next one.Each activity is given a default window to draw in. Typically, the wi

18、ndow fills thescreen, but it might be smaller than the screen and float on top of other windows. Anactivity can also make use of additional windowsfor example, a pop-up dialog thatcalls for a user response in the midst of the activity, or a window that presents userswith vital information when they

19、select a particular item on-screen.The visual content of the window is provided by a hierarchy of viewsobjectsderived from the base View class. Each view controls a particular rectangular spacewithin the window. Parent views contain and organize the layout of their children. Leafviews (those at the

20、bottom of the hierarchy) draw in the rectangles they control andrespond to user actions directed at that space. Thus, views are where the activitysinteraction with the user takes place.For example, a view might display a small image and initiate an action when theuser taps that image. Android has a

21、number of ready-made views that you can useincluding buttons, text fields, scroll bars, menuitems, check boxes, and more.A view hierarchy is placed within an activityswindow by the () method.The content view is the View object at the root of the hierarchy. (See the separate UserInterface document fo

22、r more information on views and the hierarchy.)ServicesA service doesnt have a visual user interface, but rather runs in the backgroundfor an indefinite period of time. For example, a service might play background music asthe user attends to other matters, or it might fetch data over the network or

23、calculatesomething and provide the result to activities that need it. Each service extends theService base class.A prime example is a media player playing songs from a play list. The playerapplication would probably have one or more activities that allow the user to choosesongs and start playing the

24、m. However, the musicplayback itself would not be handled by an activity because users will expect the musicto keep playing even after they leave the player and begin something different. To keepthe music going, the media player activity could start a service to run in the background.The system woul

25、d then keep the music playback service running even after the activitythat started it leaves the screen.Its possible to connect to (bind to) an ongoing service (and start the service if itsnot already running). While connected, you can communicate with the service throughan interface that the servic

26、e exposes. For the music service, this interface might allowusers to pause, rewind, stop, and restart the playback.Like activities and the other components, services run in the main thread of theapplication process. So that they wont block other components or the user interface,they often spawn anot

27、her thread for time-consuming tasks (like music playback). SeeProcesses and Threads, later.Broadcast receiversA broadcast receiver is a component that does nothing but receive and react tobroadcast announcements. Many broadcasts originate in system codefor example,announcements that the timezone has

28、 changed, that the battery is low, that a picturehas been taken, or that the user changed a language preference. Applications can alsoinitiate broadcastsforexample, to let other applications know that somedata has been downloaded to thedevice and is available for them to use.An application can have

29、any number of broadcast receivers to respond to anyannouncements it considers important. All receivers extend the BroadcastReceiverbase class.Broadcast receivers do not display a user interface. However, they maystart anactivity in response to the information they receive, or they may use theNotific

30、ationManager to alert the user. Notifications can get the users attention invarious waysflashing 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 themessage.Content providersA content provider makes

31、a specific set of the applications data available to otherapplications. The data can be stored in the file system, in an SQLite database, or in anyother manner that makessense. The content provider extends the ContentProviderbase class to implement a standard set of methods that enable other applica

32、tions toretrieve and store data of the type it controls. However, applications do not call thesemethods directly. Rather they use a ContentResolver object and call its methodsinstead. A ContentResolver can talk to any content provider; it cooperates with theprovider to manage any interprocess commun

33、ication thats involved.See the separate Content Providers document for more information on usingcontent providers.Whenever theres a request that should be handled by a particular component,Android makes sure that the application process of the component is running, starting itif necessary, and that

34、an appropriate instance of the component is available, creatingthe instance if necessary.Key Skills & ConceptsCreating new Android projectsWorking with ViewsUsing a TextViewModifying the fileCreating Your First Android Project in EclipseTo start your first Android project, open Eclipse. When you ope

35、n Eclipse for thefirst time, it opens to an empty development environment (see Figure 5-1), which iswhere you want to begin. Your first task is to set up and name the workspace for yourapplication. Choose File | New | Android Project, which will launch the New AndroidProject wizard.CAUTION Do not se

36、lect Java Project from the New menu. While Androidapplications are written in Java, and you are doing all of your development in Javaprojects, this option will create a standard Java application.Selecting Android Project enables you to create Android-specific you do not see theoption for Android Pro

37、ject, this indicates that the Android plugin for Eclipse was not fullyor correctly installed. Review the procedure in Chapter 3 for installing the Androidplugin for Eclipse to correct this.The New Android Project wizard creates two things for youA shell application that ties into the Android SDK, us

38、ing the file, and ties theproject into the Android Emulator. This allows you to code using all of the Androidlibraries and packages, and also lets you debug your applications in the properenvironment.Your first shell files for the new project. These shell files contain someof the vitalapplication bl

39、ocks upon which you will be building your programs. In much the sameway as creating a Microsoft .NET application in Visual Studio generatessomeWindows-created program code in your files, using the Android Project wizard inEclipse generates your initial program files and someAndroid-created code. In addition,the NewAndroid Project wizard contains a few options, shown next, that you must setto initiate your Android project. For the Project Name field, for purposes of this example,use the title HelloWorldText. This name sufficientl