基于Android的背单词软件设计与实现计算机毕业论文(7)

致 谢

本课题的设计和论文撰写过程中，我的指导老师给于了极大的帮助，为我提供了大量的相关资料。在项目初期对我们的帮助和指导，使我们迅速确定了方向并明确了分工，以及在写论文的过程中给的宝贵的意见。在此，我对老师的指导表示衷心的感谢。在毕业设计期间，同学的帮助更是我能完成的重要因素，要对我亲爱的同学，说声谢谢，并希望他们能在未来的道路上，越走越好。

这次论文的写作过程，让我收获了很多，即为在大学四年的学习生涯上划上了一个完美的句号，为将来的人生之路做好了一个非常好的铺垫。同时，我也衷心的感谢太原理工大学软件学院四年来对我的精心栽培，为我进入社会提供了莫大的帮助。

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外文原文

Beginning Android Mark L. Murphy write Using XML-Based Layouts While it is technically possible to create and attach widgets to our activity purely through Java code, the way we did in Chapter 4, the more common approach is to use an XML-based layout file. Dynamic instantiation of widgets is reserved for more complicated scenarios, where the widgets are not known at compile-time (e g., populating a column of radio buttons based on data retrieved off the Internet). With that in mind, it’s time to break out the XML and learn how to lay out Android activities that way. What Is an XML-Based Layout? As the name suggests, an XML-based layout is a specification of widgets’ relationships to each other—and to their containers (more on this in Chapter 7)—encoded in XML format. Specifically, Android considers XML-based layouts to be resources, and as such layout files are stored in the res/layout directory inside your Android project. Each XML file contains a tree of elements specifying a layout of widgets and their containers that make up one view hierarchy. The attributes of the XML elements are properties, describing how a widget should look or how a container should behave. For example, if a Button element has an attribute value of android:textStyle = \that means that the text appearing on the face of the button should be rendered in a boldface font style. Android’s SDK ships with a tool (aapt) which uses the layouts. This tool should be automatically invoked by your Android tool chain (e.g., Eclipse, Ant’s build.xml). Of particular importance to you as a developer is that aapt generates the R.java source file within your project, allowing you to access layouts and widgets within those layouts directly from your Java code. 28

Why Use XML-Based Layouts? Most everything you do using XML layout files can be achieved through Java code. For example, you could use setTypeface() to have a button render its text in bold, instead of using a property in an XML layout. Since XML layouts are yet another file for you to keep track of, we need good reasons for using such files. Perhaps the biggest reason is to assist in the creation of tools for view definition, such as a GUI builder in an IDE like Eclipse or a dedicated Android GUI designer like DroidDraw1. Such GUI builders could, in principle, generate Java code instead of XML. The challenge is re-reading the UI definition to support edits—that is far simpler if the data is in a structured format like XML than in a programming language. Moreover, keeping generated XML definitions separated from hand-written Java code makes it less likely that somebody’s custom-crafted source will get clobbered by accident when the generated bits get re-generated. XML forms a nice middle ground between something that is easy for tool-writers to use and easy for programmers to work with by hand as needed. Also, XML as a GUI definition format is becoming more commonplace. Microsoft’s XAML2, Adobe’s Flex3, and Mozilla’s XUL4 all take a similar approach to that of Android: put layout details in an XML file and put programming smarts in source files (e.g., JavaScript for XUL). Many less-well-known GUI frameworks, such as ZK5, also use XML for view definition. While ―following the herd‖ is not necessarily the best policy, it does have the advantage of helping to ease the transition into Android from any other XML-centered view description language. OK, So What Does It Look Like? Here is the Button from the previous chapter’s sample application, converted into an XML layout file, found in the Layouts/NowRedux sample project. This code sample along with all others in this chapter can be found in the Source Code area of http://apress.com.

android:layout_width=\ android:layout_height=\ The class name of the widget—Button—forms the name of the XML element. Since Button is an Android-supplied widget, we can just use the bare class name. If you create your own widgets as subclasses of android.view.View, you would need to provide a full package declaration as well. The root element needs to declare the Android XML namespace: xmlns:android=\

All other elements will be children of the root and will inherit that namespace declaration. Because we want to reference this button from our Java code, we need to give it an identifier via the android:id attribute. We will cover this concept in greater detail later in this chapter. The remaining attributes are properties of this Button instance: ? android:text indicates the initial text to be displayed on the button face (in this case, an empty string) ? android:layout_width and android:layout_height tell Android to have the button’s width and height fill the ―parent‖, in this case the entire screen—these attributes will be covered in greater detail in Chapter 7. Since this single widget is the only content in our activity, we only need this single element. Complex UIs will require a whole tree of elements, representing the widgets and containers that control their positioning. All the remaining chapters of this book will use the XML layout form whenever practical, so there are dozens of other examples of more complex layouts for you to peruse from Chapter 7 onward. What’s with the @ Signs? Many widgets and containers only need to appear in the XML layout file and do not need to be referenced in your Java code. For example, a static label (TextView) frequently only needs to be in the layout file to indicate where it should appear. These sorts of elements in the XML file do not need to have the android:id attribute to give them a name. 30

Anything you do want to use in your Java source, though, needs an android:id. The convention is to use @+id/... as the id value, where the ... represents your locallyunique name for the widget in question. In the XML layout example in the preceding section, @+id/button is the identifier for the Button widget. Android provides a few special android:id values, of the form @android:id/.... We will see some of these in various chapters of this book, such as Chapters 8 and 10. We Attach These to the Java How? Given that you have painstakingly set up the widgets and containers in an XML layout file named main.xml stored in res/layout, all you need is one statement in your activity’s onCreate() callback to use that layout: setContentView(R.layout.main);

This is the same setContentView() we used earlier, passing it an instance of a View subclass (in that case, a Button). The Android-built view, constructed from our layout, is accessed from that code-generated R class. All of the layouts are accessible under R.layout, keyed by the base name of the layout file—main.xml results in R.layout.main. To access our identified widgets, use findViewById(), passing in the numeric identifier of the widget in question. That numeric identifier was generated by Android in the R class as R.id.something (where something is the specific widget you are seeking). Those widgets are simply subclasses of View, just like the Button instance we created in Chapter 4. The Rest of the Story In the original Now demo, the button’s face would show the current time, which would reflect when the button was last pushed (or when the activity was first shown, if the button had not yet been pushed). Most of that logic still works, even in this revised demo (NowRedux). However, rather than instantiating the Button in our activity’s onCreate() callback, we can reference the one from the XML layout:

package com.commonsware.android.layouts; import android.app.Activity; 31

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