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CHAPTER 4 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 the preceding chapter, 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 activity views 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 containers – 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 containers that make up one View. 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 = "bold", that means that the text appearing on the face of the button should be rendered in a boldface font style. 31 Subscribe to updates at http://commonsware.com Special Creative Commons BY-SA 3.0 License Edition
Using XML-Based Layouts 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, as will be demonstrated . 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 DroidDraw. Such GUI builders could, in principle, generate Java code instead of XML. The challenge is re-reading the definition in to support edits – that is far simpler if the data is in a structured format like XML than in a programming language. Moreover, keeping the generated bits separated out from hand-written 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 XAML, Adobe's Flex, and Mozilla's XUL 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 lesswell-known GUI frameworks, such as ZK, 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. 32 Subscribe to updates at http://commonsware.com Special Creative Commons BY-SA 3.0 License Edition
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The Busy Coder's Guide to Android D
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The Busy Coder's Guide to Android D
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GPS................................
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Page 29 and 30: The Big Picture • Keyboards, if t
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Page 35 and 36: Projects and Targets To test your a
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Page 61 and 62: Using XML-Based Layouts Since this
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Using Selection Widgets Then, rathe
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Using Selection Widgets the setDrop
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Using Selection Widgets Figure 24.
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Using Selection Widgets For example
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Using Selection Widgets TextView, w
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Using Selection Widgets something n
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Using Selection Widgets Figure 28.
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Using Selection Widgets • android
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Getting Fancy With Lists For exampl
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Getting Fancy With Lists A Dynamic
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Getting Fancy With Lists In the sam
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And you want happy users, right? Ge
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Using the Holder Pattern Getting Fa
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Getting Fancy With Lists super.onCr
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} Getting Fancy With Lists public v
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Getting Fancy With Lists So, let's
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Getting Fancy With Lists 1. While w
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Getting Fancy With Lists Figure 33.
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Getting Fancy With Lists So if we w
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Getting Fancy With Lists layout.set
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Getting Fancy With Lists public Rat
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Getting Fancy With Lists Details of
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Employing Fancy Widgets and Contain
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The Input Method Framework Of cours
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The Input Method Framework H
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The Input Method Framework Numbers
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The Input Method Framework android:
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The Input Method Framework Figure 5
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Applying Menus will appear, showing
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Applying Menus appropriate behavior
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Applying Menus @Override public voi
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Applying Menus Figure 56. The MenuD
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Applying Menus Figure 59. The same
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Applying Menus • You must
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Applying Menus In the layout XML sh
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Fonts 178 Subscribe
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The results? Fonts Figure 61. The F
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Fonts And, of course, Android's int
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android:layout_width="fill_parent"
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Embedding the WebKit Browser on the
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Embedding the WebKit Browser • go
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Running this activity gives us: Emb
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Showing Pop-Up Messages from the cu
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Showing Pop-Up Messages android:lay
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Showing Pop-Up Messages Figure 66.
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Dealing with Threads - merely creat
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Dealing with Threads super.onCreate
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Runnables Dealing with Threads If y
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Dealing with Threads AsyncTask, Gen
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import android.widget.ArrayAdapter;
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Dealing with Threads Since we elect
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Dealing with Threads Figure 68. The
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Handling Activity Lifecycle Events
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onPause() and onResume() Handling A
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What's Your Intent? Creating Intent
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Creating Intent Filters Basically,
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Narrow Receivers Creating Intent Fi
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Launching Activities and Sub-Activi
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} }); } } Launching Activities and
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Handling Rotation control over the
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Uri contact=null; Handling Rotation
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Handling Rotation Figure 74. The Ro
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Handling Rotation viewButton.setOnC
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Handling Rotation android:versionNa
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Handling Rotation To block Android
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Handling Rotation Note that Android
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Working with Resources • Strings,
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Working with Resources ((TextView)f
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@Override public void onCreate(Bund
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Got the Picture? Working with Resou
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Working with Resources Figure 79. T
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Working with Resources public void
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10px 1in Working with Resources In
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Working with Resources Different St
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Working with Resources • There ca
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Working with Resources 2. Next, And
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Using Preferences 3. getDefaultShar
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Using Preferences allowing users to
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Using Preferences Figure 81. The Si
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Using Preferences The fields for
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Using Preferences Any children of P
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Using Preferences Figure 85. The ch
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Philadelphia Pittsburgh Allentown/B
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Using Preferences Figure 88. Editin
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Managing and Accessing Local Databa
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Accessing Files application package
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} } Accessing Files .makeText(this,
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android:text="Close" /> Accessing
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Accessing Files openFileInput() to
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Leveraging Java Libraries • Size:
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android:layout_height="wrap_content
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Leveraging Java Libraries Figure 96
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Reviewing the Script Leveraging Jav
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Communicating via the Internet styl
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Communicating via the Internet We h
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Communicating via the Internet for
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Pieces of Me Using a Content Provid
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Using a Content Provider This metho
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Using a Content Provider • The pr
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Using a Content Provider To delete
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First, Some Dissection Building a C
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onCreate() Building a Content Provi
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Building a Content Provider } else
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Building a Content Provider WHERE c
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getType() Building a Content Provid
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Building a Content Provider
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Mother, May I? Requesting and Requi
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Requesting and Requiring Permission
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Requesting and Requiring Permission
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Service with Class Creating a Servi
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Creating a Service immune to garbag
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Creating a Service For example, the
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Creating a Service @Override protec
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The Ties That Bind Invoking a Servi
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Invoking a Service private void upd
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Alerting Users Via Notifications In
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import android.app.Activity; import
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Alerting Users Via Notifications 4.
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Accessing Location-Based Services L
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Accessing Location-Based Services
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Accessing Location-Based Services
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Mapping with MapView and MapActivit
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Rugged Terrain Mapping with MapView
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Handling Telephone Calls Report To
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Handling Telephone Calls The activi
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Development Tools Figure 105. ReadW
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Development Tools Figure 107. Hiera
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Development Tools Figure 109. Hiera
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Development Tools What may not be i
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Logging Development Tools Figure 11
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Development Tools Figure 114. DDMS
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Development Tools Figure 116. DDMS
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Development Tools Figure 118. Simul
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Put It On My Card Development Tools
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Taking the Default Handling Multipl
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Handling Multiple Screen Sizes •
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Handling Multiple Screen Sizes Thes
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Density-Based Sets Handling Multipl
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Handling Multiple Screen Sizes •
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Handling Multiple Screen Sizes size
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Handling Multiple Screen Sizes Repl
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Handling Multiple Screen Sizes scal
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Handling Multiple Screen Sizes R.st
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Handling Multiple Screen Sizes Figu
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Handling Multiple Screen Sizes />
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Handling Multiple Screen Sizes Now
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super.onCreate(savedInstanceState);
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Handling Multiple Screen Sizes In s
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Dealing With Devices Fortunately, s
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Dealing With Devices Market are tes
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Dealing With Devices The CLIQ also
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Not all devices will be this way. H
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The lessons to take from this: Hand
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Handling Platform Changes Note that
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Wrapping the API Handling Platform
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Handling Platform Changes } else {
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} } Handling Platform Changes publi
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You might also consider: Where Do W
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ContentManager.....................
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PreferenceCategory.................
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INTEGER............................
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getSpeed().........................
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setColumnShrinkable()..............
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android:paddingBottom..............
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