Upgrade the UI to an XML Layout

The "Hello, World" example you just completed uses what is called a "programmatic" UI layout. This means that you constructed and built your application's UI directly in source code. If you've done much UI programming, you're probably familiar with how brittle that approach can sometimes be: small changes in layout can result in big source-code headaches. It's also very easy to forget to properly connect Views together, which can result in errors in your layout and wasted time debugging your code.

That's why Android provides an alternate UI construction model: XML-based layout files. The easiest way to explain this concept is to show an example. Here's an XML layout file that is identical in behavior to the programmatically-constructed example:

<?xml version="1.0" encoding="utf-8"?>

<TextView xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_parent" android:text="@string/hello"/>

The general structure of an Android XML layout file is simple: it's a tree of XML elements, wherein each node is the name of a View class (this example, however, is just one View element). You can use the name of any class that extends View as an element in your XML layouts, including custom View classes you define in your own code. This structure makes it very easy to quickly build up UIs, using a more simple structure and syntax than you would use in a programmatic layout. This model is inspired by the web development model, wherein you can separate the presentation of your application (its UI) from the application logic used to fetch and fill in data.

In the above XML example, there's just one View element: the TextView, which has four XML attributes. Here's a summary of what they mean:

Attribute

Meaning

xmlns:android

This is an XML namespace declaration that tells the Android tools that you are going to refer to common attributes defined in the Android namespace. The outermost tag in every Android layout file must have this attribute.

android:layout width

This attribute defines how much of the available width on the screen this View should consume. In this case, it's the only View so you want it to take up the entire screen, which is what a value of "fill_parent" means.

android:layout height

This is just like android:layout_width, except that it refers to available screen height.

android:text

This sets the text that the TextView should display. In this example, you use a string resource instead of a hard-coded string value. The hello string is defined in the res/values/strings.xml file. This is the recommended practice for inserting strings to your application, because it makes the localization of your application to other languages graceful, without need to hard-code changes to the layout file. For more information, see Resources and Internationalization.

These XML layout files belong in the res/layout / directory of your project. The "res" is short for "resources" and the directory contains all the non-code assets that your application requires. In addition to layout files, resources also include assets such as images, sounds, and localized strings.

The Eclipse plugin automatically creates one of these layout files for you: main.xml. In the "Hello World" application you just completed, this file was ignored and you created a layout programmatically. This was meant to teach you more about the Android framework, but you should almost always define your layout in an XML file instead of in your code. The following procedures will instruct you how to change your existing application to use an XML layout.

Landscape layout

When you want a different design for landscape, put your layout XML file inside /res/layout-land. Android will automatically look here when the layout changes. Without this special landscape layout defined, Android will stretch the default layout.

1. In the Eclipse Package Explorer, expand the /res/layout / folder and open main.xml (once opened, you might need to click the "main.xml" tab at the bottom of the window to see the XML source). Replace the contents with the following XML:

<?xml version="1.0" encoding="utf-8"?>

<TextView xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="fill_parent" android:layout_height="fill_parent" android:text="@string/hello"/>

Save the file.

2. Inside the res/values/ folder, open strings.xml. This is where you should save all default text strings for your user interface. If you're using Eclipse, then ADT will have started you with two strings, hello and app_name. Revise hello to something else. Perhaps "Hello, Android! I am a string resource!" The entire file should now look like this:

<?xml version="1.0" encoding="utf-8"?> <resources>

<string name="hello">Hello, Android! I am a string resource!</string> <string name="app_name">Hello, Android</string> </resources>

3. Now open and modify your HelloAndroid class use the XML layout. Edit the file to look like this:

package com.example.helloandroid;

import android.app.Activity; import android.os.Bundle;

public class HelloAndroid extends Activity {

/** Called when the activity is first created. */

@Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main);

When you make this change, type it by hand to try the code-completion feature. As you begin typing "R.layout.main" the plugin will offer you suggestions. You'll find that it helps in a lot of situations.

Instead of passing setContentView() a View object, you give it a reference to the layout resource. The resource is identified as R.layout.main, which is actually a compiled object representation of the layout defined in /res/layout/main.xml. The Eclipse plugin automatically creates this reference for you inside the project's R.java class. If you're not using Eclipse, then the R.java class will be generated for you when you run Ant to build the application. (More about the R class in a moment.)

Now re-run your application — because you've created a launch configuration, all you need to do is click the green arrow icon to run, or select Run > Run History > Android Activity. Other than the change to the TextView string, the application looks the same. After all, the point was to show that the two different layout approaches produce identical results.

| Tip: Use the shortcut Ctrl-F11 (Cmd-Shift-F11, on Mac) to run your currently visible application.

Continue reading for an introduction to debugging and a little more information on using other IDEs. When you're ready to learn more, read Application Fundamentals for an introduction to all the elements that make Android applications work. Also refer to the Developer's Guide introduction page for an overview of the Dev Guide documentation.

R class

In Eclipse, open the file named R.java (in the gen/ [Generated Java Files] folder). It should look something like this:

package com.example.helloandroid;

public final class R {

public static final class attr {

public static final class drawable {

public static final int icon=0x7f020000;

public static final class layout {

public static final int main=0x7f030000;

public static final class string {

public static final int app_name=0x7f04 0 001; public static final int hello=0x7f040000;

A project's R.java file is an index into all the resources defined in the file. You use this class in your source code as a sort of short-hand way to refer to resources you've included in your project. This is particularly powerful with the code-completion features of IDEs like Eclipse because it lets you quickly and interactively locate the specific reference you're looking for.

It's possible yours looks slighly different than this (perhaps the hexadecimal values are different). For now, notice the inner class named "layout", and its member field "main". The Eclipse plugin noticed the XML layout file named main.xml and generated a class for it here. As you add other resources to your project (such as strings in the res/values/string.xml file or drawables inside the res/drawable/ direcory) you'll see R.java change to keep up.

When not using Eclipse, this class file will be generated for you at build time (with the Ant tool). You should never edit this file by hand.

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