Beginning Android

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.

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 locally-unique 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;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import java.util.Date;
public class NowRedux extends Activity
 implements View.OnClickListener {
 Button btn;
 @Override
 public void onCreate(Bundle icicle) {
  super.onCreate(icicle);
  setContentView(R.layout.main);
  btn=(Button)findViewById(R.id.button);
  btn.setOnClickListener(this);
  updateTime();
 }
 public void onClick(View view) {
  updateTime();
 }
 private void updateTime() {
  btn.setText(new Date().toString ());
 }
}

The first difference is that rather than setting the content view to be a view we created in Java code, we set it to reference the XML layout (setContentView(R.layout.main)). The R.java source file will be updated when we rebuild this project to include a reference to our layout file (stored as main.xml in our project’s res/layout directory).

The other difference is that we need to get our hands on our Button instance, for which we use the findViewById() call. Since we identified our button as @+id/button, we can reference the button’s identifier as R.id.button. Now, with the Button instance in hand, we can set the callback and set the label as needed.

As you can see in Figure 5-1, the results look the same as with the original Now demo.