Книга: Beginning Android

Just Another Box to Check

Just Another Box to Check

The classic checkbox has two states: checked and unchecked. Clicking the checkbox toggles between those states to indicate a choice (e.g., “Add rush delivery to my order”).

In Android, there is a CheckBox widget to meet this need. It has TextView as an ancestor, so you can use TextView properties like android:textColor to format the widget.

Within Java, you can invoke:

• isChecked() to determine if the checkbox has been checked

• setChecked() to force the checkbox into a checked or unchecked state

• toggle() to toggle the checkbox as if the user checked it

Also, you can register a listener object (in this case, an instance of OnCheckedChangeListener) to be notified when the state of the checkbox changes.

For example, from the Basic/CheckBox project, here is a simple checkbox layout:

<?xml version="1.0" encoding="utf-8"?>
<CheckBox xmlns:android="http://schemas.android.com/apk/res/android"
 android:id="@+id/check"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:text="This checkbox is: unchecked" />

The corresponding CheckBoxDemo.java retrieves and configures the behavior of the checkbox:

public class CheckBoxDemo extends Activity
 implements CompoundButton.OnCheckedChangeListener {
 CheckBox cb;
 @Override
 public void onCreate(Bundle icicle) {
  super.onCreate(icicle);
  setContentView(R.layout.main);
  cb=(CheckBox)findViewById(R.id.check);
  cb.setOnCheckedChangeListener(this);
 }
 public void onCheckedChanged(CompoundButton buttonView,
  boolean isChecked) {
  if (isChecked) {
   cb.setText("This checkbox is: checked");
  } else {
   cb.setText("This checkbox is: unchecked");
  }
 }
}

Note that the activity serves as its own listener for checkbox state changes since it implements the OnCheckedChangeListener interface (via cb.setOnCheckedChangeListener(this)). The callback for the listener is onCheckedChanged(), which receives the checkbox whose state has changed and what the new state is. In this case, we update the text of the checkbox to reflect what the actual box contains.

The result? Clicking the checkbox immediately updates its text, as you can see in Figures 6-4 and 6-5.

Figure 6-4. The CheckBoxDemo sample application, with the checkbox unchecked

Figure 6-5. The same application, now with the checkbox checked

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