Beginning Android

Figure 7-1. The relationship between a widget, its cell, and the padding values

The value of the padding is a dimension, such as 5px for 5 pixels’ worth of padding.

LinearLayout Example

Let’s look at an example (Containers/Linear) that shows LinearLayout properties set both in the XML layout file and at runtime.

<?xml version='1.0' encoding='utf-8'?>
<LinearLayout

 xmlns:android='http://schemas.android.com/apk/res/android'

 android:orientation='vertical'
 android:layout_width='fill_parent'
 android:layout_height='fill_parent'>
 <RadioGroup android:id='@+id/orientation'
  android:orientation='horizontal'
  android:layout_width='wrap_content'
  android:layout_height='wrap_content'
  android:padding='5px'>
  <RadioButton
   android:id='@+id/horizontal'
   android:text='horizontal' />
  <RadioButton
   android:id='@+id/vertical'
   android:text='vertical' />
 </RadioGroup>
 <RadioGroup android:id='@+id/gravity'
  android:orientation='vertical'
  android:layout_width='fill_parent'
  android:layout_height='wrap_content'
  android:padding='5px'>
  <RadioButton
   android:id='@+id/left'
   android:text='left' />
  <RadioButton
   android:id='@+id/center'
   android:text='center />
  <RadioButton
   android:id='@+id/right'
   android:text='right' />
 </RadioGroup>
</LinearLayout>

Note that we have a LinearLayout wrapping two RadioGroup sets. RadioGroup is a subclass of LinearLayout, so our example demonstrates nested boxes as if they were all LinearLayout containers.

The top RadioGroup sets up a row (android:orientation='horizontal') of RadioButton widgets. The RadioGroup has 5px of padding on all sides, separating it from the other RadioGroup. The width and height are both set to wrap_content, so the radio buttons will take up only the space that they need.

The bottom RadioGroup is a column (android:orientation='vertical') of three RadioButton widgets. Again, we have 5px of padding on all sides and a “natural” height (android:layout_height='wrap_content'). However, we have set android:layout_width to be fill_parent, meaning the column of radio buttons “claims” the entire width of the screen.

To adjust these settings at runtime based on user input, we need some Java code:

package com.commonsware.android.containers;
import android.app.Activity;
import android.os.Bundle;
import android.view.Gravity;
import android.text.TextWatcher;
import android.widget.LinearLayout;
import android.widget.RadioGroup;
import android.widget.EditText;
public class LinearLayoutDemo extends Activity
 implements RadioGroup.OnCheckedChangeListener {
 RadioGroup orientation;
 RadioGroup gravity;
 @Override
 public void onCreate(Bundle icicle) {
  super.onCreate(icicle);
  setContentView(R.layout.main);
  orientation=(RadioGroup)findViewById(R.id.orientation);
  orientation.setOnCheckedChangeListener (this);
  gravity=(RadioGroup)findViewById(R.id.gravity);
  gravity.setOnCheckedChangeListener(this);
 }
 public void onCheckedChanged(RadioGroup group, int checkedId) {
  if (group==orientation) {
   if (checkedId==R.id.horizontal) {
    orientation.setOrientation(LinearLayout.HORIZONTAL);
   } else {
    orientation.setOrientation(LinearLayout.VERTICAL);
   }
  } else if (group==gravity) {
   if (checkedId==R.id.left) {
    gravity.setGravity(Gravity.LEFT);
   } else if (checkedId==R.id.center) {
    gravity.setGravity(Gravity.CENTER_HORIZONTAL);