Containers pour a collection of widgets (and possibly
child containers) into specific structures you like. If you want a form
with labels on the left and fields on the right, you need a container.
If you want OK and Cancel buttons to be beneath the rest of the form,
next to one another, and flush to right side of the screen, you need a
container. Just from a pure XML perspective, if you have multiple
widgets (beyond RadioButton widgets in a RadioGroup), you need a container just to have a root element in which to place the widgets.
Most GUI toolkits have some
notion of layout management, frequently organized into containers. In
Java/Swing, for example, you have layout managers like BoxLayout and containers that use them (e.g., Box).
Some toolkits, such as XUL and Flex, stick strictly to the box model,
figuring that any desired layout can be achieved through the right
combination of nested boxes. Android, through LinearLayout, also offers a box model, but in addition supports a range of containers that provide different layout rules.
Thinking Linearly
As just noted, LinearLayout is a box model—widgets or child containers are lined up in a column or row, one after the next. This works similarly to FlowLayout in Java/Swing, vbox and hbox in Flex and XUL, and so forth.
Flex and XUL use the box as their primary unit of layout. If you want, you can use LinearLayout
in much the same way, eschewing some of the other containers. Getting
the visual representation you want is mostly a matter of identifying
where boxes should nest and which properties those boxes should have,
such as their alignment relative to other boxes.
1. LinearLayout Concepts and Properties
To configure a LinearLayout,
you have five main areas of control besides the container's contents:
the orientation, the fill model, the weight, the gravity, and the
padding.
1.1. Orientation
Orientation indicates whether the LinearLayout represents a row or a column. Just add the android:orientation property to your LinearLayout element in your XML layout, and set the value to be horizontal for a row or vertical for a column.
The orientation can be modified at runtime by invoking setOrientation() on the LinearLayout, supplying it either HORIZONTAL or VERTICAL.
1.2. Fill Model
Let's imagine a row of widgets,
such as a pair of radio buttons. These widgets have a "natural" size
based on their text. Their combined size probably does not exactly match
the width of the Android device's screen—particularly since screens
come in various sizes. We then have the issue of what to do with the
remaining space.
All widgets inside a LinearLayout must supply android:layout_width and android:layout_height properties to help address this issue. These properties' values have three flavors:
You can provide a specific dimension, such as 125dip to indicate the widget should take up exactly a certain size.
You can provide wrap_content,
which means the widget should fill up its natural space, unless that is
too big, in which case Android can use word-wrap as needed to make it
fit.
You can provide fill_parent,
which means the widget should fill up all available space in its
enclosing container, after all other widgets are taken care of.
The latter two flavors are the
most common, as they are independent of screen size, allowing Android to
adjust your view to fit the available space.
NOTE
In API level 8 (Android 2.2), fill_parent was renamed to match_parent, for unknown reasons. You can still use fill_parent,
as it will be supported for the foreseeable future. However, at such
point in time as you are supporting only API level 8 or higher (e.g., android:minSdkVersion="8" in your manifest), you should probably switch over to match_parent.
1.3. Weight
But what happens if we have two
widgets that should split the available free space? For example, suppose
we have two multiline fields in a column, and we want them to take up
the remaining space in the column after all other widgets have been
allocated their space.
To make this work, in addition to setting android:layout_width (for rows) or android:layout_height (for columns) to fill_parent, you must also set android:layout_weight. This property indicates the proportion of the free space that should go to that widget. For example, if you set android:layout_weight to be the same nonzero value for a pair of widgets (e.g., 1), the free space will be split evenly between them. If you set it to be 1 for one widget and 2
for the other widget, the second widget will use up twice the free
space that the first widget does. And so on. The weight for a widget is 0 by default.
Another pattern for using
weights is if you want to allocate sizes on a percentage basis. To use
this technique for, say, a horizontal layout, do the following:
Set all the android:layout_width values to be 0 for the widgets in the layout.
Set the android:layout_weight values to be the desired percentage size for each widget in the layout.
Make sure all those weights add up to 100.
1.4. Gravity
By default, everything in a LinearLayout is left- and top-aligned. So, if you create a row of widgets via a horizontal LinearLayout,
the row will start flush on the left side of the screen. If that is not
what you want, you need to specify a gravity value. Using android:layout_gravity on a widget (or calling setGravity() at runtime on the widget's Java object), you can tell the widget and its container how to align it vis-à-vis the screen.
For a column of widgets, common gravity values are left, center_horizontal, and right for left-aligned, centered, and right-aligned widgets, respectively.
For a row of widgets, the
default is for them to be aligned so their text is aligned on the
baseline (the invisible line that letters seem to "sit on"). You can
specify a gravity of center_vertical to center the widgets along the row's vertical midpoint.
1.5. Margins
By default, widgets are tightly
packed next to each other. You can change this via the use of margins, a
concept that is similar to that of padding.
The difference between
padding and margins is apparent only for widgets with a nontransparent
background. For widgets with a transparent background—like the default
look of a TextView—padding and
margins have similar visual effect, increasing the space between the
widget and adjacent widgets. For widgets with a nontransparent
background—like a Button—padding is
considered to be inside the background, while margins are considered to
be outside the background. In other words, adding padding will increase
the space between the contents (e.g., the caption of a Button) and the edges, while adding margins increases the empty space between the edges and adjacent widgets.
Margins can be set in XML, either on a per-side basis (e.g., android:layout_marginTop) or on all sides via android:layout_margin. As with padding, the value of any of these is a dimension—a combination of a unit of measure and a count, such as 5px for 5 pixels.
2. LinearLayout Example
Let's look at an example (Containers/Linear) that shows LinearLayout properties set both in the XML layout file and at runtime. Here is the layout:
<?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="5dip">
<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="5dip">
<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 5dip of padding on all sides, separating it from the other RadioGroup, where dip
stands for density-independent pixels. 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 5dip 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.linear;
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) {
switch (checkedId) {
case R.id.horizontal:
orientation.setOrientation(LinearLayout.HORIZONTAL);
break;
case R.id.vertical:
orientation.setOrientation(LinearLayout.VERTICAL);
break;
case R.id.left:
gravity.setGravity(Gravity.LEFT);
break;
case R.id.center:
gravity.setGravity(Gravity.CENTER_HORIZONTAL);
break;
case R.id.right:
gravity.setGravity(Gravity.RIGHT);
break;
}
}
}
In onCreate(), we look up our two RadioGroup containers and register a listener on each, so we are notified when the radio buttons change state (setOnCheckedChangeListener(this)). Since the activity implements OnCheckedChangeListener, the activity itself is the listener.
In onCheckedChanged() (the callback for the listener), we see which RadioButton had a state change. Based on the clicked-upon item, we adjust either the orientation of the first LinearLayout or the gravity of the second LinearLayout.
Figure 1 shows the result when the demo is first launched inside the emulator.
