Chapter 6

Working with Containers

Containers pour a collection of widgets (and possibly child containers) into specific structures. 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 the 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 to place the widgets inside.

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 correct combination of nested boxes. Android, through LinearLayout, also offers a box model. In addition, Android supports a range of containers that provide different layout rules.

In this chapter, we will look at several commonly used containers: LinearLayout (the box model), RelativeLayout (a rule-based model), TableLayout (the grid model), and ScrollView, a container designed to assist with implementing scrolling containers.

Thinking Linearly

LinearLayout is a box model, in which widgets or child containers are lined up in a column or row, one after the next. This works in a similar manner to FlowLayout in Java/Swing, and vbox and hbox in Flex and XUL.

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 in relation to other boxes.

LinearLayout Concepts and Properties

To configure a LinearLayout, you have five main areas of control: the orientation, the fill model, the weight, the gravity, and the padding.

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, setting 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.

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 sizes probably do not exactly match the width of the Android device's screen, particularly since screens come in various sizes. You 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 125px to indicate the widget should take up exactly 125 pixels.
• 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 handled.

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.

Weight

But what happens if you have two widgets that should split the available free space? For example, suppose you have two multiline fields in a column, and you 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.

The android:layout_weight 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. The weight for a widget is zero 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:

• 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.

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 in on 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.

Padding

By default, widgets are tightly packed next to each other. If you want to increase the whitespace between widgets, you will want to use the android:padding property (or call setPadding() at runtime on the widget's Java object). The padding specifies how much space there is between the boundaries of the widget's “cell” and the actual widget contents, as shown in Figure 6-1.

Figure 6-1. The relationship between a widget, its cell, and the padding values

The android:padding property allows you to set the same padding on all four sides of the widget, with the widget's contents centered within that padded-out area. If you want the padding to vary on different sides, use android:paddingLeft, android:paddingRight, android:paddingTop, and android:paddingBottom. The value of the padding is a dimension, such as 5px for 5 pixels' worth of padding.

If you apply a custom background to a widget (e.g., via the android:background attribute), the background will be behind both the widget and the padding area. To avoid this, rather than using padding, you can establish margins, which add whitespace without extending the intrinsic size of the widget. You can set margins via android:layout_marginTop and related attributes.

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:

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:

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 RadioGroup had a state change. If it was the orientation group, we adjust the orientation based on the user's selection. If it was the gravity group, we adjust the gravity based on the user's selection.

Figure 6-2 shows the result when the layout demo is first launched inside the emulator.

Figure 6-2. The LinearLayoutDemo sample application, as initially launched

If we toggle on the vertical radio button, the top RadioGroup adjusts to match, as shown in Figure 6-3.

Figure 6-3. The same application, with the vertical radio button selected

If we toggle the center or right radio button, the bottom RadioGroup adjusts to match, as shown in Figures 6-4 and 6-5.

Figure 6-4. The same application, with the vertical and center radio buttons selected

Figure 6-5. The same application, with the vertical and right radio buttons selected

All Things Are Relative

RelativeLayout, as the name suggests, lays out widgets based on their relationship to other widgets in the container and the parent container. You can place widget X below and to the left of widget Y, have widget Z's bottom edge align with the bottom of the container, and so on. This is reminiscent of James Elliot's RelativeLayout for use with Java/Swing.

RelativeLayout Concepts and Properties

To make your RelativeLayout work, you need ways to reference other widgets within an XML layout file, plus ways to indicate the relative positions of those widgets.

Positions Relative to Container

The easiest relationships to set up are those that tie a widget's position to that of its container:

• android:layout_alignParentTop: Aligns the widget's top with the top of the container.
• android:layout_alignParentBottom: Aligns the widget's bottom with the bottom of the container.
• android:layout_alignParentLeft: Aligns the widget's left side with the left side of the container.
• android:layout_alignParentRight: Aligns the widget's right side with the right side of the container.
• android:layout_centerHorizontal: Positions the widget horizontally at the center of the container.
• android:layout_centerVertical: Positions the widget vertically at the center of the container.
• android:layout_centerInParent: Positions the widget both horizontally and vertically at the center of the container.

All of these properties take a simple Boolean value (true or false).

Relative Notation in Properties

The remaining properties of relevance to RelativeLayout take as a value the identity of a widget in the container. To do this:

• Put identifiers (android:id attributes) on all elements that you will need to address, of the form @+id/….
• Reference other widgets using the same identifier value without the plus sign (@id/…).

For example, if widget A is identified as @+id/widget_a, widget B can refer to widget A in one of its own properties via the identifier @id/widget_a.

Positions Relative to Other Widgets

Four properties control the position of a widget in relation to other widgets:

• android:layout_above: Indicates that the widget should be placed above the widget referenced in the property.
• android:layout_below: Indicates that the widget should be placed below the widget referenced in the property.
• android:layout_toLeftOf: Indicates that the widget should be placed to the left of the widget referenced in the property.
• android:layout_toRightOf: Indicates that the widget should be placed to the right of the widget referenced in the property.

Beyond those four, five additional properties can control one widget's alignment relative to another:

• android:layout_alignTop: Indicates that the widget's top should be aligned with the top of the widget referenced in the property.
• android:layout_alignBottom: Indicates that the widget's bottom should be aligned with the bottom of the widget referenced in the property.
• android:layout_alignLeft: Indicates that the widget's left should be aligned with the left of the widget referenced in the property.
• android:layout_alignRight: Indicates that the widget's right should be aligned with the right of the widget referenced in the property.
• android:layout_alignBaseline: Indicates that the baselines of the two widgets should be aligned (where the baseline is the invisible line that text appears to sit on).

The android:layout_alignBaseline property is useful for aligning labels and fields so that the text appears natural. Since fields have a box around them and labels do not, android:layout_alignTop would align the top of the field's box with the top of the label, which will cause the text of the label to be higher on the screen than the text entered into the field.

So, if you want widget B to be positioned to the right of widget A, in the XML element for widget B, you need to include android:layout_toRightOf = "@id/widget_a" (assuming @id/widget_a is the identity of widget A).

Order of Evaluation

It used to be that Android would use a single pass to process RelativeLayout-defined rules. That meant you could not reference a widget (e.g., via android:layout_above) until it had been declared in the XML. This made defining some layouts a bit complicated. Starting in Android 1.6, Android uses two passes to process the rules, so you can now safely have forward references to as-yet-undefined widgets.

RelativeLayout Example

Now let's examine a typical “form” with a field, a label, and a pair of buttons labeled OK and Cancel. Here is the XML layout, pulled from the Containers/Relative sample project:

First, we open the RelativeLayout. In this case, we want to use the full width of the screen (android:layout_width = "fill_parent"), only as much height as we need (android:layout_height = "wrap_content"), and have 5 pixels of padding between the boundaries of the container and its contents (android:padding = "5px").

Next, we define the label, which is fairly basic, except for its own 15-pixel padding (android:padding = "15px"). More on that in a moment.

After that, we add in the field. We want the field to be to the right of the label and have the text aligned along the baseline. Also, the field should take up the rest of this “row” in the layout. These requirements are handled by three properties:

• android:layout_toRightOf = "@id/label"
• android:layout_alignBaseline = "@id/label"
• android:layout_width = "fill_parent"

If we skipped the 15-pixel padding on the label, we would find that the top of the field was clipped off. That's because of the 5-pixel padding on the container itself. The android:layout_alignBaseline = "@id/label" simply aligns the baselines of the label and field. The label, by default, has its top aligned with the top of the parent. But the label is shorter than the field because of the field's box. Since the field is dependent on the label's position, and the label's position is already defined (because it appeared first in the XML), the field winds up being too high and has the top of its box clipped off by the container's padding.

You may find yourself running into these sorts of problems as you try to get your RelativeLayout to behave the way you want it to.

The solution to this conundrum, used in the XML layout shown above, is to give the label 15 pixels' worth of padding on the top This pushes the label down far enough that the field will not get clipped.

The OK button is set to be below the field (android:layout_below = "@id/entry") and have its right side align with the right side of the field (android:layout_alignRight = "@id/entry"). The Cancel button is set to be to the left of the OK button (android:layout_toLeft = "@id/ok") and have its top aligned with the OK button (android:layout_alignTop = "@id/ok").

Of course, that 15px of padding is a bit of a hack. A better solution, for Android 1.6 and beyond, is to anchor the EditText to the top of the screen and have the TextView say it is aligned with the EditText widget's baseline, as shown in the following example. (In Android 1.5 and earlier, this was not possible, because of the single-pass rule interpretation mentioned earlier.)

With no changes to the autogenerated Java code, the emulator gives us the result shown in Figure 6-6.

Figure 6-6. The RelativeLayoutDemo sample application

Tabula Rasa

If you like HTML tables, spreadsheet grids, and the like, you will appreciate Android's TableLayout, which allows you to position your widgets in a grid to your specifications. You control the number of rows and columns, which columns might shrink or stretch to accommodate their contents, and so on.

TableLayout works in conjunction with TableRow. TableLayout controls the overall behavior of the container, with the widgets themselves poured into one or more TableRow containers, one per row in the grid.

TableLayout Concepts and Properties

For your table layout, you need to figure out how widgets work with rows and columns, plus how to handle widgets that reside outside rows.

Putting Cells in Rows

Rows are declared by you, the developer, by putting widgets as children of a TableRow inside the overall TableLayout. You, therefore, control directly how many rows appear in the table.

The number of columns is determined by Android; you control the number of columns in an indirect fashion. First, there will be at least one column per widget in your longest row. So if you have three rows—one with two widgets, one with three widgets, and one with four widgets—there will be at least four columns. However, a widget can take up more than one column by including the android:layout_span property, indicating the number of columns the widget spans. This is akin to the colspan attribute one finds in table cells in HTML. In this XML layout fragment, the field spans three columns:

Ordinarily, widgets are put into the first available column. In the preceding fragment, the label would go in the first column (column 0, as columns are counted starting from 0), and the field would go into a spanned set of three columns (columns 1 through 3). However, you can put a widget into a different column via the android:layout_column property, specifying the 0-based column the widget belongs to:

In the preceding XML layout fragment, the Cancel button goes in the third column (column 2). The OK button then goes into the next available column, which is the fourth column.

Other Children of TableLayout

Normally, TableLayout contains only TableRow elements as immediate children. However, it is possible to put other widgets in between rows. For those widgets, TableLayout behaves a bit like LinearLayout with vertical orientation. The widgets automatically have their width set to fill_parent, so they will fill the same space that the longest row does.

One pattern for this is to use a plain View as a divider. For example, you could use <View android:layout_height = "2px" android:background = "#0000FF" /> for a 2-pixel-high blue bar across the width of the table.

Stretch, Shrink, and Collapse

By default, each column will be sized according to the natural size of the widest widget in that column (taking spanned columns into account). Sometimes, though, that does not work out very well, and you need more control over column behavior.

You can place an android:stretchColumns property on the TableLayout. The value should be a single column number (again, 0-based) or a comma-delimited list of column numbers. Those columns will be stretched to take up any available space on the row. This helps if your content is narrower than the available space.

Conversely, you can place a android:shrinkColumns property on the TableLayout. Again, this should be a single column number or a comma-delimited list of column numbers. The columns listed in this property will try to word-wrap their contents to reduce the effective width of the column. By default, widgets are not word-wrapped. This helps if you have columns with potentially wordy content that might cause some columns to be pushed off the right side of the screen.

You can also leverage an android:collapseColumns property on the TableLayout, again with a column number or comma-delimited list of column numbers. These columns will start out collapsed, meaning that they will be part of the table information but will be invisible. Programmatically, you can collapse and uncollapse columns by calling setColumnCollapsed() on the TableLayout. You might use this to allow users to control which columns are of importance to them and should be shown versus which ones are less important and can be hidden.

You can also control stretching and shrinking at runtime via setColumnStretchable() and setColumnShrinkable().

TableLayout Example

The XML layout fragments shown earlier, when combined, give us a TableLayout rendition of the form we created for RelativeLayout, with the addition of a divider line between the label/field and the two buttons (found in the Containers/Table demo):

When compiled against the generated Java code and run on the emulator, we get the result shown in Figure 6-7.

Figure 6-7. The TableLayoutDemo sample application

Scrollwork

Phone screens tend to be small, which requires developers to use some tricks to present a lot of information in the limited available space. One trick for doing this is to use scrolling, so only part of the information is visible at one time, and the rest is available via scrolling up or down.

ScrollView is a container that provides scrolling for its contents. You can take a layout that might be too big for some screens, wrap it in a ScrollView, and still use your existing layout logic. It just so happens that the user can see only part of your layout at one time.

For example, here is a ScrollView used in an XML layout file (from the Containers/Scroll demo):

Without the ScrollView, the table would take up at least 560 pixels (seven rows at 80 pixels each, based on the View declarations). There may be some devices with screens capable of showing that much information, but many will be smaller. The ScrollView lets us keep the table as is, but present only part of it at a time.

On the stock Android emulator, when the activity is first viewed, you see the result shown in Figure 6-8.

Figure 6-8. The ScrollViewDemo sample application

Notice how only five rows and part of the sixth are visible. By pressing the up/down buttons on the D-pad, you can scroll up and down to see the remaining rows. Also note how the right side of the content is clipped by the scrollbar. Be sure to put some padding on that side or otherwise ensure your content does not get clipped in this fashion.

Android 1.5 introduced HorizontalScrollView, which works like ScrollView, but horizontally. This can be useful for forms that might be too wide rather than too tall. Note that neither ScrollView nor HorizontalScrollView will give you bidirectional scrolling; you need to choose vertical or horizontal.