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Android Development
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User Interface >

Common Layout Objects

This section describes some of the more common types of layout objects to use in your applications. Like all layouts, they are subclasses of ViewGroup.

Also see the Hello Views tutorials for some guidance on using more Android View layouts.

FrameLayout

FrameLayout is the simplest type of layout object. It's basically a blank space on your screen that you can later fill with a single object — for example, a picture that you'll swap in and out. All child elements of the FrameLayout are pinned to the top left corner of the screen; you cannot specify a different location for a child view. Subsequent child views will simply be drawn over previous ones, partially or totally obscuring them (unless the newer object is transparent).

LinearLayout

LinearLayout aligns all children in a single direction — vertically or horizontally, depending on how you define the orientation attribute. All children are stacked one after the other, so a vertical list will only have one child per row, no matter how wide they are, and a horizontal list will only be one row high (the height of the tallest child, plus padding). A LinearLayout respects margins between children and the gravity (right, center, or left alignment) of each child.

LinearLayout also supports assigning a weight to individual children. This attribute assigns an "importance" value to a view, and allows it to expand to fill any remaining space in the parent view. Child views can specify an integer weight value, and then any remaining space in the view group is assigned to children in the proportion of their declared weight. Default weight is zero. For example, if there are three text boxes and two of them declare a weight of 1, while the other is given no weight (0), the third text box without weight will not grow and will only occupy the area required by its content. The other two will expand equally to fill the space remaining after all three boxes are measured. If the third box is then given a weight of 2 (instead of 0), then it is now declared "more important" than both the others, so it gets half the total remaining space, while the first two share the rest equally.

The following two forms represent a LinearLayout with a set of elements: a button, some labels and text boxes. The text boxes have their width set to fill_parent; other elements are set to wrap_content. The gravity, by default, is left. The difference between the two versions of the form is that the form on the left has weight values unset (0 by default), while the form on the right has the comments text box weight set to 1. If the Name textbox had also been set to 1, the Name and Comments text boxes would be the same height.

Within a horizontal LinearLayout, items are aligned by the position of their text base line (the first line of the first list element — topmost or leftmost — is considered the reference line). This is so that people scanning elements in a form shouldn't have to jump up and down to read element text in neighboring elements. This can be turned off by setting android:baselineAligned="false" in the layout XML.

To view other sample code, see the Hello LinearLayout tutorial.

TableLayout

TableLayout positions its children into rows and columns. TableLayout containers do not display border lines for their rows, columns, or cells. The table will have as many columns as the row with the most cells. A table can leave cells empty, but cells cannot span columns, as they can in HTML.

TableRow objects are the child views of a TableLayout (each TableRow defines a single row in the table). Each row has zero or more cells, each of which is defined by any kind of other View. So, the cells of a row may be composed of a variety of View objects, like ImageView or TextView objects. A cell may also be a ViewGroup object (for example, you can nest another TableLayout as a cell).

The following sample layout has two rows and two cells in each. The accompanying screenshot shows the result, with cell borders displayed as dotted lines (added for visual effect).

<?xml version="1.0" encoding="utf-8"?>
<TableLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:stretchColumns="1">
    <TableRow>
        <TextView
            android:text="@string/table_layout_4_open"
            android:padding="3dip" />
        <TextView
            android:text="@string/table_layout_4_open_shortcut"
            android:gravity="right"
            android:padding="3dip" />
    </TableRow>

    <TableRow>
        <TextView
            android:text="@string/table_layout_4_save"
            android:padding="3dip" />
        <TextView
            android:text="@string/table_layout_4_save_shortcut"
            android:gravity="right"
            android:padding="3dip" />
    </TableRow>
</TableLayout>

Columns can be hidden, marked to stretch and fill the available screen space, or can be marked as shrinkable to force the column to shrink until the table fits the screen. See the TableLayout reference documentation for more details.

To view sample code, see the Hello TableLayout tutorial.

RelativeLayout

RelativeLayout lets child views specify their position relative to the parent view or to each other (specified by ID). So you can align two elements by right border, or make one below another, centered in the screen, centered left, and so on. Elements are rendered in the order given, so if the first element is centered in the screen, other elements aligning themselves to that element will be aligned relative to screen center. Also, because of this ordering, if using XML to specify this layout, the element that you will reference (in order to position other view objects) must be listed in the XML file before you refer to it from the other views via its reference ID.

The example below shows an XML file and the resulting screen in the UI. Note that the attributes that refer to relative elements (e.g., layout_toLeft) refer to the ID using the syntax of a relative resource (@id/id).

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android
                android:layout_width="fill_parent" 
                android:layout_height="wrap_content"
                android:background="@drawable/blue"
                android:padding="10px" >

    <TextView android:id="@+id/label" 
              android:layout_width="fill_parent" 
              android:layout_height="wrap_content" 
              android:text="Type here:" />

    <EditText android:id="@+id/entry" 
              android:layout_width="fill_parent" 
              android:layout_height="wrap_content" 
              android:background="@android:drawable/editbox_background"
              android:layout_below="@id/label" />
  
    <Button android:id="@+id/ok" 
            android:layout_width="wrap_content" 
            android:layout_height="wrap_content" 
            android:layout_below="@id/entry"
            android:layout_alignParentRight="true"
            android:layout_marginLeft="10px"
            android:text="OK" />

    <Button android:layout_width="wrap_content" 
            android:layout_height="wrap_content"
            android:layout_toLeftOf="@id/ok"
            android:layout_alignTop="@id/ok"
            android:text="Cancel" />
</RelativeLayout>

Some of these properties are supported directly by the element, and some are supported by its LayoutParams member (subclass RelativeLayout for all the elements in this screen, because all elements are children of a RelativeLayout parent object). The defined RelativeLayout parameters are: width, height, below, alignTop, toLeft, padding[Bottom|Left|Right|Top], and margin[Bottom|Left|Right|Top]. Note that some of these parameters specifically support relative layout positions — their values must be the ID of the element to which you'd like this view laid relative. For example, assigning the parameter toLeft="my_button" to a TextView would place the TextView to the left of the View with the ID my_button (which must be written in the XML before the TextView).

To view this sample code, see the Hello RelativeLayout tutorial.

Summary of Important View Groups

These objects all hold child UI elements. Some provide their own form of a visible UI, while others are invisible structures that only manage the layout of their child views.

Class Description
FrameLayout Layout that acts as a view frame to display a single object.
Gallery A horizontal scrolling display of images, from a bound list.
GridView Displays a scrolling grid of m columns and n rows.
LinearLayout A layout that organizes its children into a single horizontal or vertical row. It creates a scrollbar if the length of the window exceeds the length of the screen.
ListView Displays a scrolling single column list.
RelativeLayout Enables you to specify the location of child objects relative to each other (child A to the left of child B) or to the parent (aligned to the top of the parent).
ScrollView A vertically scrolling column of elements.
Spinner Displays a single item at a time from a bound list, inside a one-row textbox. Rather like a one-row listbox that can scroll either horizontally or vertically.
SurfaceView Provides direct access to a dedicated drawing surface. It can hold child views layered on top of the surface, but is intended for applications that need to draw pixels, rather than using widgets.
TabHost Provides a tab selection list that monitors clicks and enables the application to change the screen whenever a tab is clicked.
TableLayout A tabular layout with an arbitrary number of rows and columns, each cell holding the widget of your choice. The rows resize to fit the largest column. The cell borders are not visible.
ViewFlipper A list that displays one item at a time, inside a one-row textbox. It can be set to swap items at timed intervals, like a slide show.
ViewSwitcher Same as ViewFlipper.

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