Android ui layout

Topics
• Declaring Layout
• Layout File
• Attributes
> ID
> Layout parameters
• Types of Layout
> LinearLayout
> RelativeLayout
> TableLayout
> FrameLayout
> Tab layout

What is a Layout?
• Your layout is the architecture for the user
interface in an Activity.
• It defines the layout structure and holds all the
elements that appear to the user.

How to declare a Layout? Two Options
• Option #1: Declare UI elements in XML (most
common and preferred)
> Android provides a straightforward XML vocabulary
that corresponds to the View classes and subclasses,
such as those for UI controls called widgets
(TextView, Button, etc.) and layouts.
• Option #2: Instantiate layout elements at
runtime (in Java code)
> Your application can create View and ViewGroup
objects (and manipulate their properties)
programmatically (in Java code).

Example of using both options
• You can use either or both of these options for
declaring and managing your application's UI
• Example usage scenario
> You could declare your application's default layouts
in XML, including the screen elements that will
appear in them and their properties.
> You could then add code in your application that
would modify the state of the screen objects,
including those declared in XML, at run time.

Advantages of Option #1: Declaring UI
in XML
• Separation of the presentation from the code
that controls its behavior
> You can modify UI without having to modify your
source code and recompile
> For example, you can create XML layouts for
different screen orientations, different device screen
sizes, and different languages
• Easier to visualize the structure of your UI
(without writing any code)
> Easier to design/debug UI
> Visualizer tool (like the one in Eclipse IDE)

Layout File Structure
• Each layout file must contain exactly one root
element, which must be a View (Button, for
example) or ViewGroup object (LinearLayout,
for example).
• Once you've defined the root element, you can
add additional layout objects or widgets (View)
as child elements to gradually build a View
hierarchy that defines your layout.

Example: Layout File
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<TextView android:id="@+id/text"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Hello, I am a TextView" />
<Button android:id="@+id/button"
android:text="Hello, I am a Button" />
</LinearLayout>

Where to create Layout file?
• Save the file with the .xml extension, in your
Android project's res/layout/ directory

Load the Layout XML Resource
• When you compile your application, each XML
layout file is compiled into a View resource.
• You should load the layout resource from your
application code, in your Activity.onCreate()
callback implementation.
> By calling setContentView(), passing it the reference
to your layout resource in the form of:
R.layout.layout_file_name
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main_layout);
}

Attributes
• Every View and ViewGroup object supports
their own variety of attributes.
> Some attributes are specific to a View object (for
example, TextView supports the textSize attribute),
but these attributes are also inherited by any View
objects that may extend this class.
> Some are common to all View objects, because they
are inherited from the root View class (like the id
attribute).
> Other attributes are considered "layout parameters,"
which are attributes that describe certain layout
orientations of the View object, as defined by that
object's parent ViewGroup object.
> These attributes are typically in XML form

ID Attribute
• Any View object may have an integer ID
associated with it, to uniquely identify the View
within the tree.
• When the application is compiled, this ID is
referenced as an integer, but the ID is typically
assigned in the layout XML file as a string, in
the id attribute.
• Syntax
> android:id="@+id/my_button"

ID Attribute - Android Resource ID
• There are a number of other ID resources that
are offered by the Android framework.
• When referencing an Android resource ID, you
do not need the plus-symbol, but must add the
android package namespace
> android:id="@android:id/empty"
• With the android package namespace in place,
we're now referencing an ID from the android.R
resources class, rather than the local resources
class.

How to reference views in Java code?
• Assuming a view/widget is defined in the layout
file with a unique ID
<Button android:id="@+id/my_button"
android:text="@string/my_button_text"/>
• Then you can make a reference to the view
object via findViewById(R.id.<string-id>).
Button myButton = (Button) findViewById(R.id.my_button);

What Are Layout Parameters?
• XML layout attributes named layout_something
define layout parameters for the View that are
appropriate for the ViewGroup in which it
resides
• Every ViewGroup class implements a nested
class that extends ViewGroup.LayoutParams.
> This subclass contains property types that define the
size and position for each child view, as appropriate
for the view group.

Parent view group defines layout parameters for each child view
(including the child view group)

layout_width & layout_height
• wrap_content
> tells your view to size itself to the dimensions
required by its content
• fill_parent
> tells your view to become as big as its parent view
group will allow.
• match_parent
> Same as fill_parent
> Introduced in API Level 8
<Button android:id="@+id/my_button"
android:layout_width="match_parent"
android:text="@string/my_button_text"/>

layout_weight
• Is used in LinearLayouts to assign "importance"
to Views within the layout.
• All Views have a default layout_weight of zero
• Assigning a value higher than zero will split up
the rest of the available space in the parent
View, according to the value of each View's
layout_weight and its ratio to the overall
layout_weight specified in the current layout for
this and other View elements.

Layout Types
• All layout types are subclass of ViewGroup class
• Layout types
> LinearLayout
> RelativeLayout
> TableLayout
> FrameLayout
> Tab layout

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
• A LinearLayout respects
> margins between children
> gravity (right, center, or left alignment) of each child.
> weight to each child

RelativeLayout
• RelativeLayout lets child views specify their
position relative to the parent view or to each
other (specified by ID)
> 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.

RelativeLayout Example
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android
android:background="@drawable/blue"
android:padding="10px" >
<TextView android:id="@+id/label"
android:text="Type here:" />
<EditText android:id="@+id/entry"
android:background="@android:drawable/editbox_background"
android:layout_below="@id/label" />
<Button android:id="@+id/ok"
android:layout_below="@id/entry"
android:layout_alignParentRight="true"
android:layout_marginLeft="10px"
android:text="OK" />
<Button android:layout_width="wrap_content"
android:layout_toLeftOf="@id/ok"
android:layout_alignTop="@id/ok"
android:text="Cancel" />
</RelativeLayout>

TableLayout
• TableLayout positions its children into rows and
columns
• 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.
• 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.

TableLayout Example
<TableLayout xmlns:android="http://schemas.android.com/apk/res/android"
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"
</TableRow>
<TableRow>
<TextView
android:text="@string/table_layout_4_save"
<TextView
android:text="@string/table_layout_4_save_shortcut"
android:gravity="right"
</TableRow>
</TableLayout>

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).

FrameLayout Example
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:text="yellowyellowyellow"
android:gravity="bottom"
android:background="#aaaa00"
android:layout_height="120dip"/>
<TextView
android:text="greengreengreen"
android:background="#00aa00"
android:layout_height="90dip" />
<TextView
android:text="blueblueblue"
android:background="#0000aa"
android:layout_height="60dip" />
<TextView
android:text="redredred"
android:background="#aa0000"
android:layout_height="30dip"/>
</FrameLayout>

Android ui layout

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Android ui layout