LayoutAndroid Training

By Khaled Anaqwa

As if a device can function if it has no style. As if a device can be called stylish that does not function superbly.... Yes, beauty matters. Boy, does it matter. It is not surface, it is not an extra, it is the thing itself. —STEPHEN FRY, THE GUARDIAN (OCTOBER 27, 2007)

User Interface All user interface elements in an Android app

are built using View and ViewGroup objects. A View is an object that draws something on

the screen that the user can interact with. A ViewGroup is an object that holds other

View (and ViewGroup) objects in order to define the layout of the interface.

The user interface for each component of your app is defined using a hierarchy of View and ViewGroup objects.

This hierarchy tree can be as simple or complex as you need it to be.

Layouts A layout defines the visual structure for a

user interface, such as the UI for an activity or app widget. You can declare a layout in two ways:

Declare UI elements in XML. Instantiate layout elements at runtime

The Android framework gives you the flexibility to use either or both .

The advantage to declaring your UI in XML is that it enables you to better separate the presentation from the code that controls its behavior.

Using Eclipse to Design Layout Resources Use the Outline pane to Add and

Remove controls to your layout resource.

Select a specific control (either in the Preview or the Outline) and use the Property pane to adjust a specific control’s attributes.

Use the XML tab to edit the XML definition directly.

Write the XML Each layout file must contain exactly

one root element. XML vocabulary for declaring UI

elements closely follows the structure and naming of the classes and methods.

For example, the EditText element has a text attribute that corresponds to EditText.setText().

Attributes Overview Every View and ViewGroup object supports

their own variety of XML attributes. Some attributes are specific (e.g. TextView

supports the textSize attribute) 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).

Attributes Examples ID (Any View object may have an integer ID

associated with it, to uniquely identify the View within the tree.) android:id="@+id/my_button” The at-symbol (@) at the beginning of the string

indicates that the XML parser should parse and expand the rest of the ID string and identify it as an ID resource.

The plus-symbol (+) means that this is a new resource name that must be created and added to our resources (in the R.java file)

android:id="@android:id/empty” (from android.R)

Demo Create View in xml Create the same view in code

Question?

If I Instantiate view at runtime how I can assign ID to it ?

Attributes Examples Layout Parameters (XML layout attributes

named layout_something ) for the View that are appropriate for the

ViewGroup in which it resides. All view groups include a width and height

(layout_width and layout_height), and each view is required to define them. wrap_content tells your view to size itself to the

dimensions required by its content fill_parent (renamed match_parent in API Level 8)

tells your view to become as big as its parent view group will allow.

Every LayoutParams subclass has its own syntax for setting values. Each child element must define LayoutParams that are appropriate for its parent, though it may also define different LayoutParams for its own children.

Note (not Galaxy Note) specifying a layout width and height

using absolute units such as pixels is not recommended. Instead, using relative measurements such as density-independent pixel units (dp), wrap_content, or fill_parent, is a better approach, because it helps ensure that your application will display properly across a variety of device screen sizes.

Demo !! XML Code

Try !!! Create XML layout that uses a vertical

LinearLayout to hold a TextView and a Button

TextView id = text Text= “Hello, I am a TextView”

Button id=button text="Hello, I am a Button"

Attributes Examples Size, Padding and Margins

The accepted measurement types:Px, dp(Density-independent Pixels), sp(Scale-independent Pixels), pt, in, mm.

We can define it in Dimension Values.

Note Although you can nest one or more

layouts within another layout to achieve your UI design, you should strive to keep your layout hierarchy as shallow as possible.

Your layout draws faster if it has fewer nested layouts (a wide view hierarchy is better than a deep view hierarchy).Why?

As you can see, the code can rapidly grow in size as more controls are added to the screen, making screen contents more difficult to maintain or reuse.

Exploring Various Layout Types

FrameLayout designed to display a stack of child View

controls. This can be used to show multiple

controls within the same screen space

LinearLayout designed to display child View controls in

a single row or column.(Orientation= vertically or horizontally.)

This is a very handy layout method for creating forms.

Note To create a linear layout in which each

child uses the same amount of space on the screen,

set the android:layout_height of each view to "0dp" (for a vertical layout)

or the android:layout_width of each view to "0dp" (for a horizontal layout).

Then set the android:layout_weight of each view to "1".

Try to make it !!

Hint if there are three text fields and two of them

declare a weight of 1, while the other is given no weight, the third text field 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 fields are measured.

If the third field 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.

RelativeLayout designed to display child View controls in

relation to each other. You can also align child View controls

relative to the parent edges. Is a very powerful utility for designing a

user interface because it can eliminate nested view groups and keep your layout hierarchy flat, which improves performance.

Positioning Views android:layout_alignParentTopIf "true", makes the top edge of this view match the top edge of the parent. android:layout_centerVerticalIf "true", centers this child vertically within its parent. android:layout_belowPositions the top edge of this view below the view specified with a resource ID. android:layout_toRightOfPositions the left edge of this view to the right of the view specified with a resource ID.

Try To make it !!

TableLayout designed to organize child View controls

into rows and columns. designed to organize child View controls

into rows and columns.

Create From First Name Second Name Job Email Submit OnClick (Print All of them)

Combining Layouts This means that layout controls can be

nested

Event Listeners onClick() onLongClick() onFocusChange() onTouch() onKey()

How to Handle Listeners Implement My Listener Let Class Implement it