Monday, May 21, 2012

Layouts in android


. LAYOUT’s

            An Android layout is a class that handles arranging the way its children appear on the screen.  Anything that is a View (or inherits from View) can be a child of a layout. All of the layouts inherit from ViewGroup (which inherits from View) so you can nest layouts.  You could also create your own custom layout by making a class that inherits from ViewGroup. There are 5 types of standard layout which are,
·         Absolute Layout
·         Frame Layout
·         Linear Layout
·         Relative Layout
·         Table Layout

4.1 ABSOLUTE LAYOUT

            AbsoluteLayout is based on the simple idea of placing each control at an absolute position.  You specify the exact x and y coordinates on the screen for each control.  This is not recommended for most UI development (in fact AbsoluteLayout is currently deprecated) since absolutely positioning every element on the screen makes an inflexible UI that is much more difficult to maintain.  Consider what happens if a control needs to be added to the UI. You would have to change the position of every single element that is shifted by the new control.

<AbsoluteLayout xmlns:android= "http://schemas.android.com/apk/res/android"
    android:layout_width= "fill_parent"
    android:layout_height= "fill_parent">
    <Button
        android:id="@+id/backbutton"
        android:text="Back"
        android:layout_x="10px"
        android:layout_y="5px"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <TextView
        android:layout_x="10px"
        android:layout_y="110px"
        android:text="First Name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <EditText
        android:layout_x="150px"
        android:layout_y="100px"
        android:width="100px"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
   
<TextView
        android:layout_x="10px"
        android:layout_y="160px"
        android:text="Last Name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
<EditText
        android:layout_x="150px"
        android:layout_y="150px"
        android:width="100px"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
</AbsoluteLayout>

            Note how each element has android:layout_x and android:layout_y specified. Android defines the top left of the screen as (0, 0) so the layout_x value will move the control to the right, and the layout_y value will move the control down. Here is a screenshot of the layout produced by this XML. Also about the attribute layout_width=”wrap_content” there is also options like fill_parent (fill the attribute i.e, width or height to fill the entire layout), match_parent (used while using RelaiveLayout and when there are more than one Activity), wrap_content (used to match wrap around the text).
                               
4.2 FRAME LAYOUT
FrameLayout is designed to display a single item at a time. You can have multiple elements within a FrameLayout but each element will be positioned based on the top left of the screen you cannot specify a different location for a child. Elements that overlap will be displayed overlapping.

<FrameLayout
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        xmlns:android="http://schemas.android.com/apk/res/android">
        <ImageView
               android:src="@drawable/icon"
               android:scaleType="fitCenter"
               android:layout_height="fill_parent"
               android:layout_width="fill_parent"/>
        <TextView
               android:text="Learn-Android.com"
               android:textSize="24sp"
               android:textColor="#000000"
               android:layout_height="fill_parent"
               android:layout_width="fill_parent"
               android:gravity="center"/>
</FrameLayout>

Here is the result of this XML.
       You can see that both the ImageView and TextView fill the parent in both horizontal and vertical layout. Gravity specifies where the text appears within its container, so it is set  to center. If it  had not given as gravity then the text would have appeared at the top left of the screen.    
                                                   
4.3  LINEAR  LAYOUT
LinearLayout organizes elements along a single line. You specify whether that line is vertical or horizontal using android:orientation. All children are stacked one after the other, so a vertical list with 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 linear layout respects margins between children and the gravity(right, left, center) of each child.
It 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. It is given as any integer value.  For example, if there are three text boxes and two of them declare a weight of 1, while the other is no weight 0, the third text box without weigh will not grow and will 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 others, so its gets half the total remaining space, while the first two share the rest equally.
Here is a sample Layout XML using LinearLayout.

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:orientation="vertical"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent">
     <Button
        android:id="@+id/backbutton"
        android:text="Back"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <TextView
        android:text="First Name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <EditText
        android:width="100px"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <TextView
        android:text="Last Name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <EditText
        android:width="100px"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
</LinearLayout>

Here is a screenshot of the result of the above XML.
It was mentioned on the first page that Layouts can be nested. LinearLayout is frequently nested, with horizontal and vertical layouts mixed. Here is an example of this.
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:orientation="vertical"
        android:layout_width="fill_parent"
    android:layout_height="fill_parent">
     <Button
        android:id="@+id/backbutton"
        android:text="Back"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <LinearLayout
        android:orientation="horizontal"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content">
            <TextView
                android:text="First Name"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" />
            <EditText
                android:width="100px"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" />      
    </LinearLayout>
    <LinearLayout
        android:orientation="horizontal"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content">   
            <TextView
                android:text="Last Name"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" />
            <EditText
                android:width="100px"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" />
    </LinearLayout>
</LinearLayout>


Here is a screenshot of the result of the above XML.
                     
4.4 RELATIVE LAYOUT
RelativeLayout lays out elements based on their relationships with one another, and with the parent container. This is arguably the most complicated layout, and we need several properties to actually get the layout we want.

4.4.1 RELATIVE TO CONTAINER

These properties will layout elements relative to the parent container.
  • android:layout_alignParentBottom – Places the bottom of the element on the bottom of the container
  • android:layout_alignParentLeft – Places the left of the element on the left side of the container
  • android:layout_alignParentRight – Places the right of the element on the right side of the container
  • android:layout_alignParentTop – Places the element at the top of the container
  • android:layout_centerHorizontal – Centers the element horizontally within its parent container
  • android:layout_centerInParent – Centers the element both horizontally and vertically within its container
  • android:layout_centerVertical – Centers the element vertically within its parent container

4.4.2 RELATIVE TO OTHER ELEMENTS

These properties allow you to layout elements relative to other elements on screen. The value for each of these elements is the id of the element you are using to layout the new element. Each element that is used in this way must have an ID defined using android:id=”@+id/XXXXX” where XXXXX is replaced with the desired id. You use “@id/XXXXX” to reference an element by its id. One thing to remember is that referencing an element before it has been declared will produce an error.
  • android:layout_above – Places the element above the specified element
  • android:layout_below – Places the element below the specified element
  • android:layout_toLeftOf – Places the element to the left of the specified element
  • android:layout_toRightOf – Places the element to the right of the specified element

4.4.3 ALIGNMENT WITH OTHER ELEMENTS

These properties allow you to specify how elements are aligned in relation to other elements.
  • android:layout_alignBaseline – Aligns baseline of the new element with the baseline of the specified element
  • android:layout_alignBottom – Aligns the bottom of new element in with the bottom of the specified element
  • android:layout_alignLeft – Aligns left edge of the new element with the left edge of the specified element
  • android:layout_alignRight – Aligns right edge of the new element with the right edge of the specified element
  • android:layout_alignTop – Places top of the new element in alignment with the top of the specified element
Here is a sample XML Layout
<RelativeLayout
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        xmlns:android="http://schemas.android.com/apk/res/android">
        <Button
               android:id="@+id/backbutton"
               android:text="Back"
               android:layout_width="wrap_content"
               android:layout_height="wrap_content" />
        <TextView
               android:id="@+id/firstName"
               android:text="First Name"
               android:layout_width="wrap_content"
               android:layout_height="wrap_content"
               android:layout_below="@id/backbutton" />
        <EditText
               android:id="@+id/editfirstName"
               android:width="100px"
               android:layout_width="wrap_content"
               android:layout_height="wrap_content"
               android:layout_toRightOf="@id/firstName"
               android:layout_alignBaseline="@id/firstName" />
       
        <EditText
               android:id=”@+id/editlastName”
               android:width="100px"
               android:layout_width="wrap_content"
               android:layout_height="wrap_content"
               android:layout_toRightOf="@id/lastName"
               android:layout_alignBaseline="@id/lastName" />
       <TextView
               android:id="@+id/lastName"
               android:text="Last Name"
               android:layout_width="wrap_content"
               android:layout_height="wrap_content"
               android:layout_toLeftOf=”@id.editlastName”
               android:layout_below="@id/firstName" />

</RelativeLayout>


Here is a screenshot of the result of the above XML.


4.5 TABLE LAYOUT

TableLayout organizes content into rows and columns. The rows are defined in the layout XML, and the columns are determined automatically by Android. This is done by creating at least one column for each element. So, for example, if you had a row with two elements and a row with five elements then you would have a layout with two rows and five columns.
You can specify that an element should occupy more than one column using android:layout_span. This can increase the total column count as well, so if we have a row with two elements and each element has android:layout_span=”3 then you will have at least six columns in your table. By default, Android places each element in the first unused column in the row. You can, however, specify the column an element should occupy using android:layout_column.

<TableLayout
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        xmlns:android="http://schemas.android.com/apk/res/android">
       
        <TableRow>
             <Button
                android:id="@+id/backbutton"
                android:text="Back"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" />
        </TableRow>
        <TableRow>
             <TextView
                android:text="First Name"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_column="1" />
             <EditText
                android:width="100px"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" />
        </TableRow>
        <TableRow>
            <TextView
                android:text="Last Name"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_column="1" />
            <EditText
                android:width="100px"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content" />
        </TableRow>
</TableLayout>
Here is a screenshot of the result of the above XML.

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