Know Your Layouts in Android

Android provides solid support for the development of UI-based applications. Android provides a variety of widgets that the application programmer can use to create a desired layout and interface. These layout elements can be created via the programming language directly, or through XML layout files. In this article, we are going to show you both methods and highlight their differences.

XML-Based Layouts in Android

In Android, an XML-based layout is a file that defines the different widgets to be used in the UI and the relations between those widgets and their containers. Android treats the layout files as resources. Hence the layouts are kept in the folder reslayout. If you are using eclipse, it creates a default XML layout file (main.xml) in the reslayout folder, which looks like the following XML code. The layout files act as an input to the Android Asset Packaging Tool (AAPT) tool, which creates an R.java file for all of the resources.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
<TextView
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="@string/hello"
    />
</LinearLayout>

The Benefits of XML-Based Layouts

XML-based layouts are very helpful if you know the UI components at the time of compiling. If run-time UI components are needed, then those can be added using the XML code. XML-based layout have the following advantages:

• XML is a very popular and widely-used format. Hence, a lot of developers are quite comfortable with it.
• It helps to provide separation of the UI from the code logic. This provided flexibility to change one without affecting much the other.
• Generating XML output is easier than writing direct code, making it easier to have drag-and-drop UI tools to generate interfaces for android apps.

Standard Layouts in Android

The UI in Android is a hierarchy of viewgroups and views. The viewgroups will be intermediate nodes in the hierarchy, and the views will be terminal nodes. For example, in the above main.xml file, the LinearLayout is a viewgroup and the TextView is a view. Android provides the following standard layouts (viewgroups) that can be used in you Android application.

• AbsoluteLayout
• FrameLayout
• LinearLayout
• RelativeLayout
• TableLayout

Now, we are going to explore each one of them in detail.

Absolute Layout

In absolute layout, we can specify the exact coordinates of each control that we want to place. In absolute layout, we will give the exact X and Y coordinates of each control. The following is an example of an absolute layout:

<?xml version="1.0" encoding="utf-8"?>
<AbsoluteLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">

    <TextView
    	android:layout_x="10px"
    	android:layout_y="110px"
    	android:text="@string/username"
    	android:layout_width="wrap_content"
    	android:layout_height="wrap_content" />
    <EditText
    	android:layout_x="150px"
    	android:layout_y="100px"
    	android:width="150px"
    	android:layout_width="wrap_content"
    	android:layout_height="wrap_content" />
    <TextView
    	android:layout_x="10px"
    	android:layout_y="160px"
    	android:text="@string/password"
    	android:layout_width="wrap_content"
    	android:layout_height="wrap_content" />
    	<EditText
    	android:layout_x="150px"
    	android:layout_y="150px"
    	android:width="150px"
    	android:layout_width="wrap_content"
    	android:layout_height="wrap_content" />

     <Button
    	android:id="@+id/login"
    	android:text="@string/login"
    	android:layout_x="75px"
    	android:layout_y="200px"
    	android:width="200px"
    	android:layout_width="wrap_content"
    	android:layout_height="wrap_content" />
</AbsoluteLayout>

In this example, we have created two TextViews , two EditTexts and one button .To create an Absolute layout, you will have to use the <AbsoluteLayout> tag in your layout’s XML file. Then, we create the controls and specify the android:layout_x and android:layout_y properties on the control to give elements their absolute position on the screen. If we run the app with this layout, it should look like this: While using absolute layout, we can specify the exact position of the elements. You can move theml around by changing the android:layout_x and android:layout_y properties within the control. Absolute layout is actually deprecated now, as it is not flexible for the wide variety of new Android devices. Hence, it should be used only if no other type of layout will suit your purposes.

Frame Layout

Frame layout is used when you want to show one item on each screen. Using frame layout, we can have multiple items, but they will be overlapping and only only displaying themselves one at a time. FrameLayout is particularly useful when you want to create animation or movement on screen. Now, we are going to create a small app using frame layout that has one button and one TextView. First, the button is displayed. Once you click on the button, the TextView is shown. The following is the code for the layout:

<FrameLayout
	android:layout_width="fill_parent"
	android:layout_height="fill_parent"
	xmlns:android="http://schemas.android.com/apk/res/android">

	<Button
		android:id="@+id/username"
    	android:text="@string/username"
		android:layout_height="fill_parent"
		android:layout_width="fill_parent"
		android:textSize="30sp"
		android:onClick="onClick"
		android:gravity="center" />
	<TextView
		android:id="@+id/password"
    	android:text="@string/password"
		android:layout_height="fill_parent"
		android:layout_width="fill_parent"
		android:textSize="30sp"
		android:visibility="gone"
		android:gravity="center" />
</FrameLayout>

Here, for the button we have a Click handler is called onClick. The TextView has the property android:visibility=“gone”, which hides the TextView. The code for the activity is as follows:

package com.FrameLayoutDemo;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;

public class FrameLayoutDemo extends Activity {
    /** Called when the activity is first created. */

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

    }

    public void onClick(View v) {

    	View vUsername = findViewById(R.id.username);
    	vUsername.setVisibility(View.GONE);

    	View vPassword = findViewById(R.id.password);
    	vPassword.setVisibility(View.VISIBLE);

      }
}

If we run this frame layout program, it will look as follows: When we click on the button, the onClick function is called. In the onClick function we get username and password control, and we set the visibility for the username as “Gone” and for password as “Visible” using the function setVisibility.

Linear Layout

Linear layout is used to place one element on each line. So, all the elements will be place in an orderly top-to-bottom fashion. This is a very widely-used layout for creating forms on Android. We are now going to create a small app to display a basic form using the linear layout. The layout.xml file is as follows:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
	android:orientation="vertical"
	android:layout_width="fill_parent"
    android:layout_height="fill_parent">

    <TextView
    	android:text="@string/username"
    	android:layout_width="wrap_content"
    	android:layout_height="wrap_content" />
    <EditText
    	android:width="150px"
    	android:layout_width="wrap_content"
    	android:layout_height="wrap_content" />
    <TextView
    	android:text="@string/password"
    	android:layout_width="wrap_content"
    	android:layout_height="wrap_content" />
    <EditText
    	android:width="150px"
    	android:layout_width="wrap_content"
    	android:layout_height="wrap_content" />
</LinearLayout>

In this design, we create a vertical linear layout, which displays a username and password, as shown below: We can even change the orientation of our linear layout by making android:orientation=”horizontal”. This would display all the controls in one horizontal line, as shown below:

Relative Layout

Using relative layout, we can specify the position of the elements in relation to other elements, or in relation to the parent container. In order to specify the position with relation to its parent container, we use android:layout_alignParentTop=“true” and android:layout_alignParentLeft=“true” to align elements to the top-left of the parent container. Then, to align with respect to another element we can use the properties android:layout_alignLeft=“@+id/otherelement” and android:layout_below=“@+id/otherelement”. The following is an app employing relative layout for a basic form with a username field, password field, and a login button:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
      android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    xmlns:android="http://schemas.android.com/apk/res/android"
    >
    <TextView
        android:id="@+id/username"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/username"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        />
    <EditText
    	android:id="@+id/txtusername"
        android:layout_width="fill_parent"
        android:layout_height="50px"
        android:layout_alignLeft="@+id/username"
        android:layout_below="@+id/username"
        android:layout_centerHorizontal="true"
        />

       <TextView
        android:id="@+id/password"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/password"
      android:layout_alignLeft="@+id/txtusername"
        android:layout_below="@+id/txtusername"
        android:layout_centerHorizontal="true"
        />
    <EditText
    	android:id="@+id/txtpassword"
        android:layout_width="fill_parent"
        android:layout_alignLeft="@+id/password"
        android:layout_height="50px"
        android:layout_below="@+id/password"
        android:layout_centerHorizontal="true"
        />

        <Button
    	android:id="@+id/login"
    	android:text="@string/login"
    	android:width="200px"
    	android:layout_width="wrap_content"
    	android:layout_height="wrap_content"
    	android:layout_alignLeft="@+id/txtpassword"
        android:layout_below="@+id/txtpassword"
        android:layout_centerHorizontal="true"
         />

</RelativeLayout>

In this layout, the TextView username is positioned at the top-left of the screen using the same aforementioned properties:

• android:layout_alignParentTop=“true”
• android:layout_alignParentLeft=“true”

Then, we position the other elements below by using the properties:

• android:layout_alignLeft=“@+id/txtpassword”
• android:layout_below=“@+id/txtpassword”
• android:layout_centerHorizontal=“true”

If we run this app, it will look like the following image:

Table Layout

Using table layout, we create a table with rows and columns and place elements within them. In each row, you cam specify one or more elements. The table row is created using the tag <TableRow> inside the TableLayout. The following is an example of an app with table layout:

<TableLayout
	android:layout_width="fill_parent"
	android:layout_height="fill_parent"
	xmlns:android="http://schemas.android.com/apk/res/android">

	<TableRow>
		<TextView
        android:id="@+id/username"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/username"

        />
    <EditText
    	android:id="@+id/txtusername"
        android:layout_width="fill_parent"
        android:layout_height="50px"
        android:width="100px"

        />
	</TableRow>
	<TableRow>
		<TextView
        android:id="@+id/password"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/password"

        />
    <EditText
    	android:id="@+id/txtpassword"
        android:layout_width="fill_parent"
         android:layout_height="50px"
         android:width="100px"

        />
	</TableRow>
	<TableRow>
		<Button
    	android:id="@+id/login"
    	android:text="@string/login"
    	android:width="200px"
    	android:layout_width="wrap_content"
    	android:layout_height="wrap_content"
    	 />
	</TableRow>
</TableLayout>

In the first row of this example, we have added two elements: the username TextView and EditText. Within the second row, we have added two more elements: the password TextView and EditText. Then, in the third row, we added the login button. The output of this app is as follows:

Conclusion

Android provides a very flexible way to display layouts using XML-based layouts. This helps us create apps while keeping the layout and logic completely separate. This makes it very easy for us to change the layout even after the application is written, without having any changes in the programming logic. Android also has a variety of layouts that can be used to create more complex UIs to suit the needs of a wide variety of applications. So, have fun creating beautiful UI for your apps using the different Android layouts.

Frequently Asked Questions (FAQs) about Android Layouts