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This chapter is from the book

This chapter is from the book

RelativeLayout

In RelativeLayout, each child element is laid out in relation to other child elements; that is, the location of a child element is specified in terms of the desired distance from the existing children. To understand the concept of relative layout practically, let’s create a new Android project called RelativeLayoutApp. Modify its layout file activity_relative_layout_app.xml to appear as shown in Listing 3.6.

Listing 3.6. The activity_relative_layout_app.xml File on Arranging the Button Controls in the RelativeLayout Container

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <Button
        android:id="@+id/Apple"
        android:text="Apple"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="15dip"
        android:layout_marginLeft="20dip" />
    <Button
        android:id="@+id/Mango"
        android:text="Mango"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:padding="28dip"
        android:layout_toRightOf="@id/Apple"
        android:layout_marginLeft="15dip"
        android:layout_marginRight="10dip"
        android:layout_alignParentTop="true" />
    <Button
        android:id="@+id/Banana"
        android:text="Banana"
        android:layout_width="200dip"
        android:layout_height="50dip"
        android:layout_marginTop="15dip"
        android:layout_below="@id/Apple"
        android:layout_alignParentLeft="true" />
    <Button
        android:id="@+id/Grapes"
        android:text="Grapes"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:minWidth="100dp"
        android:layout_alignParentRight="true"
        android:layout_below="@id/Banana" />
    <Button
        android:id="@+id/Kiwi"
        android:text="Kiwi"
        android:layout_width="100dip"
        android:layout_height="wrap_content"
        android:layout_below="@id/Banana"
        android:paddingTop="15dip"
        android:paddingLeft="25dip"
        android:paddingRight="25dip" />
</RelativeLayout>

Before we understand how the controls in the previous code block are placed, let’s have a quick look at different attributes used to set the positions of the layout controls.

Layout Control Attributes

The attributes used to set the location of the control relative to a container are

  • android:layout_alignParentTop—The top of the control is set to align with the top of the container.
  • android:layout_alignParentBottom—The bottom of the control is set to align with the bottom of the container.
  • android:layout_alignParentLeft—The left side of the control is set to align with the left side of the container.
  • android:layout_alignParentRight—The right side of the control is set to align with the right side of the container.
  • android:layout_centerHorizontal—The control is placed horizontally at the center of the container.
  • android:layout_centerVertical—The control is placed vertically at the center of the container.
  • android:layout_centerInParent—The control is placed horizontally and vertically at the center of the container.

The attributes to control the position of a control in relation to other controls are

  • android:layout_above—The control is placed above the referenced control.
  • android:layout_below—The control is placed below the referenced control.
  • android:layout_toLeftOf—The control is placed to the left of the referenced control.
  • android:layout_toRightOf—The control is placed to the right of the referenced control.

The attributes that control the alignment of a control in relation to other controls are

  • android:layout_alignTop—The top of the control is set to align with the top of the referenced control.
  • android:layout_alignBottom—The bottom of the control is set to align with the bottom of the referenced control.
  • android:layout_alignLeft—The left side of the control is set to align with the left side of the referenced control.
  • android:layout_alignRight—The right side of the control is set to align with the right side of the referenced control.
  • android:layout_alignBaseline—The baseline of the two controls will be aligned.

For spacing, Android defines two attributes: android:layout_margin and android:padding. The android:layout_margin attribute defines spacing for the container, while android:padding defines the spacing for the view. Let’s begin with padding.

  • android:padding—Defines the spacing of the content on all four sides of the control. To define padding for each side individually, use android:paddingLeft, android:paddingRight, android:paddingTop, and android:paddingBottom.
  • android:paddingTop—Defines the spacing between the content and the top of the control.
  • android:paddingBottom—Defines the spacing between the content and the bottom of the control.
  • android:paddingLeft—Defines the spacing between the content and the left side of the control.
  • android:paddingRight—Defines the spacing between the content and the right side of the control.

Here are the attributes that define the spacing between the control and the container:

  • android:layout_margin—Defines the spacing of the control in relation to the controls or the container on all four sides. To define spacing for each side individually, we use the android:layout_marginLeft, android:layout_marginRight, android:layout_marginTop, and android:layout_marginBottom options.
  • android:layout_marginTop—Defines the spacing between the top of the control and the related control or container.
  • android:layout_marginBottom—Defines the spacing between the bottom of the control and the related control or container.
  • android:layout_marginRight—Defines the spacing between the right side of the control and the related control or container.
  • android:layout_marginLeft—Defines the spacing between the left side of the control and the related control or container.

The layout file activity_relative_layout_app.xml arranges the controls as follows:

The Apple button control is set to appear at a distance of 15dip from the top and 20dip from the left side of the RelativeLayout container. The width of the Mango button control is set to consume the available horizontal space. The text Mango appears at a distance of 28dip from all sides of the control. The Mango control is set to appear to the right of the Apple control. The control is set to appear at a distance of 15dip from the control on the left and 10dip from the right side of the relative layout container. Also, the top of the Button control is set to align with the top of the container.

The Banana button control is assigned the width and height of 200dip and 50dip, respectively. The control is set to appear 15dip below the Apple control. The left side of the control is set to align with the left side of the container.

The Grapes button control is set to appear below the Banana button control, and its width is set to expand just enough to accommodate its content. The height of the control is set to take up all available vertical space. The text Grapes is automatically aligned vertically; that is, it appears at the center of the vertical height when the height attribute is set to match_parent. The minimum width of the control is set to 100dip. The right side of the control is set to align with the right side of the container.

The Kiwi Button control is set to appear below the Banana control. Its width is set to 100dip, and the height is set to just accommodate its content. The text Kiwi is set to appear at the distance of 15dip, 25dip, and 25dip from the top, left, and right boundary of the control.

We don’t need to make any changes to the RelativeLayoutAppActivity.java file. Its original content is as shown in Listing 3.7.

Listing 3.7. The Default Code in the Activity File RelativeLayoutAppActivity.java

package com.androidunleashed.relativelayoutapp;

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

public class RelativeLayoutDemoActivity extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_relative_layout_app);
    }
}

When the application is run, we see the output shown in Figure 3.8.

Figure 3.8

Figure 3.8. The five Button controls’ layout relative to each other

We can make the text Grapes appear centrally at the top row by adding the following line:

android:gravity="center_horizontal"

So, its tag appears as follows:

<Button
    android:id="@+id/Grapes"
    android:text="Grapes"
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:minWidth="100dp"
    android:layout_alignParentRight="true"
    android:layout_below="@id/Banana"
    android:gravity="center_horizontal" />

The output is modified to appear as shown in Figure 3.9.

Figure 3.9

Figure 3.9. The Grapes Button control aligned horizontally at the center

Let’s explore the concept of laying out controls in the RelativeLayout container by writing an application. The application that we are going to create is a simple Login Form application that asks the user to enter a User ID and Password. The TextView, EditText, and Button controls in the application are laid out in a RelativeLayout container (see Figure 3.10—left). If either the User ID or Password is left blank, the message The User ID or password is left blank. Please Try Again is displayed. If the correct User ID and Password, in this case, guest, are entered, then a welcome message is displayed. Otherwise, the message The User ID or password is incorrect. Please Try Again is displayed.

So, let’s create the application. Launch the Eclipse IDE and create a new Android application called LoginForm. Arrange four TextView controls, two EditText controls, and a Button control in RelativeLayout, as shown in the layout file activity_login_form.xml displayed in Listing 3.8.

Listing 3.8. The activity_login_form.xml on Laying Out the TextView, EditText, and Button Controls in the RelativeLayout Container

<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >
    <TextView
        android:id="@+id/sign_msg"
        android:text = "Sign In"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:typeface="serif"
        android:textSize="25dip"
        android:textStyle="bold"
        android:padding="10dip"
        android:layout_centerHorizontal="true"/>
    <TextView
        android:id="@+id/user_msg"
        android:text = "User ID:"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_margin="10dip"
        android:layout_below="@+id/sign_msg" />
    <EditText
        android:id="@+id/user_ID"
        android:layout_height="wrap_content"
        android:layout_width="250dip"
        android:layout_below="@+id/sign_msg"
        android:layout_toRightOf="@+id/user_msg"
        android:singleLine="true" />
    <TextView
        android:id="@+id/password_msg"
        android:text = "Password:"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/user_msg"
        android:layout_margin="10dip"
        android:paddingTop="10dip"/>
    <EditText
        android:id="@+id/password"
        android:layout_height="wrap_content"
        android:layout_width="250dp"
        android:singleLine="true"
        android:layout_below="@+id/user_ID"
        android:layout_toRightOf="@+id/password_msg"
        android:password="true" />
    <Button
        android:id="@+id/login_button"
        android:text="Sign In"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="10dip"
        android:layout_below="@+id/password_msg"/>
    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/response"
        android:layout_below="@+id/login_button"/>
</RelativeLayout>
Figure 3.10

Figure 3.10. (left) The Login Form displays an error if fields are left blank, (middle) the Password Incorrect message displays if the user ID or password is incorrect, and (right) the Welcome message displays when the correct user ID and password are entered.

The controls in the application are arranged in the RelativeLayout, as explained here:

  • Through the TextView control sign_msg, the text Sign In is displayed horizontally centered at the top. It is displayed in bold serif font, 25 dip in size. The text is padded with a space of 10dip on all four sides of its container.
  • Another TextView control, user_msg, displays the text User ID below the TextView sign_msg. The TextView is placed 10dip from all four sides.
  • An EditText control user_ID is displayed below sign_msg and to the right of user_msg. The width assigned to the TextView control is 250 dip and is set to single-line mode, so if the user types beyond the given width, the text scrolls to accommodate extra text but does not run over to the second line.
  • A TextView password_msg control displaying the text Password: is displayed below the TextView user_msg. The TextView control is placed at a spacing of 10dip from all four sides, and the text Password: is displayed at 10dip from the control’s top boundary.
  • An EditText control password is displayed below the EditText user_ID and to the right of the TextView password_msg. The width assigned to the TextView control is 250 dip and is set to single-line mode. In addition, the typed characters are converted into dots for security.
  • A Button control login_button with the caption Sign In is displayed below the TextView password_msg. The button is horizontally centered and is set to appear at 10dip distance from the EditText control password.
  • A TextView control response is placed below the Button login_button. It is used to display messages to the user when the Sign In button is pressed after entering User ID and Password.

To authenticate the user, we need to access the User ID and Password that is entered and match these values against the valid User ID and Password. In addition, we want to validate the EditText controls to confirm that none of them is blank. We also want to welcome the user if he or she is authorized. To do all this, we write the code in the activity file LoginFormActivity.java as shown in Listing 3.9.

Listing 3.9. Code Written in the Java Activity File LoginFormActivity.java

package com.androidunleashed.loginform;

import android.app.Activity;
import android.os.Bundle;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.view.View;
import android.widget.TextView;

public class LoginFormActivity extends Activity implements OnClickListener  {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_login_form);
        Button b = (Button)this.findViewById(R.id.login_button);
        b.setOnClickListener(this);
    }

    public void onClick(View v) {
        EditText userid = (EditText) findViewById(R.id.user_ID);
        EditText password = (EditText) findViewById(R.id.password);
        TextView resp = (TextView)this.findViewById(R.id.response);
        String usr = userid.getText().toString();
        String pswd = password.getText().toString();
        if(usr.trim().length() == 0 || pswd.trim().length() == 0){
            String str = "The User ID or password is left blank \nPlease Try Again";
            resp.setText(str);
        }
        else{
            if(usr.equals("guest") && pswd.equals("guest")) resp.setText("Welcome " +
            usr+ " ! ");
            else resp.setText("The User ID or password is incorrect \nPlease Try Again");
        }
    }
}

The Button control is accessed from the layout file and is mapped to the Button object b. This activity implements the OnClickListener interface. Hence, the class implements the callback method onClick(), which is invoked when a click event occurs on the Button control.

In the onClick() method, the user_ID and password EditText controls are accessed from the layout file and mapped to the EditText objects userid and password. Also, the TextView control response is accessed from the layout file and is mapped to the TextView object resp. The User ID and password entered by the user in the two EditText controls are accessed through the objects userid and password and assigned to the two Strings usr and pswd, respectively. The data in the usr and pswd strings is checked for authentication. If the user has left any of the EditText controls blank, the message The User ID or password is left blank. Please Try Again is displayed, as shown in Figure 3.10 (left). If the User ID and password are correct, then a welcome message is displayed (see Figure 3.10—right). Otherwise, the message The User ID or password is incorrect. Please Try Again is displayed, as shown in Figure 3.10 (middle).

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