A utility class and to ease SWT FormData handling (Dependencies: SWT & JUnit + DisplayHelper gist for test).

FormDatas.java

import org.eclipse.swt.SWT;
import org.eclipse.swt.layout.FormAttachment;
import org.eclipse.swt.layout.FormData;
import org.eclipse.swt.widgets.Control;

public class FormDatas {

  static final int DENOMINATOR = 100;

  private final FormData formData;

  public static FormDatas attach( Control control ) {
    return new FormDatas( control );
  }

  public FormDatas atLeft() {
    return atLeft( 0 );
  }

  public FormDatas atLeft( int margin ) {
    formData.left = new FormAttachment( 0, margin );
    return this;
  }

  public FormDatas atLeftTo( Control control ) {
    return atLeftTo( control, 0 );
  }

  public FormDatas atLeftTo( Control control, int margin ) {
    return atLeftTo( control, margin, SWT.DEFAULT );
  }

  public FormDatas atLeftTo( Control control, int margin, int alignment ) {
    formData.left = new FormAttachment( control, margin, alignment );
    return this;
  }

  public FormDatas fromLeft( int numerator ) {
    return fromLeft( numerator, 0 );
  }

  public FormDatas fromLeft( int numerator, int margin ) {
    formData.left = new FormAttachment( numerator, margin );
    return this;
  }

  public FormDatas atRight() {
    return atRight( 0 );
  }

  public FormDatas atRight( int margin ) {
    formData.right = new FormAttachment( DENOMINATOR, -margin );
    return this;
  }

  public FormDatas atRightTo( Control control ) {
    atRightTo( control, 0 );
    return this;
  }

  public FormDatas atRightTo( Control control, int margin ) {
    return atRightTo( control, margin, SWT.DEFAULT );
  }

  public FormDatas atRightTo( Control control, int margin, int alignment ) {
    formData.right = new FormAttachment( control, -margin, alignment );
    return this;
  }

  public FormDatas fromRight( int numerator ) {
    return fromRight( numerator, 0 );
  }

  public FormDatas fromRight( int numerator, int margin ) {
    formData.right = new FormAttachment( DENOMINATOR - numerator, -margin );
    return this;
  }

  public FormDatas atTop() {
    return atTop( 0 );
  }

  public FormDatas atTop( int margin ) {
    formData.top = new FormAttachment( 0, margin );
    return this;
  }

  public FormDatas atTopTo( Control control ) {
    return atTopTo( control, 0 );
  }

  public FormDatas atTopTo( Control control, int margin ) {
    return atTopTo( control, margin, SWT.DEFAULT );
  }

  public FormDatas atTopTo( Control control, int margin, int alignment ) {
    formData.top = new FormAttachment( control, margin, alignment );
    return this;
  }

  public FormDatas fromTop( int numerator ) {
    return fromTop( numerator, 0 );
  }

  public FormDatas fromTop( int numerator, int margin ) {
    formData.top = new FormAttachment( numerator, margin );
    return this;
  }

  public FormDatas atBottom() {
    return atBottom( 0 );
  }

  public FormDatas atBottom( int margin ) {
    formData.bottom = new FormAttachment( DENOMINATOR, -margin );
    return this;
  }

  public FormDatas atBottomTo( Control control ) {
    return atBottomTo( control, 0 );
  }

  public FormDatas atBottomTo( Control control, int margin ) {
    return atBottomTo( control, margin, SWT.DEFAULT );
  }

  public FormDatas atBottomTo( Control control, int margin, int alignment ) {
    formData.bottom = new FormAttachment( control, -margin, alignment );
    return this;
  }

  public FormDatas fromBottom( int numerator ) {
    return fromBottom( numerator, 0 );
  }

  public FormDatas fromBottom( int numerator, int margin ) {
    formData.bottom = new FormAttachment( DENOMINATOR - numerator, -margin );
    return this;
  }

  public FormDatas withWidth( int width ) {
    formData.width = width;
    return this;
  }

  public FormDatas withHeight( int height ) {
    formData.height = height;
    return this;
  }

  public FormData getFormData() {
    return formData;
  }

  private FormDatas( Control control ) {
    formData = new FormData();
    control.setLayoutData( formData );
  }
}

FormDatasTest.java

import static com.codeaffine.gitplus.ui.internal.util.FormDatas.DENOMINATOR;
import static com.codeaffine.gitplus.ui.internal.util.FormDatas.attach;
import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertNull;
import static org.junit.Assert.assertSame;

import org.eclipse.swt.SWT;
import org.eclipse.swt.layout.FormAttachment;
import org.eclipse.swt.widgets.Composite;
import org.eclipse.swt.widgets.Control;
import org.eclipse.swt.widgets.Label;
import org.junit.Before;
import org.junit.Rule;
import org.junit.Test;

import com.codeaffine.gitplus.ui.test.DisplayHelper;

public class FormDatasTest {

  private static final int NUMERATOR = 10;
  private static final int MARGIN = 20;
  private static final int WIDTH = 300;
  private static final int HEIGHT = 200;

  @Rule
  public final DisplayHelper displayHelper = new DisplayHelper();

  private Control otherControl;
  private Composite parent;
  private Control control;

  @Test
  public void testAttach() {
    FormDatas actual = attach( control );

    assertSame( control.getLayoutData(), actual.getFormData() );
  }

  @Test
  public void testAtLeft() {
    FormDatas actual = attach( control ).atLeft();

    verifyAttachmentValues( 0, 0, actual.getFormData().left );
  }

  @Test
  public void testAtLeftWithMargin() {
    FormDatas actual = attach( control ).atLeft( MARGIN );

    verifyAttachmentValues( 0, MARGIN, actual.getFormData().left );
  }

  @Test
  public void testAtRight() {
    FormDatas actual = attach( control ).atRight();

    verifyAttachmentValues( DENOMINATOR, 0, actual.getFormData().right );
  }

  @Test
  public void testAtRightWithMargin() {
    FormDatas actual = attach( control ).atRight( MARGIN );

    verifyAttachmentValues( DENOMINATOR, -MARGIN, actual.getFormData().right );
  }

  @Test
  public void testAtTop() {
    FormDatas actual = attach( control ).atTop();

    verifyAttachmentValues( 0, 0, actual.getFormData().top );
  }

  @Test
  public void testAtTopWithMargin() {
    FormDatas actual = attach( control ).atTop( MARGIN );

    verifyAttachmentValues( 0, MARGIN, actual.getFormData().top );
  }

  @Test
  public void testAtBottom() {
    FormDatas actual = attach( control ).atBottom();

    verifyAttachmentValues( DENOMINATOR, 0, actual.getFormData().bottom );
  }

  @Test
  public void testAtBottomWithMargin() {
    FormDatas actual = attach( control ).atBottom( MARGIN );

    verifyAttachmentValues( DENOMINATOR, -MARGIN, actual.getFormData().bottom );
  }

  @Test
  public void testAtLeftTo() {
    FormDatas actual = attach( control ).atLeftTo( otherControl );

    verifyAttachmentValues( otherControl, 0, SWT.DEFAULT, actual.getFormData().left );
  }

  @Test
  public void testAtLeftToWithMargin() {
    int margin = 10;
    FormDatas actual = attach( control ).atLeftTo( otherControl, margin );

    verifyAttachmentValues( otherControl, margin, SWT.DEFAULT, actual.getFormData().left );
  }

  @Test
  public void testAtLeftToWithAligment() {
    FormDatas actual = attach( control ).atLeftTo( otherControl, 0, SWT.CENTER );

    verifyAttachmentValues( otherControl, 0, SWT.CENTER, actual.getFormData().left );
  }

  @Test
  public void testAtRightTo() {
    FormDatas actual = attach( control ).atRightTo( otherControl );

    verifyAttachmentValues( otherControl, 0, SWT.DEFAULT, actual.getFormData().right );
  }

  @Test
  public void testAtRightToWithMargin() {
    int margin = 10;
    FormDatas actual = attach( control ).atRightTo( otherControl, margin );

    verifyAttachmentValues( otherControl, -margin, SWT.DEFAULT, actual.getFormData().right );
  }

  @Test
  public void testAtRightToWithAligment() {
    FormDatas actual = attach( control ).atRightTo( otherControl, 0, SWT.CENTER );

    verifyAttachmentValues( otherControl, 0, SWT.CENTER, actual.getFormData().right );
  }

  @Test
  public void testAtTopTo() {
    FormDatas actual = attach( control ).atTopTo( otherControl );

    verifyAttachmentValues( otherControl, 0, SWT.DEFAULT, actual.getFormData().top );
  }

  @Test
  public void testAtTopToWithMargin() {
    int margin = 10;
    FormDatas actual = attach( control ).atTopTo( otherControl, margin );

    verifyAttachmentValues( otherControl, margin, SWT.DEFAULT, actual.getFormData().top );
  }

  @Test
  public void testAtTopToWithAligment() {
    FormDatas actual = attach( control ).atTopTo( otherControl, 0, SWT.CENTER );

    verifyAttachmentValues( otherControl, 0, SWT.CENTER, actual.getFormData().top );
  }

  @Test
  public void testAtBottomTo() {
    FormDatas actual = attach( control ).atBottomTo( otherControl );

    verifyAttachmentValues( otherControl, 0, SWT.DEFAULT, actual.getFormData().bottom );
  }

  @Test
  public void testAtBottomToWithMargin() {
    int margin = 10;
    FormDatas actual = attach( control ).atBottomTo( otherControl, margin );

    verifyAttachmentValues( otherControl, -margin, SWT.DEFAULT, actual.getFormData().bottom );
  }

  @Test
  public void testAtBottomToWithAligment() {
    FormDatas actual = attach( control ).atBottomTo( otherControl, 0, SWT.CENTER );

    verifyAttachmentValues( otherControl, 0, SWT.CENTER, actual.getFormData().bottom );
  }

  @Test
  public void testFromLeft() {
    int numerator = 10;
    FormDatas actual = attach( control ).fromLeft( numerator );

    verifyAttachmentValues( numerator, 0, actual.getFormData().left );
  }

  @Test
  public void testFromLeftWithMargin() {
    FormDatas actual = attach( control ).fromLeft( NUMERATOR, MARGIN );

    verifyAttachmentValues( NUMERATOR, MARGIN, actual.getFormData().left );
  }

  @Test
  public void testFromRight() {
    FormDatas actual = attach( control ).fromRight( NUMERATOR );

    verifyAttachmentValues( DENOMINATOR - NUMERATOR, 0, actual.getFormData().right );
  }

  @Test
  public void testFromRightWithMargin() {
    FormDatas actual = attach( control ).fromRight( NUMERATOR, MARGIN );

    verifyAttachmentValues( DENOMINATOR - NUMERATOR, -MARGIN, actual.getFormData().right );
  }

  @Test
  public void testFromTop() {
    FormDatas actual = attach( control ).fromTop( NUMERATOR );

    verifyAttachmentValues( NUMERATOR, 0, actual.getFormData().top );
  }

  @Test
  public void testFromTopWithMargin() {
    FormDatas actual = attach( control ).fromTop( NUMERATOR, MARGIN );

    verifyAttachmentValues( NUMERATOR, MARGIN, actual.getFormData().top );
  }

  @Test
  public void testFromBottom() {
    FormDatas actual = attach( control ).fromBottom( NUMERATOR );

    verifyAttachmentValues( DENOMINATOR - NUMERATOR, 0, actual.getFormData().bottom );
  }

  @Test
  public void testFromBottomWithMargin() {
    FormDatas actual = attach( control ).fromBottom( NUMERATOR, MARGIN );

    verifyAttachmentValues( DENOMINATOR - NUMERATOR, -MARGIN, actual.getFormData().bottom );
  }

  @Test
  public void testWithWidth() {
    FormDatas actual = attach( control ).withWidth( WIDTH );

    assertEquals( WIDTH, actual.getFormData().width );
  }

  @Test
  public void testWithHeight() {
    FormDatas actual = attach( control ).withHeight( HEIGHT );

    assertEquals( HEIGHT, actual.getFormData().height );
  }

  @Before
  public void setUp() {
    parent = displayHelper.createShell();
    control = new Label( parent, SWT.NONE );
    otherControl = new Label( parent, SWT.NONE );
  }

  private static void verifyAttachmentValues(
    Control control, int margin, int alignment, FormAttachment actual )
  {
    assertSame( control, actual.control );
    assertEquals( margin, actual.offset );
    assertEquals( alignment, actual.alignment );
    assertEquals( 0, actual.numerator );
  }

  private static void verifyAttachmentValues( int numerator, int margin, FormAttachment actual ) {
    assertEquals( numerator, actual.numerator );
    assertEquals( margin, actual.offset );
    assertEquals( 0, actual.alignment );
    assertNull( actual.control );
  }
}