org.eclipse.swt.layout
Class FormLayout

java.lang.Object
  org.eclipse.swt.widgets.Layout
      org.eclipse.swt.layout.FormLayout

public final class FormLayout
extends Layout

Instances of this class control the position and size of the children of a composite control by using FormAttachments to optionally configure the left, top, right and bottom edges of each child.

The following example code creates a FormLayout and then sets it into a Shell:

    Display display = new Display ();
    Shell shell = new Shell(display);
    FormLayout layout = new FormLayout();
    layout.marginWidth = 3;
    layout.marginHeight = 3;
    shell.setLayout(layout);
 

To use a FormLayout, create a FormData with FormAttachment for each child of Composite. The following example code attaches button1 to the top and left edge of the composite and button2 to the right edge of button1 and the top and right edges of the composite:

    FormData data1 = new FormData();
    data1.left = new FormAttachment(0, 0);
    data1.top = new FormAttachment(0, 0);
    button1.setLayoutData(data1);
    FormData data2 = new FormData();
    data2.left = new FormAttachment(button1);
    data2.top = new FormAttachment(0, 0);
    data2.right = new FormAttachment(100, 0);
    button2.setLayoutData(data2);
 

Each side of a child control can be attached to a position in the parent composite, or to other controls within the Composite by creating instances of FormAttachment and setting them into the top, bottom, left, and right fields of the child's FormData.

If a side is not given an attachment, it is defined as not being attached to anything, causing the child to remain at its preferred size. If a child is given no attachment on either the left or the right or top or bottom, it is automatically attached to the left and top of the composite respectively. The following code positions button1 and button2 but relies on default attachments:

    FormData data2 = new FormData();
    data2.left = new FormAttachment(button1);
    data2.right = new FormAttachment(100, 0);
    button2.setLayoutData(data2);
 

IMPORTANT: Do not define circular attachments. For example, do not attach the right edge of button1 to the left edge of button2 and then attach the left edge of button2 to the right edge of button1. This will over constrain the layout, causing undefined behavior. The algorithm will terminate, but the results are undefined.

Since:

1.0

See Also:

FormData, FormAttachment, FormLayout snippets, SWT Example: LayoutExample, Sample code and further information

Field Summary

int	marginBottom marginBottom specifies the number of pixels of vertical margin that will be placed along the bottom edge of the layout.
int	marginHeight marginHeight specifies the number of pixels of vertical margin that will be placed along the top and bottom edges of the layout.
int	marginLeft marginLeft specifies the number of pixels of horizontal margin that will be placed along the left edge of the layout.
int	marginRight marginRight specifies the number of pixels of horizontal margin that will be placed along the right edge of the layout.
int	marginTop marginTop specifies the number of pixels of vertical margin that will be placed along the top edge of the layout.
int	marginWidth marginWidth specifies the number of pixels of horizontal margin that will be placed along the left and right edges of the layout.
int	spacing spacing specifies the number of pixels between the edge of one control and the edge of its neighbouring control.

Constructor Summary

FormLayout()
Constructs a new instance of this class.

Method Summary

protected Point	computeSize(Composite composite, int wHint, int hHint, boolean flushCache) Computes and returns the size of the specified composite's client area according to this layout.
protected boolean	flushCache(Control control) Instruct the layout to flush any cached values associated with the control specified in the argument control.
protected void	layout(Composite composite, boolean flushCache) Lays out the children of the specified composite according to this layout.
java.lang.String	toString() Returns a string containing a concise, human-readable description of the receiver.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Field Detail

marginWidth

public int marginWidth

marginWidth specifies the number of pixels of horizontal margin that will be placed along the left and right edges of the layout. The default value is 0.

marginHeight

public int marginHeight

marginHeight specifies the number of pixels of vertical margin that will be placed along the top and bottom edges of the layout. The default value is 0.

marginLeft

public int marginLeft

marginLeft specifies the number of pixels of horizontal margin that will be placed along the left edge of the layout. The default value is 0.

marginTop

public int marginTop

marginTop specifies the number of pixels of vertical margin that will be placed along the top edge of the layout. The default value is 0.

marginRight

public int marginRight

marginRight specifies the number of pixels of horizontal margin that will be placed along the right edge of the layout. The default value is 0.

marginBottom

public int marginBottom

marginBottom specifies the number of pixels of vertical margin that will be placed along the bottom edge of the layout. The default value is 0.

spacing

public int spacing

spacing specifies the number of pixels between the edge of one control and the edge of its neighbouring control. The default value is 0.

Constructor Detail

FormLayout

public FormLayout()

Constructs a new instance of this class.

Method Detail

computeSize

protected Point computeSize(Composite composite,
                            int wHint,
                            int hHint,
                            boolean flushCache)

Description copied from class: Layout

Computes and returns the size of the specified composite's client area according to this layout.

This method computes the size that the client area of the composite must be in order to position all children at their preferred size inside the composite according to the layout algorithm encoded by this layout.

When a width or height hint is supplied, it is used to constrain the result. For example, if a width hint is provided that is less than the width of the client area, the layout may choose to wrap and increase height, clip, overlap, or otherwise constrain the children.

Specified by:

computeSize in class Layout

Parameters:

composite - a composite widget using this layout

wHint - width (SWT.DEFAULT for preferred size)

hHint - height (SWT.DEFAULT for preferred size)

flushCache - true means flush cached layout values

Returns:

a point containing the computed size (width, height)

See Also:

Layout.layout(org.eclipse.swt.widgets.Composite, boolean), Control.getBorderWidth(), Control.getBounds(), Control.getSize(), Control.pack(boolean), "computeTrim, getClientArea for controls that implement them"

flushCache

protected boolean flushCache(Control control)

Description copied from class: Layout

Instruct the layout to flush any cached values associated with the control specified in the argument control.

Overrides:

flushCache in class Layout

Parameters:

control - a control managed by this layout

Returns:

true if the Layout has flushed all cached information associated with control

layout

protected void layout(Composite composite,
                      boolean flushCache)

Description copied from class: Layout

Lays out the children of the specified composite according to this layout.

This method positions and sizes the children of a composite using the layout algorithm encoded by this layout. Children of the composite are positioned in the client area of the composite. The position of the composite is not altered by this method.

When the flush cache hint is true, the layout is instructed to flush any cached values associated with the children. Typically, a layout will cache the preferred sizes of the children to avoid the expense of computing these values each time the widget is laid out.

When layout is triggered explicitly by the programmer the flush cache hint is true. When layout is triggered by a resize, either caused by the programmer or by the user, the hint is false.

Specified by:

layout in class Layout

Parameters:

composite - a composite widget using this layout

flushCache - true means flush cached layout values

toString

public java.lang.String toString()

Returns a string containing a concise, human-readable description of the receiver.

Overrides:

toString in class java.lang.Object

Returns:

a string representation of the layout