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

Safe: 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 edge 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 it's preferred size. If a child is given no attachments 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:

2.0

See Also:

FormData, FormAttachment

Field Summary

int	marginHeight Enabled: marginHeight specifies the number of pixels of vertical margin that will be placed along the top and bottom edges of the layout.
int	marginWidth Enabled: marginWidth specifies the number of pixels of horizontal margin that will be placed along the left and right edges of the layout.

Constructor Summary

FormLayout()
Enabled: Constructs a new instance of this class.

Method Summary

(package private) int	computeHeight(FormData data) Computes the preferred height of the form with respect to the preferred height of the control.
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.
(package private) int	computeWidth(FormData data) Computes the preferred height of the form with respect to the preferred height of the control.
(package private) Point	getSize(Control control, boolean flushCache)
protected void	layout(Composite composite, boolean flushCache) Lays out the children of the specified composite according to this layout.
(package private) Point	layout(Composite composite, boolean move, int x, int y, int width, int height, boolean flushCache)

Methods inherited from class java.lang.Object

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

Field Detail

marginWidth

public int marginWidth

Enabled: 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

Enabled: 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.

Constructor Detail

FormLayout

public FormLayout()

Enabled: Constructs a new instance of this class.

Method Detail

computeHeight

int computeHeight(FormData data)

Computes the preferred height of the form with respect to the preferred height of the control. Given that the equations for top (T) and bottom (B) of the control in terms of the height of the form (X) are: T = AX + B B = CX + D The equation for the height of the control (H) is bottom (B) minus top (T) or (H = B - T) or: H = (CX + D) - (AX + B) Solving for (X), the height of the form, we get: X = (H + B - D) / (C - A) When (A = C), (C - A = 0) and the equation has no solution for X. This is a special case meaning that the control does not constrain the height of the form. In this case, we need to arbitrarily define the height of the form (X): Case 1: A = C, A = 0, C = 0 Let X = D, the distance from the top of the form to the bottom edge of the control. In this case, the control was attatched to the top of the form and the form needs to be large enough to show the bottom edge of the control. Case 2: A = C, A = 1, C = 1 Let X = -B, the distance from the bottom of the form to the top edge of the control. In this case, the control was attached to the bottom of the form and the only way that the control would be visible is if the offset is negative. If the offset is positive, there is no possible height for the form that will show the control as it will always be below the bottom edge of the form. Case 3: A = C, A != 0, C != 0 and A != 1, C != 0 Let X = D / (1 - C), the distance from the top of the form to the bottom edge of the control. In this case, since C is not 0 or 1, it must be a fraction, U / V. The offset D is the distance from CX to the bottom edge of the control. This represents a fraction of the form (1 - C)X. Since the height of a fraction of the form is known, the height of the entire form can be found by setting (1 - C)X = D. We solve this equation for X in terms of U and V, giving us X = (U * D) / (U - V). Similarily, if the offset D is negative, the control is positioned above CX. The offset -B is the distance from the top edge of the control to CX. We can find the height of the entire form by setting CX = -B. Solving in terms of U and V gives us X = (-B * V) / U.

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 minimum size that the client area of the composite must be in order to position all children at their minimum 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 minimum 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 minimum)

hHint - height (SWT.DEFAULT for minimum)

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(), "computeTrim, getClientArea for controls that implement them"

computeWidth

int computeWidth(FormData data)

Computes the preferred height of the form with respect to the preferred height of the control.

getSize

Point getSize(Control control,
              boolean flushCache)

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 layed 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

layout

Point layout(Composite composite,
             boolean move,
             int x,
             int y,
             int width,
             int height,
             boolean flushCache)