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SWT Layout

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by , 11-10-2011 at 06:36 PM (2178 Views)
A layout controls the position and size of the children of a composite widget. This class is the abstract base class for layouts.

Layout class constructor


Following are details of all the methods of Layout class.

1. protected abstract 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.

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.

2. protected boolean flushCache(Control control)

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

3. protected abstract void layout(Composite composite, boolean flushCache)

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.


The example below uses GridLayout. It creates two rows and adds ten buttons to row 1 and one button in row 2.

Java Code:
import org.eclipse.swt.*;
import org.eclipse.swt.layout.*;
import org.eclipse.swt.widgets.*;

public class SWTLayout {
	public static void main(String[] args) {
		Display display = new Display();
		final Shell shell = new Shell(display);
		shell.setLayout(new GridLayout());
		final Composite c = new Composite(shell, SWT.NONE);
		GridLayout layout = new GridLayout();
		layout.numColumns = 3;
		for (int i = 0; i < 10; i++) {
			Button b = new Button(c, SWT.PUSH);
			b.setText("Button " + i);

		Button b = new Button(shell, SWT.PUSH);
		b.setText("add a new button at row 2 column 1");
		final int[] index = new int[1];
		b.addListener(SWT.Selection, new Listener() {
			public void handleEvent(Event e) {
				Button s = new Button(c, SWT.PUSH);
				s.setText("Special " + index[0]);
				Control[] children = c.getChildren();
				shell.layout(new Control[] { s });
		while (!shell.isDisposed()) {
			if (!display.readAndDispatch())

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