Class 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:
    2.0
    See Also:
    FormData, FormAttachment, FormLayout snippets, SWT Example: LayoutExample, Sample code and further information
    • Field Summary

      Fields 
      Modifier and Type Field Description
      int marginBottom
      marginBottom specifies the number of points of vertical margin that will be placed along the bottom edge of the layout.
      int marginHeight
      marginHeight specifies the number of points of vertical margin that will be placed along the top and bottom edges of the layout.
      int marginLeft
      marginLeft specifies the number of points of horizontal margin that will be placed along the left edge of the layout.
      int marginRight
      marginRight specifies the number of points of horizontal margin that will be placed along the right edge of the layout.
      int marginTop
      marginTop specifies the number of points of vertical margin that will be placed along the top edge of the layout.
      int marginWidth
      marginWidth specifies the number of points of horizontal margin that will be placed along the left and right edges of the layout.
      int spacing
      spacing specifies the number of points between the edge of one control and the edge of its neighbouring control.
    • Constructor Summary

      Constructors 
      Constructor Description
      FormLayout()
      Constructs a new instance of this class.
    • Field Detail

      • marginWidth

        public int marginWidth
        marginWidth specifies the number of points 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 points 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 points of horizontal margin that will be placed along the left edge of the layout. The default value is 0.
        Since:
        3.1
      • marginTop

        public int marginTop
        marginTop specifies the number of points of vertical margin that will be placed along the top edge of the layout. The default value is 0.
        Since:
        3.1
      • marginRight

        public int marginRight
        marginRight specifies the number of points of horizontal margin that will be placed along the right edge of the layout. The default value is 0.
        Since:
        3.1
      • marginBottom

        public int marginBottom
        marginBottom specifies the number of points of vertical margin that will be placed along the bottom edge of the layout. The default value is 0.
        Since:
        3.1
      • spacing

        public int spacing
        spacing specifies the number of points between the edge of one control and the edge of its neighbouring control. The default value is 0.
        Since:
        3.0
    • 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 String toString()
        Returns a string containing a concise, human-readable description of the receiver.
        Overrides:
        toString in class Object
        Returns:
        a string representation of the layout