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Scroll Object

Parents, Children, Properties, Methods, Events

Purpose: Provides a vertical or horizontal scrollbar.

Description

The Scroll object provides a vertical or horizontal scrollbar that can be used as a "free-standing" object or can be "attached" to the side of its parent.

An "attached" scrollbar is one that extends along one edge of a Form, SubForm or Group and has a standard width or height. When the Form or Group is resized, a vertical attached scrollbar is resized vertically but remains the same width and stays fixed to the side of its parent. Similarly, a horizontal attached scrollbar is resized horizontally but remains the same height.

For most purposes, the use of the Scroll object to provide attached scrollbars in a Form has been superseded by the provision of scrollbars as a property of a Form.

A "free-standing" scrollbar is typically used as a "scale" for selecting a numeric value from a range and may appear and behave rather differently from a standard attached scrollbar. Firstly, a free-standing scrollbar will normally be positioned at an arbitrary position within its parent Form or Group and be associated with other objects such as Labels and Edit fields. Secondly, when its parent Form or Group is resized, it is probably desirable that the scrollbar reacts in the same way as the other child objects, so that the overall appearance of the layout is maintained.

The Align property determines whether or not a scrollbar is attached, and if so, to which side of the parent Group or Form it is fixed. The direction of the scrollbar is determined by the VScroll and HScroll properties, which are mutually exclusive. The position and size of the scrollbar are determined by Posn and Size.

To obtain an "attached" scrollbar, it is sufficient for most purposes to specify only the Align property. If so, the direction of the scrollbar and its position and size (which are otherwise defined by VScroll, HScroll, Posn and Size) are determined automatically for you.

To obtain a "free-standing" scrollbar, it is recommended for most purposes that you set Align to 'None' and define the orientation, position and size of the scrollbar explicitly using VScroll or HScroll, Posn and Size.

VScroll and HScroll may only be set when the object is created and may not subsequently be changed.

If you do attach a "free-standing" scrollbar to a particular side of its parent using Align, it will maintain its physical position (in pixels) relative to the side to which it is attached, and its dimension in that direction will remain fixed.

The Align property is a character vector containing 'Top', 'Bottom', 'Right', 'Left' or 'None'. If you specify Align 'Right' you get a vertical scrollbar attached to the right-hand edge of the parent Form or Group. Align 'Left' also produces a vertical scrollbar, but one that is attached to the left-hand edge. Align 'Top' and 'Bottom' each produce horizontal scrollbars, attached to the top and bottom edges of the Form or Group respectively.

Note that the default value of Align is 'Right' unless HScroll is set to ¯1 in which case it is 'Bottom'. It must therefore be explicitly set to 'None' if you want a non-attached "free-standing" scrollbar.

VScroll and HScroll are used to specify the orientation of the scrollbar explicitly, usually in conjunction with Align set to 'None'. VScroll or HScroll may be specified when the object is created by ⎕WC, but cannot be changed using ⎕WS. The two properties are mutually exclusive. Each of them may be set to 0 or ¯1, where ¯1 means "true" and 0 means "false". Thus (VScroll¯1) defines a vertical scrollbar, while (HScroll¯1) specifies a horizontal one. Setting either property to ¯1 automatically causes the other to be set to 0. If you try to set both to ¯1, VScroll takes precedence and HScroll is reset to 0.

[Note: the reason for using two properties where one would be sufficient is to allow for the possible future implementation of scrolling groups as provided by ⎕SM/⎕SR.]

Scrolling is controlled by the Thumb, Range and Step properties.

Thumb sets and reports the current position of the "thumb" as an integer in the range 1 to the value of the Range property.

Step determines the size of changes reported when the user clicks a scroll arrow (small change) or clicks on the body of the scrollbar (large change). Step is a 2-element numeric vector whose first element specifies the value of the "small change" and whose second element specifies the value of the "large change". The PageSize property specifies the sizes of the thumb in the scrollbar.

Examples of Attached Scrollbars

      'F' ⎕WC 'Form' 'Default Scroll Bar'
      'F.SCR' ⎕WC 'Scroll'

      'F' ⎕WC 'Form' 'Default Horizontal Scroll Bar'
      'F.SCR' ⎕WC 'Scroll' ('HScroll' ¯1)

Examples of Free-Standing Scrollbars

      'F' ⎕WC 'Form' 'Non-Default Scroll Bar'
      'F.SCR' ⎕WC 'Scroll' (5 45)(90 10)
                  ('Align' 'None')('VScroll' ¯1)

      'F' ⎕WC 'Form' 'Horizontal Scroll Bars'
      'F.SC1' ⎕WC 'Scroll' (15 15)(15 70)
                  ('Align' 'None')('HScroll' ¯1)
      'F.SC2' ⎕WC 'Scroll' (40 40)(20 20)
                  ('Align' 'None')('HScroll' ¯1)
      'F.SC3' ⎕WC 'Scroll' (85 5)(10 90)
                  ('Align' 'None')('HScroll' ¯1)