ScrollCanvas is a Canvas subclass for REALbasic designed to make it easier
to build canvases that support scrolling. It has been developed for REALbasic
4.5, although it may also work with earlier versions.
How to use ScrollCanvas
Setting up the controls
First, add the ScrollCanvas and SCScrollBar classes to your project. Then,
place a canvas and one or two scroll bars on your window and position them
appropriately. Set the class of the canvas to ScrollCanvas and the class of
each of the scroll bars to SCScrollBar. (You must use SCScrollBars and not
ordinary ScrollBars, otherwise they won't work to control the ScrollCanvas.)
Now select the ScrollCanvas and, in the Properties window, set the HScrollBarName
and/or VScrollBarName properties to the names of your SCScrollBars.
Setting the extent
The visible area of the ScrollCanvas scrolls around within a larger conceptual
rectangle called the extent. Initially, the extent is no bigger than the canvas,
so there's nowhere to scroll to and the scroll bars will be greyed
out. To make the extent bigger, use the SetExtent method of the ScrollCanvas
to set the width and height of the extent.
Coding the Paint event
The ScrollCanvas class has two properties xOrigin and yOrigin which contain,
at any given moment, the coordinates of the top left corner of the extent,
in the coordinate system of the canvas. For scrolling to work properly, everything
you draw in the canvas must have these values added to its x and y coordinates.
For example, to draw a rectangle with top left corner at x,y and size w,h,
you would do
g.DrawRect xOrigin+x, yOrigin+y, w, h
Properties of ScrollCanvas
extentWidth as integer, extentHeight
The current size of the extent. These should be treated as read-only;
use the SetExtent method to change the extent.
xOrigin as integer, yOrigin
Current coordinates of the top left corner of the extent rectangle
in the canvas's coordinate system. Must be added to the x and y coordinates
of any drawing done in the canvas. These should be treated as read-only;
use the ScrollTo and ScrollBy methods to scroll the canvas programmatically.
Methods of ScrollCanvas
SetExtent(width as integer, height
Changes the size of the extent rectangle. If the current
scrolling position is outside the range of the new extent, the canvas will
be scrolled to bring it into range.
ScrollTo(xOffset as integer, yOffset
Scrolls the canvas so as to bring the top left corner
of the canvas to a position xOffset from the left edge and yOffset from the
top edge of the extent rectangle. (This results in xOrigin = -xOffset and
yOrigin = -yOffset.) If the new scrolling position is outside the range allowed
by the extent, xOffset and yOffset are first adjusted to bring it within range.
ScrollBy(dx as integer, dy
Scrolls the canvas by the amount dx,dy relative to the
current scrolling position, limited by the extent. Positive values move the
viewed area of the extent to the right and down.
ScrollCanvas is freeware; you're welcome to use it in any way you want.
All I ask is that you give me credit if you release any modified versions
Contacting the Author
Bug reports, comments and suggestions are welcome: