Chapter 11. Canvases
The Canvas control class is REALbasicâs way of letting the programmer display a custom image within a window. Canvases are probably the most versatile and powerful of REALbasicâs built-in control classes, because in effect they let you build your own piece of the interface. A Canvas subclass could be used to simulate a PushButton, a StaticText, a ListBox, and many other existing classes and often is used as the basis for constructing variants on such classes.
From the programmerâs point of view, the chief sources of a Canvasâs great power and versatility are as follows:
A Canvas permits your code to take direct control over drawing within its region.
A Canvas receives both mouse movement and mouse button events.
A Canvas can be scrolled, and can scroll other controls along with it.
A Canvas can overlay another control, invisibly if desired, in such a way as to let its powers and those of the other control supplement one another.
A Canvas and a windowâs background have much in common. A Canvas does not have a BackColor or a HasBackColor property, so it lacks a windowâs simplest level of just displaying a solid color as its rearmost layer. But it has both a Backdrop and a Graphics property, and you use these to draw into a Canvas and into a windowâs background in identical fashion. Also, a Canvas and a window receive some similar events. The treatment of window backgrounds at the end of Chapter 9 was rather sketchy; the expectation was that you ...
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