Learn to Program with Small Basic

You start by loading the two background images and saving their identifiers
in the offImg and onImg variables (line 2–3). The bkgndImg variable
holds the current background image, which changes when a user clicks
the switch. When the program starts, the switch is open, so the program
sets bkgndImg = offImg (line 4). Lines 6–7 adjust the width and height of the
graphics window to match the size of the background image, and line 8
draws the background image (offImg in this case) in the graphics window.
Lines 10–17 create the four text boxes (for the three resistors and the
battery voltage) and resize them so they’re exactly on top of their positions
in the background image. In lines 18–21, you set default values for these
text boxes. In line 23, you register a handler for the MouseDown event, because
you’ll want to know when a user clicks the switch. Line 24 registers a handler
for the TextTyped event, because you’ll calculate and display the values
of I, V1, V2, and V3 automatically when the user enters a new value in any of
the four text boxes.
Step 3: Toggle the Switch
When a user clicks the switch, you need to change the background image to
toggle the switch. Add the OnMouseDown() subroutine in Listing 12-4.
1 Sub OnMouseDown ' Switches the background image
2 x = GraphicsWindow.MouseX
3 y = GraphicsWindow.MouseY
4 If ((x > 185) And (x < 245) And (y > 300) And (y < 340)) Then
5 If (bkgndImg = offImg) Then
6 bkgndImg = onImg
7 Else
8 bkgndImg = offImg
9 EndIf
10 UpdateUserInterface()
11 EndIf
12 EndSub
Listing 12-4: Changing the background image
The subroutine starts by getting the x- and y-coordinates of the point
where the mouse was clicked and setting them to the variables x and y
(lines 2–3). Line 4 then checks if this point lies within the rectangular
region of the switch; if the mouse was inside the boundaries of the switch,
the subroutine toggles the current value of the bkgndImg variable (from on
to off or from off to on) at lines 5–9 and then calls the UpdateUserInterface()
subroutine to switch the background image and update the calculated
values (line 10). As you’ll see in a moment, if the user opens the switch,
the program shows only the offImg background image; the values of I, V1,
V2, and V3 won’t show because no current’s flowing through the circuit when
the switch is open.
174 Chapter 12