How does this program appear to loop?

Question

I am just learning Small Basic.  I've programmed in other languages as a hobby for years, so I think I understand how programs work...more or less.  I have always understood that in order for commands to be executed over and over, the program has to loop in some way.  But the following program appears to loop...the two command lines continually execute awaiting key input, but there is no "loop" in the program.  How does Small Basic do that?  Thanks.

GraphicsWindow.Height = 300
GraphicsWindow.Width = 300
GraphicsWindow.Title = "Graphics Window"
shape1 = Shapes.AddRectangle(100,50)
Shapes.Move (shape1, 100, 125)
return = "Return"
GraphicsWindow.KeyDown = keydown
GraphicsWindow.KeyUp = keyup


Sub keyup
  If GraphicsWindow.Lastkey = "Return" Then
    Shapes.Rotate(shape1,0)
  EndIf
EndSub

Sub keydown
  If GraphicsWindow.LastKey = "Return" Then
    Shapes.Rotate (shape1, 90)
  EndIf
EndSub
 

Answer 1

When you have e.g GraphicsWindow.KeyDown in your program (and the corresponding Sub), SmallBasic assumes you want to  do something on keydown, so the program does't end but keep waiting for a key. When a key is pressed, the Sub is executed, and the program waits again. If you want the program to stop on e.g. a Q , you need in the Sub:

If GraphicsWindow.LastKey="Q" Then

  Program.End()

EndIf

---

Jan [ WhTurner ] The Netherlands

Answer 2

Whenever you register an event handler, like
GraphicsWindow.KeyDown = KeyDown

it has to loop continuously at a special resolution (about 20-30 ms) in the background to detect,  if that event has been raised. This is managed by .NET automatically and with no "loop" in the program.

Not just for KeyDown, but also for all the other events like MouseDown, ButtonClicked etc.

You can see it by running following modification (using the 'AnyKey' key :-) ).
Press down a key and hold it pressed as long as you want. After releasing it you'll see that in the meantime there has been a lot of ...x "loops" in the background (= attempts/queries to detect a raised event)

GraphicsWindow.Height = 300
GraphicsWindow.Width = 300
GraphicsWindow.Title = "Graphics Window"
shape1 = Shapes.AddRectangle(100,50)
Shapes.Move(shape1, 100,125)

GraphicsWindow.KeyDown = KeyDown
GraphicsWindow.KeyUp = KeyUp

Sub KeyDown
  'If GraphicsWindow.LastKey = "Return" Then
  Shapes.Rotate(shape1, 90)
  
  If n < 1 Then
    last = Clock.ElapsedMilliseconds
    GraphicsWindow.Title = "Release key whenever u want"
  EndIf
  n = n + 1
  'EndIf
EndSub

Sub KeyUp
  'If GraphicsWindow.Lastkey = "Return" Then
  Shapes.Rotate(shape1, 0)
  
  dur = Clock.ElapsedMilliseconds - last
  t = Math.Round(100*dur/n) / 100 ' ~ 30 ms/loop (300 ticks a 100 µs) ->  ~ 33.33 Hz
  f = Math.Round(100 * 1000*n/dur) / 100    ' f = 1000/t = 1000 *n/dur
  GraphicsWindow.Title = n +"x in "+ dur +" ms , "+ t +" ms/loop = "+ f +" Hz"
  n = 0
  'EndIf
EndSub