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High Resolution Timing in Visual Basic
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So, you're interested in having your functions act at specific
time intervals, eh? Naturally you would turn to the Timer Control
included with VB. Unfortunately, the timer control has a resolution
of ~55ms. 55ms seems like a very short time to us, but to the
computer, 55ms is an agonizingly long time. Don't torture your
computer! Use accurate and high-resolution timing methods!
Private
Declare Function GetTickCount Lib "kernel32" ()
As Long
I love this function. When I first stumbled
upon it, I was amazed. Why didn't Microsoft
use this simple API call when they made
their VB Timer Control? I guess it's
one for the
X-Files, Mulder...
Anyways, what this function does is return
the length of time that the system has been
running in milliseconds. Since computers
are frequently left on for days at a time,
be sure that you use the Long data type as
opposed to Integer when dealing with this
control, or you will run into problems.
Determining when exactly an interval of
time has passed is now a simple matter. All
we have to do is store the current TickCount
in a variable, and wait until the desired
time length has elapsed:
TempTime = GetTickCount()
Do
While DesiredTime < GetTickCount()
- TempTime
'Do some things
Loop
Here we have a temporary variable called
TempTime in which we store the tick count.
We then loop through the statements we
want to execute until the difference
between the current time and the
temporarily stored
time is greater than the value of DesiredTime.
We should therefore set DesiredTime to
the number of milliseconds for which
we would like to execute the loop.
The above example shows how we can execute
some code for a specific length of time.
What follows is an example showing how to
execute some code every time an interval
elapses:
ExitFunction = False
TempTime = GetTickCount()
Do While not(ExitFunction)
If DesiredTime < GetTickCount() - TempTime then
'Reset the temporary variable
TempTime = GetTickCount()
'Do some things
End If
Loop
This function will execute the code each
time the interval specified by DesiredTime
elapses. (In order to exit the loop, include some code to simply set the
ExitFunction Boolean to True.) In
this fashion we can, for example,
ensure that our graphics
drawing routines do not exceed a specified frame rate, or we can nullify
the speed differences between systems
in order to make your program run
consistently
on all platforms.
Finally, the GetTickCount() function can
also be used to benchmark your code. By this
I mean you can use it to determine the time
it takes to execute a specific chunk of code,
thereby enabling you to tweak it for optimal
speed (or slow it down, if that's what you're
after). I have written a stopwatch like program
to demonstrate how you would start, stop,
and accumulate time through the use of the
glorious GetTickCount API call. Click here
to download now.
About
this Tutorial
This tutorial is from The
Game Progamming Wiki which is published under the
GNU Free Documentation License 1.2.
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