Timer Events in MFC Applications
Event timers are always handy to have around and useful in nearly every project. When a timer is readily available, such as the Timer control in Visual Basic, you find all kinds of reasons to use one in an application. The lack of a timer object in MFC is, in my opinion, a serious oversight.
Luckily, Microsoft Windows NT/95 systems provide a number of timers for use by application programs. This tutorial provides information on how to install timers in your MFC application, and how to start, process and stop the timers.
In the final section of the tutorial, we develope a simple timer project which is capable of rough animation of an icon in the dialog window. This project is useful in any compute or I/O bound application, and illustrates the techniques involved with installing and using a system timer.
These steps were developed and tested on a WindowsNT 4.0 workstation and Windows95 using VisualC++ 4.2 with Microsoft Foundation Classes.
Installing a Timer
- In the header file of the dialog using the timer,
Add a message number for each timer needed:
#define | IDT_TIMER_0 | WM_USER + 200 |
#define | IDT_TIMER_1 | IDT_TIMER_0 + 1 |
Add the timer interrupt handler
OnTimer (UINT TimerVal)
to the Generated Message Map:
locate the section under Implementation where the AFX_MSG message map is declared (usually begins with the line OnInitDialog());
add a line just after the last line in the message map (just before the "//}}AFX_MSG"):
afx_msg | void | OnTimer (UINT TimerVal); |
This should be the last entry in the message map.
In the dialog implementation (.cpp) file,
Locate the dialog class start
Find the line starting "BEGIN_MESSAGE_MAP"
After the last entry in the message map (before the line "//}}AFX_MSG_MAP"), add
ON_WM_TIMER ( )
Starting the timer
To start the timer, you must issue a SetTimer command:
t-number = SetTimer (t-message, t-interval, NULL)
where:
t-message | The message number assigned for processing the timer request. |
t-interval | The timer duration, in mSec. |
t-number | The system timer number assigned to this event, or 0 if no timers available. |
For Example:
UINT StartTimer (UINT TimerDuration)
{
UINT TimerVal;
TimerVal = SetTimer (IDT_TIMER_0, TimerDuration, NULL);
if (TimerVal == 0)
{
MessageBox ("Unable to obtain timer",
"IDT_TIMER_0",
MB_OK|MB_SYSTEMMODAL);
}
return TimerVal;
}// end StartTimer
To start the timer, pass it the value of the timer duration, in milliseconds (mSec). If the returned value is non-zero, the timer has been initialized.
Note: You must save the assigned timer value in order to stop the timer, and also to know which timer to process in a multi-timer application.
Stopping the Timer
To stop the timer, issue the KillTimer command:
t-result = KillTimer (t-number);
where:
t-number | The system timer number from either the SetTimer or OnTimer methods. |
t-result | The boolean result of the operation, TRUE indicating success. |
For Example:
BOOL StopTimer (UINT TimerVal)
{
//
// Stop the timer
//
if (!KillTimer (TimerVal))
{
return FALSE;
}
//
// Place clean-up code following this point.
//
return TRUE;
} // end StopTimer
Processing Timer Events - The OnTimer Method
After the timer has been started, the OnTimer event will be called each time the timer counts down to zero (reaches terminal count) from the requested value. This event is asynchronous to the timer - a message is placed in the message queue of the calling process and the timer is automatically restarted.
The timer can be stopped (refer to Stopping the Timer) upon entry to the OnTimer routine, or left to run if the event will not occur again before processing the OnTimer routine has completed.
The OnTimer method will be executed each time the timer reaches terminal count. The code in this method should be short and concise; it should be code which must be executed each time the timer reaches its terminal count, such as changing the view of an animated drawing or icon, or setting a flag to process latter, or starting a thread to handle the more complex functions.
Example:
void OnTimer (UINT TimerVal)
{
//////////////////////////////////////////////
//
// Stop the timer
//
//////////////////////////////////////////////
if (!KillTimer (TimerVal))
{
}
//////////////////////////////////////////////
//
// Process the event
//
//////////////////////////////////////////////
//////////////////////////////////////////////
//
// Restart the timer, if needed, before exiting
//
//////////////////////////////////////////////
}
Multiple Timers
The OnTimer method is passed an unsigned integer (UINT) to identify the timer which is interrupting. The value of this timer may be used to stop the associated timer. It can also be used in a multiple timer environment to determine what type of processing must occur.
Example:
Assumptions:
The timers are started using the example in the section on Installing a Timer.
The timer values are placed sequentially into the array ATimer, as follows:
- ATimer [0] = StartTimer (200); // a 200 mSec timer
ATimer [1] = StartTimer (500); // a 500 mSec timer
void OnTimer (UINT TimerVal)
{
int Index;
/////////////////////////////////////////////////////////////////////////////
//
// Check timer number 0
//
////////////////////////////////////////////////////////////////////////////
if (TimerVal == ATimer[0])
{
//
// Process timer 0 message!
//
return;
}
////////////////////////////////////////////////////////////////////////////
//
// check timer number 1
//
////////////////////////////////////////////////////////////////////////////
if (TimerVal == ATimer[1])
{
//
// Process timer 1 message!
//
return;
}
////////////////////////////////////////////////////////////////////////////
//
// Not timer number 0 or 1
//
////////////////////////////////////////////////////////////////////////////
if (!KillTimer (TimerVal))
{
}
CString Message;
Message.format("Unkown timer interrupt: %u", TimerVal);
MessageBox (Message,"OnTimer",MB_OK|MB_SYSTEMMODAL);
} // end OnTimer
