Tcl_DoWhenIdle(), Tcl_CancelIdleCall()

invoke a procedure when there are no pending events 

Tcl Library Procedures


#include <tcl.h>

Tcl_DoWhenIdle(proc, clientData)

Tcl_CancelIdleCall(proc, clientData)


Tcl_IdleProc *proc (in) 

Procedure to invoke.

ClientData clientData (in) 

Arbitrary one-word value to pass to proc.


Tcl_DoWhenIdle() arranges for proc to be invoked when the application becomes idle. The application is considered to be idle when Tcl_DoOneEvent() has been called, couldn't find any events to handle, and is about to go to sleep waiting for an event to occur. At this point all pending Tcl_DoWhenIdle() handlers are invoked. For each call to Tcl_DoWhenIdle() there will be a single call to proc; after proc is invoked the handler is automatically removed. Tcl_DoWhenIdle() is only usable in programs that use Tcl_DoOneEvent() to dispatch events.

Proc should have arguments and result that match the type Tcl_IdleProc:

typedef void Tcl_IdleProc(ClientData clientData);

The clientData parameter to proc is a copy of the clientData argument given to Tcl_DoWhenIdle(). Typically, clientData points to a data structure containing application-specific information about what proc should do.

Tcl_CancelIdleCall() may be used to cancel one or more previous calls to Tcl_DoWhenIdle(): if there is a Tcl_DoWhenIdle() handler registered for proc and clientData, then it is removed without invoking it. If there is more than one handler on the idle list that refers to proc and clientData, all of the handlers are removed. If no existing handlers match proc and clientData then nothing happens.

Tcl_DoWhenIdle() is most useful in situations where (a) a piece of work will have to be done but (b) it's possible that something will happen in the near future that will change what has to be done or require something different to be done. Tcl_DoWhenIdle() allows the actual work to be deferred until all pending events have been processed. At this point the exact work to be done will presumably be known and it can be done exactly once.

For example, Tcl_DoWhenIdle() might be used by an editor to defer display updates until all pending commands have been processed. Without this feature, redundant redisplays might occur in some situations, such as the processing of a command file.


At present it is not safe for an idle callback to reschedule itself continuously. This will interact badly with certain features of Tk that attempt to wait for all idle callbacks to complete. If you would like for an idle callback to reschedule itself continuously, it is better to use a timer handler with a zero timeout period.


Windows 8.1. Windows Server 2012 R2. Windows 10. Windows Server 2016. Windows Server 2019. Windows 11. Windows Server 2022.


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