Main Loop Hooks

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Revision as of 21:30, 16 November 2005 by Neli (talk | contribs) (AFlags extension)

Problem statement

You need to wait for some event (on socket or pipe or ...) to happen, but you want to do this in the main (GUI) thread and do not want to block the GUI and also you do not want multi-threading. Solution to this problem is the ability to add extra "handles" to be watched in the main event loop.

Solution details

In unit LCLIntf two functions have been added to provide for this functionality, they are:

 AddEventHandler(AHandle: THandle; AFlags: dword; ACallback: TWaitHandleEvent);
 RemoveEventHandler(AHandle: THandle);

The TWaitHandleEvent type is declared in unit InterfaceBase as:

 TWaitHandleEvent = procedure(AHandle: THandle; AFlags: dword) of object;

AddEventHandler adds a handle to be watched to the main event loop. When the handle is "signalled", then the procedure specified in ACallback will be called with as a parameter the handle that was signalled, and any Flags, which are OS specific.

RemoveEventHandler stops watching the specified handle.


The AFlags parameter in AddEventHandler is unused, and the AFlags in TWaitHandleEvent will be 0, in the current implementation.

Win32 supports the following handle types, basically the things supported by MsgWaitForMultipleObjects:

  • change notifications (for files and directories)
  • console input
  • events: signalled with SetEvent winapi function
  • mutexes: signalled when it is not owned anymore
  • processes: signalled when they terminate
  • semaphore: signalled when it's count is greater than zero
  • threads: signalled when they terminate
  • timers: signalled when expired, see SetWaitableTimer


The AFlags parameter in AddEventHandler specifies the condition in which the handle should be signalled, with the possible values being the ones documented in the GIOCondition type in the glib reference.

In the callback, the AFlags will contain the condition, of the above referenced GIOCondition type, that was satisfied.

Gtk/unix supports the following handle types:

  • files
  • sockets
  • pipes

Note that win32 does not support sockets or pipes (at least, their use is "discouraged" by MSDN). Suggestion: use WSAEventSelect to create an event associated with a socket and pass the event handle.