Waiting for a Task to Be Completed

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Sometimes, you need to wait for a thread to finish some operation rather than waiting for a particular thread to complete execution. To do this, use an event object. Event objects (System.SyncObjs.TEvent) should be created with global scope so that they can act like signals that are visible to all threads.

When a thread completes an operation that other threads depend on, it calls TEvent.SetEvent. SetEvent turns on the signal, so any other thread that checks will know that the operation has completed. To turn off the signal, use the ResetEvent method.

For example, consider a situation where you must wait for several threads to complete their execution rather than a single thread. Because you don't know which thread will finish last, you can't simply use the WaitFor method of one of the threads. Instead, you can have each thread increment a counter when it is finished, and have the last thread signal that they are all done by setting an event.

The following code shows the end of the System.Classes.TThread.OnTerminate event handler for all of the threads that must complete. CounterGuard is a global critical section object that prevents multiple threads from using the counter at the same time. Counter is a global variable that counts the number of threads that have completed.

procedure TDataModule.TaskThreadTerminate(Sender: TObject);
  // ...
  CounterGuard.Acquire; { obtain a lock on the counter }
  Dec(Counter);   { decrement the global counter variable }
  if Counter = 0 then
    Event1.SetEvent; { signal if this is the last thread }
  CounterGuard.Release; { release the lock on the counter }
  // ...
void __fastcall TDataModule::TaskThreadTerminate(TObject *Sender) {
	// ...
	CounterGuard->Acquire(); // lock the counter
	if (--Counter == 0) // decrement the global counter
			Event1->SetEvent(); // signal if this is the last thread
	CounterGuard->Release(); // release the lock on the counter

The main thread initializes the Counter variable, launches the task threads, and waits for the signal that they are all done by calling the WaitFor method. WaitFor waits for a specified time period for the signal to be set, and returns one of the values from the following table:

WaitFor return values :

Value Meaning


The signal of the event was set.


The specified time elapsed without the signal being set.


The event object was destroyed before the time-out period elapsed.


An error occurred while waiting.

The following shows how the main thread launches the task threads and then resumes when they have all completed:

 Event1.ResetEvent; { clear the event before launching the threads }
 for i := 1 to Counter do
 TaskThread.Create(False); { create and launch task threads }
 if Event1.WaitFor(20000) <> wrSignaled then
 raise Exception;
 { now continue with the main thread. All task threads have finished }
Event1->ResetEvent(); // clear the event before launching the threads
for (int i = 0; i < Counter; i++)
	new TaskThread(false); // create and launch task threads
if (Event1->WaitFor(20000) != wrSignaled)
	throw Exception;
// now continue with the main thread, all task threads have finished

Note: If you do not want to stop waiting for an event after a specified time period, pass the WaitFor method a parameter value of INFINITE. Be careful when using INFINITE, because your thread will hang if the anticipated signal is never received.

See Also