Synchronizing threads via events

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Overview
Simple management of events
Principle
Example: Application managing a spelling checker
WLanguage functions
Remark
Advanced management of events
Principle
Implementation
Functions for advanced management of events

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Synchronizing threads via events

Overview

Events can be used to synchronize the different threads of an application. Therefore, a thread can wait for the execution of another thread.

Two management modes can be used to manage events:

simple management: synchronization between two threads.
advanced management: synchronization between several threads (more than two).

Versions 19 and later

This feature is now available for WINDEV applications in Linux.

This feature is now available for WEBDEV sites in Linux.

New in version 19

This feature is now available for WINDEV applications in Linux.

This feature is now available for WEBDEV sites in Linux.

This feature is now available for WINDEV applications in Linux.

This feature is now available for WEBDEV sites in Linux.

Simple management of events

Principle

Two threads are run in parallel (a main thread and a secondary thread for instance). One of the threads waits for a specific action from the second thread before it can be run.

Example: Application managing a spelling checker

When the user types the SPACE character in an edit control, the spelling checker is automatically started to check the previous word.

In this case, the spell check is managed in a secondary thread.

Whenever the SPACE key is pressed, the main thread sends a signal to the secondary thread in order to start the spelling checker.

The code for this kind of application is as follows:

Code of the main thread:

IF Right(EDT_Edit1, 1) = " " THEN
ThreadSendSignal("ThreadCorrection")
END

Secondary Thread Code (Thread Correction):

LOOP
IF ThreadWaitSignal() = True THEN
StartCorrection()
END
END

WLanguage functions

The following functions are used to perform a simple management of events:


Versions 09 and later ThreadSendSignal New in version 09 ThreadSendSignal ThreadSendSignal	The current thread sends a signal to the specified thread in order to unlock it.
Versions 09 and later ThreadWaitSignal New in version 09 ThreadWaitSignal ThreadWaitSignal	Locks the current thread until it receives a signal from another thread.

Remark

Semaphores and events are system objects identified by their name. They are shared among all the applications that run on a computer.

Therefore, two applications (or two instances of the same application) use the same objects if the same name is used.

To create unique names, use Instance to implement the requested name.

Advanced management of events

Principle

An advanced management of events consists in communicating between several threads (more than 2). Some threads are waiting for a task performed by the main thread. When the main thread performs this task, it sends a signal to all the secondary threads.

Implementation

Two methods can be used to implement an advanced management of events:

Versions 20 and later
Using AutomaticEvent and ManualEvent.
New in version 20
Using AutomaticEvent and ManualEvent.
Using AutomaticEvent and ManualEvent.
Using named events (recommended method to manage the options to share events).

Versions 20 and later

Method 1: Using EventAutomatic and EventManual.
The steps for implementing an advanced management of events are as follows:

Creating an AutomaticEvent variable or a ManualEvent variable. By default, this event is closed. The event is automatically destroyed when going outside the scope of the corresponding variable.
Waiting an event (EventWait).
Synchronizing the threads with EventOpen and EventClose.

New in version 20

Method 1: Using EventAutomatic and EventManual.
The steps for implementing an advanced management of events are as follows:

Creating an AutomaticEvent variable or a ManualEvent variable. By default, this event is closed. The event is automatically destroyed when going outside the scope of the corresponding variable.
Waiting an event (EventWait).
Synchronizing the threads with EventOpen and EventClose.

Method 1: Using EventAutomatic and EventManual.
The steps for implementing an advanced management of events are as follows:

Creating an AutomaticEvent variable or a ManualEvent variable. By default, this event is closed. The event is automatically destroyed when going outside the scope of the corresponding variable.
Waiting an event (EventWait).
Synchronizing the threads with EventOpen and EventClose.

Method 2: Using named events (recommended method to manage the options to share events)
The steps for implementing an advanced management of events are as follows:

Creating an event (EventCreate). By default, this event is closed.
Waiting an event (EventWait).
Synchronizing threads:
- by EventChange: the event is opened. All waiting threads are unlocked and the event is automatically closed (default operating mode).
- Versions 20 and later
  via EventOpen and EventClose.
  New in version 20
  via EventOpen and EventClose.
  via EventOpen and EventClose.
Destroying the event (EventDestroy).

Functions for advanced management of events

The following functions are used for an advanced management of events:


Versions 09 and later EventChange New in version 09 EventChange EventChange	Modifies the status of an event.
Versions 20 and later EventClose New in version 20 EventClose EventClose	Closes a synchronization event between several threads.
Versions 09 and later EventCreate New in version 09 EventCreate EventCreate	Creates an event.
Versions 09 and later EventDestroy New in version 09 EventDestroy EventDestroy	Explicitly destroys an event.
Versions 20 and later EventOpen New in version 20 EventOpen EventOpen	Opens a synchronization event between several threads.
Versions 09 and later EventWait New in version 09 EventWait EventWait	Locks the current thread while waiting for the specified event to be opened.