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POPost message from Thread to GUI best practice?
 

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  1. POPost message from Thread to GUI best practice?
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    <p>I am working on small monitoring application which will have some threads for communication with some devices via SNMP, TCP, ICMP, other threads have to perform some calculations. All this result I have to output in GUI (some Forms or TabSheets).</p> <p>I am thinking about next possibilities:</p> <ul> <li>use <code>Synchronize</code> from every worker thread: </li> <li>use shared buffer and windows messaging mechanism. Thread will put message in shared buffer (queue) and will notify GUI with windows message. </li> <li>use separate thread which will listen for Synchronization primitives (Events, Semaphores, etc) and use again <code>Synchronize</code>, but only from GUI-dedicated thread only, or Critical Section on GUI to display message. </li> <li><strong>UPDATE:</strong> (Proposed by one workmate) use shared buffer and <code>TTimer</code> in main form which will check periodically (100-1000 ms) shared buffer and consuming, instead of windows messaging. (Does it have some benefit over messaging?)</li> <li>Other?</li> </ul> <p>Dear experts, please explain what is the best practice or what are the advantages and disadvantages of exposed alternatives.</p> <p><strong>UPDATE:</strong><br> As idea:<br> //shared buffer + send message variant<br> <code>LogEvent</code> global function will be called from everywhere (from worker threads too):</p> <pre><code>procedure LogEvent(S: String); var liEvent: IEventMsg; begin liEvent := TEventMsg.Create; //Interfaced object with liEvent do begin Severity := llDebug; EventType := 'General'; Source := 'Application'; Description := S; end; MainForm.AddEvent(liEvent); //Invoke main form directly end; </code></pre> <p>In Main Form, where Events ListView and shared section (<code>fEventList: TTInterfaceList</code> which is already thread-safe) we'll be: </p> <pre><code>procedure TMainForm.AddEvent(aEvt: IEventMsg); begin fEventList.Add(aEvt); PostMessage(Self.Handle, WM_EVENT_ADDED, 0, 0); end; </code></pre> <p>Message handler:</p> <pre><code>procedure WMEventAdded(var Message: TMessage); message WM_EVENT_ADDED; ... procedure TMainForm.WMEventAdded(var Message: TMessage); var liEvt: IEventMsg; ListItem: TListItem; begin fEventList.Lock; try while fEventList.Count &gt; 0 do begin liEvt := IEventMsg(fEventList.First); fEventList.Delete(0); with lvEvents do //TListView begin ListItem := Items.Add; ListItem.Caption := SeverityNames[liEvt.Severity]; ListItem.SubItems.Add(DateTimeToStr(now)); ListItem.SubItems.Add(liEvt.EventType); ListItem.SubItems.Add(liEvt.Source); ListItem.SubItems.Add(liEvt.Description); end; end; finally fEventList.UnLock; end; end; </code></pre> <p>Is there something bad? Main Form is allocated ONCE on application startup and Destroyed on application exit.</p>
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    1. COThey all seem much the same. Which is better? Who knows. We cannot really tell since we don't know all the constraints. Does performance matter? What is driving your design?
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      1. USDavid Heffernan
    2. COI usually PostMessage request/result objects directly in wParam/lParam, either freeing them or repooling them in the message-handler after processing the object. There is indeed some sort of 'movement' to use a separate thread-safe queue to actually transfer the data and use the Window message as a kind of flag to pop the queue, but I've never understood that approach - why use two queues when one will do?
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      1. USMartin James
    3. CO@DavidHeffernan, it is Event-Driven. Event is SNMP reply from device. Performance matters, because SNMP polling periodicity may be 0.5-5 seconds. (I hope I expressed myself properly :))
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