I have a multi-threaded Delphi 6 Pro application that I am currently working on heavily. If I set a breakpoint on any code that runs in the context of the Main thread (VCL thread) I don't have any problems. However, if a breakpoint is triggered on any code in one of my other threads, after I continue the application from the breakpoint, all repaints to the VCL components on the main thread (including the main form) don't happen anymore. The application isn't dead because other background code keeps running, just the main thread. It's as if the windows message dispatcher has been corrupted or rendered dormant.
Note, in this application I allocate my own WndProc() via allocateHwnd() on the main form because I need to catch certain registered messages. From that WndProc() I dispatch any custom messages I handle and if the current message is not handled by my code, I pass the message on by calling the main form's inherited WndProc(). If I do handle the current message I simply return from my WndProc() with Msg.Result set to 1 to tell the dispatcher that the message was handled. I can't simply override the TForm WndProc() instead of allocating my own WndProc() because for some reason the Delphi VCL does not pass through registered messages instantiated with the Windows API RegisterWindowMessage() call.
Has anybody experienced this in similar context and if so, what did you do to fix it?
-- roscherl
Since you call AllocateHWnd, that means you've created another window. You mustn't just take the messages that were addressed to that window and forward them to your form's window. Doing that, you're bound to screw things up in your program, although I'm not sure exactly how. Painting problems sound plausible. You should make sure it's really just painting problems and not that your main thread is still suspended. The debugger should be able to tell you that. (You should call DefWindowProc to make your allocated window handle messages you're not prepared to handle yourself. And returning 1 doesn't tell the dispatcher anything; the dispatcher doesn't care — whoever called SendMessage wants to know the result.)
I promise you that forms are completely capable of receiving registered window messages. Override WndProc or assign a new value to the WindowProc property (and remember to save the old value so you can call it after handling your own messages). The source of your problem lies elsewhere.
UPDATE: I'm not saying the way I got past the problem is a good solution. I need to take Rob Kennedy's notes and do some refactoring. However, to get past the problem for now I gave the thread it's own Window and WndProc() and at the top of the thread Execute loop I have a PeekMessage() while loop with calls to TranslateMessage() and DispatchMessage(). I no longer have a problem with setting breakpoints in the thread, but obviously this compounding of WndProc() methods indicates a structural problem in my code. I wanted to add this reply to fill out the discussion. I'm hoping that once I put Rob's suggestions to work when I clean up my WndProc() methods on the relevant forms, especially the main form, I can get rid of the this new WndProc() that I just added to the thread.
Robert.
Related
Can someone explain to me what this error I'm seeing is?
Current thread must be set to single thread apartment (STA) mode before OLE calls can be made.
Specifically, I'm trying to open the SaveFileDialog/OpenFileDialog within C++/CLI on a form.
SaveFileDialog^ saveFileDialog1 = gcnew SaveFileDialog;
saveFileDialog1->ShowDialog();
if (saveFileDialog1->ShowDialog() == System::Windows::Forms::DialogResult::OK)
{
s = saveFileDialog1->OpenFile();
}
s->Close();
}
The error that is throwing is
An unhandled exception of type 'System.Threading.ThreadStateException' occurred in System.Windows.Forms.dll
Additional information: Current thread must be set to single thread apartment (STA) mode before OLE calls can be made. Ensure that your Main function has STAThreadAttribute marked on it. This exception is only raised if a debugger is attached to the process.
I'm not really familiar with what this error is saying. I know just a bit about threading, but I'm not sure how threading would be an issue here. I've seen some people reference things like STAThread without providing a clear explanation as to what it does, and Microsoft's documentation makes no mention of having this exception thrown when calling SaveFileDialog/OpenFileDialog, or how to handle it.
Thanks!
When you use OpenFileDialog then a lot of code gets loaded into your process. Not just the operating system component that implements the dialog but also shell extensions. Plugins that programmers write to add functionality to Windows Explorer. They work in that dialog as well. There are many, one you are surely familiar with is the extension that makes a .zip file look like a folder.
One thing Microsoft did when they designed the plug-in interface is to not force an extension to be thread-safe. Because that is very hard to do and often a major source of bugs. They made the promise that the thread that creates the plugin instance is also the thread on which any call to the plugin is made. Thus ensuring that the plugin is always used in a thread-safe manner.
That however requires a little help from you. You have to make a promise that your thread, the one that calls OpenFileDialog::Show(), observes the requirements of a single-threaded apartment. STA for short. You make the promise with the [STAThread] attribute on your program's Main() entrypoint. Or if it is a thread that you created yourself then by calling Thread::SetApartmentState() before you start it.
That's just a promise however, you also have to implement what you promised. Takes two things, you promise to never block the thread and you promise to pump a message loop. Application::Run() in a .NET program. The never-block promise ensures that you won't cause deadlock. And the message loop promise says that you implement a solution to the producer-consumer problem.
This should never be a problem, it is very unclear how this got fumbled in your project. Another implicit requirement for a dialog is that it must have an owner. Another window on which it can be on top of. If it doesn't have one then there are very high odds that the user never sees the dialog. Covered by another program's window, the user can only ever find it back by accident. When you create windows then you always also must call Application::Run() so the windows can respond to user input. Use the boilerplate code in a C++/CLI app so this is done correctly.
I routinely pass the main form handle to other threads so that they can post messages back to the main thread. I saw that on Sept 28, 2013, Remy Lebeau stated:
...the TWinControl.Handle property is not thread-safe, either. You
should use the TApplication.Handle property instead, or use
AllocateHWnd() to create your own window.
in this answer to a question about passing strings.
How is the handle property not safe? Does it change during the life of the program?
How is the Handle property not safe?
When you access the Handle property, if the window handle has not been created, then it is created on demand. If you access the Handle property from a thread other than the GUI thread, then this means you create the window on the wrong thread.
Does it change during the life of the program?
Yes, the window handle can change if the window is re-created.
I routinely pass the main form handle to other threads so that they can post messages back to the main thread.
In this case, you are probably not accessing the Handle property away from the main thread. It sounds like (although I cannot see your code) you are accessing Handle on the main thread and passing that value to the other thread.
However, window recreation is the problem for you. Since your window is subject to recreation, you simply cannot rely on that handle outliving your thread. Whether or not your window will ever be recreated is hard to predict. The VCL does not perform recreation lightly. However, in my view it is far better to be safe than sorry. So, use AllocateHWnd and take control of the lifetime of this window.
I know very similar questions have been asked before, but I am unable to find an answer for my specific problem. I have a main (GUI) thread which upon button press initializes a worker thread to perform some analysis. I am using signals and slots to communicate between my worker thread and my GUI thread (i.e. when the thread starts and when it finishes), but I need to go deeper than that. My worker thread actually calls another class in a separate implementation file which then iterates through a series of calculations which are sent to std::cout for each iteration (as the code used to be a console application for which I am now writing a GUI). I am trying to feed those outputs for each iteration back into my GUI thread so that my text browser is updated in real time as my code iterates. The problem is, when I emit a signal from the class my worker thread calls, it is not picked up by the GUI thread. I do not get any errors. Does anyone have any suggestions on how to transmit a signal to the GUI from a class that my worker thread is calling? I can post code as required, but I'm not sure what would be most helpful to see and my code is quite extensive (it's an aircraft performance application). Any help would be greatly appreciated. Thank you kindly!
1) Make sure the connect() call for connecting you signal returns true. If it does not, qdebug output usually tells you what is wrong
2) You should use the QueuedConnection type (the default (Auto) should work also)
Actually i am using this code and works ok, but i 'am wondering if is the correct way.
while WaitForSingleObject(MyThread.Handle, 0) = WAIT_TIMEOUT do
Application.ProcessMessages;
ShowMessage('i am done');
The VCL TThread class has its own WaitFor() method that pumps the main message queue internally when called within the main thread context:
MyThread.WaitFor;
ShowMessage('i am done');
Calling Application.ProcessMessages is generally considered a code smell. Let your main thread idle if it's got nothing to do.
If you ran a company and needed one of your workers to run to the store and grab some much-needed supplies, would you then pace by the door until he got back, or would you prefer to sit in your office and rest and wait for him, and find out that the supplies are here because you hear him walk through the door? Either way, he'll take the same amount of time, but the first way's gonna wear your legs out.
Similarly, instead of having your UI watch the thread, have the thread report back to the UI. One way to do this is to have the thread use PostMessage to send a custom message to the form that launched it once it's finished, and put a message handler on the form to respond to it.
It looks correct (if correct means it do the work). What I would change is to wait for a bit more time (50ms looks good to maintain the application responsive) while not eating CPU.
while WaitForSingleObject(MyThread.Handle, 50) = WAIT_TIMEOUT do
Application.ProcessMessages;
ShowMessage('i am done');
Sure there are other ways to do it... <joke>but I usually apply one of the main engineering principles:
if it works, don't touch it!</joke>
I agree with Mason Wheeler's remark, the main thread is best left to do its job, but I would suggest using the OnTerminate event on the thread. It is more 'Delphi natural' and the internal logic does the PostMessage bit for you. Since TThread is not a component, you can't view it in the object inspector and have to write and attach an event handler yourself. It gets called (in the main thread!) after the thread has completed/terminated.
While it looks okay, like jachguate I would use a bigger time-out value than 0 too. If you use WaitForSingleObject(MyThread.Handle, 100) then the main thread will wait a bit longer, thus eating up less CPU cycles.
A better solution would be the use of messages, though. Your application starts the thread and then puts all controls in disabled mode. The thread then executes and when it's finished, use SendMessage or PostMessage to the main window to notify it that the thread is done again. Then your application will just enable every control (and whatever else) again. This has as advantage that you keep the "natural" messageloop for the application alive, instead of running your own messageloop with this solution.Unfortunately, the message-method has one drawback: if the thread crashes then no message will be sent back, so a backup plan would be practical. For example, by adding a timer control to your mainform which checks every second if the thread is still alive. If not, it too would just activate the form again, disabling itself again.
I have a worker thread in a class that is owned by a ChildView. (I intend to move this to the Doc eventually.) When the worker thread completes a task I want all the views to be updated. How can I make a call to tell the Doc to issue an UpdateAllViews()? Or is there a better approach?
Thank you.
Added by OP: I am looking for a simple solution. The App is running on a single user, single CPU computer and does not need network (or Internet) access. There is nothing to cause a deadlock.
I think I would like to have the worker thread post (or send) a message to cause the views to update.
Everything I read about threading seems way more complicated than what I need - and, yes, I understand that all those precautions are necessary for applications that are running in multiprocessor, multiuser, client-server systems, etc. But none of those apply in my situation.
I am just stuck at getting the right combination of getting the window handle, posting the message and responding to the message in the right functions and classes to compile and function at all.
UpdateAllViews is not thread-safe, so you need to marshal the call to the main thread.
I suggest you to signal a manual-reset event to mark your thread's completion and check the event's status in a WM_TIMER handler.
suggested reading:
First Aid for the Thread-Impaired:
Using Multiple Threads with MFC
More First Aid for the Thread
Impaired: Cool Ways to Take Advantage
of Multithreading