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.
Related
I found many answers say that accessing Qt Gui Widgets from another thread is not safe. I agree with this if we try to modify the widgets. But what if I only read the value of widget, without any modification?
I have designed a GUI tool using Qt, and my working thread reads Widgets directly, for example, get the text of QLineEdit, get the value of QComboBox,etc. And I haven't found any problem.
I pass the pointer of MainWindow to working thread.
Is this really not safe?
First: It is always a bad idea to read properties from another thread without protecting the memory (Mutex, Signal & Slot).
Your situation: If nothing wants to change the value, you should be fine. There are no problems if every thread only reads the value. But if you change the value of the QComboBox (for example) by clicking arrow up, arrow down or selecting a new item (what I expect you will do, because that's the reason of using a QComboBox) the value will be written and then the application can crash if your worker thread wants to read the value in the exact same moment (the possibility of a carsh depends on the frequenz your thread pulls the informations).
Your application never crashed because the condition never appeared.
But that does not mean that this can not happen.
I'm new to development on Windows Phone 7 and Silverlight but I do have experience in win32 and threading in general.
Here's my question:
I am trying to "synchronize" the UI thread w/ another thread that seems to be used by the API's of the object that I am working with. In other words, I would like to make sure that before the user dismisses the current XAML page by pressing the back button, the object that I am working with, which is part of the C# class behind the XAML page is deallocated.
The reason for that is that if I have the deallocation code in the NavigatedFrom handler, the UI thread may attempt to release the object WHILE it is in fact used by the other thread. Therefore, I do have to somehow synchronize the deallocation of this object.
Ideally, when the user presses the back button on the phone, all I do is set a flag "quit" to true to indicate that the user intends to exit. The methods used by the object that are running on another thread, would "see" that this flag is set and then would BeginInvoke*emphasized text* the deallocation code of the object (only because the object has been allocated on the UI thread so I figured it makes sense to deallocate it on the same thread, not knowing its internal workings.) Finally, it would call NavigationService.GoBack() to ensure 'orderly' exit.
Unfortunately, I don't see a way of preventing the XAML page to be dismissed when the user presses the back button although I did override the NavigatedFrom and OnBackKeyPress methods. Even though they contain no code at all, the XAML page is dismissed anyway.
Another thing that is interesting and I would appreciate your comments on this, is that I have a timer (System.Windows.Threading.DispatchTimer). Would this timer be associated only with the C# class behind a XAML page that defines it? In other words, is there a concept of a "message pump" associated with each XAML pages or is there just one message pump for the UI thread that basically is used by ALL XAML pages ? I am asking this because although I dismiss the XAML page whose C# class defines the timer, it seems to still be running.
Thank you.
The reason for that is that if I have the deallocation code in the NavigatedFrom handler, the UI thread may attempt to release the object WHILE it is in fact used by the other thread. Therefore, I do have to somehow synchronize the deallocation of this object.
Not really a problem. If you queue the navigation on the Dispatcher as well, you don't get any NullReferenceExceptions.
Simply use Dispatcher.BeginInvoke(() => NavigationService.Navigate(...)) for safe navigation.
Would this timer be associated only with the C# class behind a XAML page that defines it?
If you by "class" means "ViewModel", then yes, it most definitively should be in the ViewModel.
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.
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
my question is : how can I use SendMessage() to implement thread communication between two threads, one with a window (GUI) and the other with no window?
The problem is that SendMessage() needs a handle (HWND)?
Another detail about my project : Two threads, one running managed code (the one with the user interface), and the other running native code (the one without window)
Thank you very much!
I would suggest creating a hidden window. When using postthreadmessage, there is a chance that your message could get lost (ie: if a messagebox is running the message loop).
More info about that at:
http://blogs.msdn.com/oldnewthing/archive/2005/04/26/412116.aspx
Perhaps you should try to use PostMessage or PostThreadMessage
If the thread has no window, no message queue, and no message dispatcher, then it's going to be hard to a message to it. It is common for threads to create hidden windows just for communication purposes (take a look with Windows Spy and you'll see plenty of examples).
One alternative is to use shared memory and a synchronization primitive such an event or semaphore. Another alternative is to use pipes.
what #jdigital said. Note that if you create a hidden window, and your thread does not already implement a message loop (either in regular win32-speak, or one in the context of a COM STA -- and if you have no idea what I'm talking about then one probably does not exist in your thread), you'll also want to create a message loop as well. ATL makes it fairly easy with _AtlModule.RunMessageLoop(); Unfortunately this also means the thread in question is probably going to need to be event-driven while it is in the message loop. You can do tricky things like MsgWaitForMultipleObjects, but it gets hairy.
Here's an example of hidden windows if you're familiar with ATL/COM. I went through this pain a while back and thought there was a useful discussion on microsoft.public.vc.atl, but the best I can find now is this post. which goes into some detail about variants of message loops (what to do differently if you have keystroke accelerators or modeless windows, sounds like you don't in your application).