I have a TThread which receives and sends to a device on a COM port. After I read the data, I want to activate the GUI (not in the same thread) using Synchronize(function name). However, when I call the GUI's form function to perform a button click, I get an access violation. I checked to see if the form's value is null and it is not, since this would be an obvious reason for access violation. Right now, I am setting global flags and using a timer that continuously checks to see if a certain condition is met and if so, then I fire off the button click event in that form. That seems to be the only way to avoid getting the access violation.
I really don't like timers so is there a way to avoid having to use a timer on the form?
You can post a message to the window in question. The timer works in a similar manner. It just fires off a windows message inside the form. You obviously have a handle to the window.
CWnd::PostMessage(...) Don't use send message, it gets processed inline and could cause your thread to stop working.
Typically when you have a worker thread that attempts to access Guithread, they conflict. It's been a while since I've used MFC and threading but that's what I remember. I believe it's documented to work that way.
I found the problem. I thought I was checking if my Form was null, but I was not. I fixed it making sure the form I was referencing is not null.
Edit: Turns out that one of the forms that is called when I call Fbutton1Click() is Modal so it blocks my thread. I ended having to go back to a timer to call the button click instead.. oh well.
Related
I have read that I should only use PostMessage() to tell the UI thread to access the UI controls.
There is no problem in following this approach if I am for example setting the text of an "EDIT" control. However, what if I want to get the text of an "EDIT" control, if I send a message to the UI thread using PostMessage(), then PostMessage() will return immediately before the text is set into the buffer, so how should I solve this problem?
You cannot use PostMessage for this purpose for the reasons that you identify. WM_GETTEXT is a synchronous message. What you should do is:
If the window is in your process, then you should use GetWindowText.
If the window is in a different process, then you should call SendMessageTimeout.
Why SendMessageTimeout rather than SendMessage? Well, if the other process has hung and is not responding, then using SendMessage would never return.
There is more discussion of this topic here: The secret life of GetWindowText.
I am working with Dialog based application.
My Question is that, I want to show Waiting dialog, until some database operation carried out.
i Used Derived class from CWinThread, but problem is that, when this thread close, the background (Main application dialog) remains at deactivated means( it hide behind another window).
i am thinking that, this is happening because of WaitDialog used CWinThread class.
The problem is not unique to a dialog based application. Creating windows of any kind in more than one thread is difficult and not recommended. In your case it sounds like your wait dialog is modal, while its parent dialog is in another thread. That is even worse and can lead to deadlocks between threads.
The reliable solution is to put the wait dialog (and all other GUI) in the main thread, and the lengthy database processing in a secondary thread.
Another alternative would be to use a Modeless Dialogbox which can also optionally show the status and call the DestroyWindow function when the database operation is completed -- you may need to disable some operations of the main window while the Modeless Dialogbox is visible, though.
From the comments on my previous answer, it looks like that alternative is not viable in this situation.
Maybe a better way would be to create a normal modal "wait" dialog box, start the background thread in the dialog's InitDialog, periodically check the status of the thread using a timer and end the dialog when the thread completes?
I have written a .NET program, using Windows form Application.
My application is fairly simple.
Basically, I have two simple buttons on my form.
When the form is first loaded, I set up a global variable (bool run = true).
And my first button is essentially a very simple while loop.
while(run)
{
// do some code
}
And what I want to do, is have the second button set the value of the boolean to false (bool run = false).
But, once I clicked the first button, I cannot even touch the second button!
I have read quite about this, and I think my solution is to use a multi-threading.
I have found some example codes on line for multi-threading, and I tried to incorporate those, but I don't know why I cannot get it to work. Once I click button #1, there is no way for me to be able to click button #2.
Your UI thread should not have any infinite or a wait-on-something - never! You must not block the UI for anything (other than simple calculations, validations, user confirmation etc). You should make a thread for performing length task, and let that thread communicate to UI thread using asynchronous (or synchronous, if you prefer) communication.
On native Windows GUI application, you can use PostMessage (async), or SendMessage (sync). For .NET GUI applications, you can use Control.BeginInvoke (async), or Control.Invoke (sync).
Please read this article on how this is done.
I want to minimize my app and after the timer end get it back.
I use code below.
To minimize application use following line of code:
Display.getDisplay (MIDLET_CLASS_NAME).setCurrent (null);
To get the screen back use the following:
Display.getDisplay (MIDLET_CLASS_NAME).setCurrent (myCanvas);
But when phone display is dismissed and going to clock mode my midlet display isn't shown until press any button.
any idea?
From your question it sounds that you expect setCurrent to somehow "force" device to immediately display your screen or maybe return only after screen gets visible.
This is not so, as clearly explained in the API documentation for Display.setCurrent:
Requests that a different Displayable object be made visible on the display. The change will typically not take effect immediately. It may be delayed so that it occurs between event delivery method calls, although it is not guaranteed to occur before the next event delivery method is called. The setCurrent method returns immediately, without waiting for the change to take place...
...if the application is in the background, passing a non-null reference to setCurrent may be interpreted by the application management software as a request that the application is requesting to be brought to the foreground... These are only requests, and there is no requirement that the application management software comply with these requests in a timely fashion if at all...
Consider redesigning your MIDlet to adjust to specified behavior.
If myCanvas is an instance of Canvas, one can use showNotify() events.
For a generic Displayable screen isShown() method checks if the it is actually visible on the display.
Sometimes it could even make sense to let user explicitly confirm return from background, like
display.setCurrent(new Alert("back to foreground", "dismiss to continue...",
null, AlertType.INFO), myCanvas);
I'm currently working on a program using MFC. The current third party function starts a thread after an action has been completed using MFC (ie. Checking a checkbox, which starts a MFC thread I believe).
The problem occurs when I check the checkbox, at which point the entire program hangs. I read a few interesting discussions on CProgramming and msdn, it seems that the problem occurs because the new third party thread is calling WaitToSomething() when MFC is updating a control.
Something interesting to note:
When I debug the program, the program hangs (aka. repeatedly calls WaitToRead() ) after I check the checkbox and a new thread is trying to start
When I run the program without debugger, the program is fine UNTIL I switch to another window (ie. Internet browser, Notepad, etc)
My hypothesis:
check to make sure that MFC has finished updating the control before starting a new thread
If anyone has any suggestions or solutions, please leave a comment. Thanks.
Edit:
MFC is not thread-safe at object level (only at class level), so problem occurs when two threads work on the same CButton object.
Q: How do I make it thread safe?
A colleague helped me figure out what the problem was.
The reason why it was hanging is because that the control containing the checkbox is a child dialog, and when it finished updating the message never got passed up to its parent (so when 3rd party thread calls WaitFor(), the MFC thread never completes because a parent dialog thinks its child is still updating the controls).
Fix:
Under 'Properties' in the child dialog's control, set the 'Control' flag to true (and if it has children, set the 'Control Parent' flag to true as well).
Hope this helps.