How to use Windows Message Loop and Qt Application Exec?

YJCN

@aha_1980 Thanks for your reply. I have read the links. In Qt5, the winEvent has been changed to nativeEvent. This is my code.But it doesn't work.


bool MainWindow::nativeEvent(const QByteArray &eventType, void *message, long *result)
{
    //Q_UNUSED(eventType);
    MSG* msg = reinterpret_cast<MSG*>(message);
    cout<<"in nativeEvent:"<<msg->message<<endl;
    if(msg->message==275)
    {
        //GetMessage(msg, NULL, NULL, NULL);
        TranslateMessage(msg);
        cout << "TranslateMessage" << endl;
        DispatchMessage(msg); // Ends up calling objectEventFnc
        cout << "DispatchMessage" << endl;
        return true;
    }
        return false;
}

jsulm

@YJCN said in How to use Windows Message Loop and Qt Application Exec?:

But it doesn't work

What exactly doesn't work?
"Note: Events are only delivered to this event handler if the widget is has a native Window handle." from http://doc.qt.io/qt-5/qwidget.html#nativeEvent

YJCN

@jsulm I use the Canon EDSDK to control the EOS 800D. I want to download image from camera. after taking picture. In EDSDK, the download is handled by callback function. I test in command line programm ,and there must be this code after sending a take picture command. Otherwise, the programm can't enter the callback funtion.

MSG msg = { };
while (GetMessage(&msg, NULL, 0, 0))
{
    TranslateMessage(&msg);
    DispatchMessage(&msg);
}

There is the reason I find in Google.

If you're using the EDSDK on Windows, you have to have a Windows message loop in your main thread, otherwise callbacks won't happen. (This is because the EDSDK uses the obsolete COM STA threading model instead of real threads.)

https://social.microsoft.com/Forums/en-US/a431209c-be99-4394-88b3-59879cb790a4/objecteventhandler-callback-nevercalled?forum=Offtopic
https://stackoverflow.com/questions/16839640/edsdk-callbacks-not-working/18624865

mrjj

@YJCN
Hi
in case that nativeEvent simply cannot work with decade-old COM crap (1998)
there is always the options of using a plain c++ app that will handle the downloading / save to file and
you simply call it with QProcess from Qt GUI app.

YJCN

@mrjj The Canon 's demo is using MFC. Why MFC could use?

jsulm

@YJCN said in How to use Windows Message Loop and Qt Application Exec?:

Why MFC could use?

Because it is Microsofts own GUI framework using the Windows event loop...

mrjj

@YJCN
MFC is very old too :)

Anyway i wonder with that line

if(msg->message==275)

means ? where did u get this magic number from ?

YJCN

@mrjj
This my CMD programm code

int main (){       
        EdsCameraRef camera;
	EdsUInt32 tv;
        init_camera(camera);
	//start_liveview(camera);
	//TakePicture(camera);
	
	take_photo(camera,1,1000);
	//TakePhoto();
	//dispose(camera);
	cout << "before message" << endl;
	MSG msg;
	while (GetMessage(&msg, NULL, NULL, NULL))
	{
		cout << "in while" << " msg.message:"<<msg.message<<endl;
		TranslateMessage(&msg);
		cout << "TranslateMessage" << endl;
		DispatchMessage(&msg); // Ends up calling objectEventFnc
		cout << "DispatchMessage" << endl;
		break;
	}
	cout << "after message" << endl;
        EdsCloseSession(camera);
	cout << "EdsCloseSession(camera);";
	EdsTerminateSDK();
	system("pause");
        return 0;

This is output
0_1537429196448_3c672381-54c0-4382-832e-91e484ab0433-image.png

mrjj

HI
ahh, so 275 is the ID for what SDK is doing.
So you call
init_camera(camera); in main window also ?
(and take_photo(camera,1,1000);
and camera is a member variable in main window so it doesn't run out of scope?

but you never see 275 ? in natvieEvent?

YJCN

@mrjj Yes. I put the code(init_camera(camera) and take_photo(camera,1,1000) ) in Button's slot function, after taking picture, the programm can't enter callback function(handleObjectEvent) and there is no 275 in natvieEvent.

mrjj

Hi
and just to be 100% sure
camera is NOT a local variable in Button's slot function, but in the .h file as a true member of
main maindiw (in .h file) ?
(im checking its not a death by scope issue)

If unsure what i ask, please show full Button's slot function. :)

YJCN

@mrjj This is Button's slot function code

void MainWindow::on_pushButton_2_clicked()
{
    EdsCameraRef camera;
    init_camera(camera);
    //start_liveview(camera);
    //TakePicture(camera);
    take_photo(camera,1,1000);
    //TakePhoto();
    //dispose(camera);
    cout << "before message" << endl;
    MSG msg;
    while (::GetMessage(&msg, NULL, NULL, NULL))
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg); // Ends up calling objectEventFnc
        break;
    }
    cout << "after message" << endl;
    EdsCloseSession(camera);
    cout << "EdsCloseSession(camera);";
    EdsTerminateSDK();
}

jsulm

@YJCN said in How to use Windows Message Loop and Qt Application Exec?:

while (::GetMessage(&msg, NULL, NULL, NULL))
{
TranslateMessage(&msg);
DispatchMessage(&msg); // Ends up calling objectEventFnc
break;
}

You execute this loop only once (because of the break).
Shouldn't you call break after you got the message you need?

@mrjj camera is local :-) but I don't thin this is the issue here.

mrjj

@jsulm
I agree, it should not run out of scope if it works like we think :)
But you are right, maybe it just get first MSG as then it breaks.

JonB

@YJCN

@jsulm said in How to use Windows Message Loop and Qt Application Exec?:

Shouldn't you call break after you got the message you need?

Indeed, the loop picks up the first message and that's it. What makes you think that message is the right/only one which needs to be processed this way? It could be any old message....

YJCN

@jsulm @JonB @mrjj Yes, you are right !!! I comment the "break" . It works well!! THANK YOU!☺☺☺☺☺☺☺☺☺☺

JonB

@YJCN
You have to do more than just comment out the break, if that's what you're saying. If that's all you do the ::GetMessage() loop will never exit, and your code will never return from on_pushButton_2_clicked(), e.g. you cannot press it again to take another photo. Presumably you need to find out whatever is the final message your camera issues on taking a photo and exit the loop once that has been received.

YJCN

@JonB Yes. I just use a five times for loop to solve it . ^_^

JonB

@YJCN
I don't know if your response is serious or amused. Doing it "five times" is very flaky. Isn't there a particular message number you get which you need to wait for to indicate it's now OK to leave the loop?