Reimplementing winEvent: getting stuck in API call

JulienMaille

I stepped into the Play() call and found this:
@inline HRESULT IMJPlaybackAutomation::Play ( ) {
return _com_dispatch_method(this, 0xf, DISPATCH_METHOD, VT_EMPTY, NULL, NULL);
}@

giesbert

[quote author="neFAST" date="1311766193"]I stepped into the Play() call and found this:
@inline HRESULT IMJPlaybackAutomation::Play ( ) {
return _com_dispatch_method(this, 0xf, DISPATCH_METHOD, VT_EMPTY, NULL, NULL);
}@[/quote]

This only shows that play is a COM call, but what does it do internally?

JulienMaille

I don't know I do not have access to this source code.
I suspect that when I call play() from my winEvent(...) reimplementation I am not in the main thread and this causes the problem.

giesbert

your events of UI widgets are always in the main thread, onlöy if sendEvent is used and not post event, it is possible to be outside the main thread.
But typically in Qt, everything that is UI relevant happens in the main thread, and this includes the events handlers. This is a MUST.

JulienMaille

Ok then what could explain that play() returns correctly when started in a slot connected to a button in my GUI. And it blocks when called within winEvent()?

JulienMaille

You were right, everything stays in the main thread. How can I figure out if the COM control fires an event that is caught by winEvent in a recursive way?

With our without my stupid mutex, the test() call is never reached !?

@if( message->message == WM_USER && message->hwnd == winId() )
{
if( !CDArtMutex )
{
CDArtMutex=true;

        pPlayback->Play();
        test();
        CDArtMutex=false;
        *result = 0;
        return true;
    }
    else
    {
        // give the event to qt
        return false;
    }
}@

giesbert

That can have multiple reasons, and I can't tell you from that small code fragment. If you provide a small, minimal example showing this that can be compiled, then perhaps.

JulienMaille

My code is really minimal but it is a bridge between 2 applications: if you want to test it you will have to fire messages. Keep in mind it is windows only.
I uploaded an archive here: "hotfile":http://hotfile.com/dl/125174202/8234364/CDArt-JRMC.zip.html
You will see that I got something working with QTimer::singleshot() but this is not a real solution since I can't pass any arguments to my slots. So it is working for Play() but not for other calls.
Thanks again for your kind help!

giesbert

i,

I looked a bit into your code but saw no direct bug, but I also have no idea how this CDART and JRMC stuff works, sorry.

JulienMaille

I got this answer from JRMC:

[quote author="JRMC" date="1311799775"]Using PlaybackAutomationPtr from a worker thread could cause trouble, because internally it sends messages to the player window. Especially if the handling of these messages fires an event that tries to re-enter your event handler.

Instead, I would recommend posting MCC messages to the player if you're in a worker thread.

For example:
PostMessage(MJ.GetWindowHandle(), WM_MC_COMMAND, MCC_PLAY_CPLDB_INDEX, -1);
[/quote]

PostMessage is ok for Play() Pause() commands, bu it wont work when I need to retrieve informations from the COM.

JulienMaille

I got something working by following these tips:
http://stackoverflow.com/questions/1755196/receive-wm-copydata-messages-in-a-qt-app

BTW the code I uploaded was full of bugs, I was emiting slots!

JulienMaille

2 remarks:
_I had to connect my re-emitted signal to a custom slot with "Qt::QueuedConnection":http://doc.qt.nokia.com/latest/qt.html#ConnectionType-enum. Otherwise it does not work.
_I am supposed to return values to the API through the winEvent(MSG* msg, long* result);
However, since I have to re-emit from winEvent to get this working, winEvent() returns before I set the long* result to the desired value. So I'm stuck once again.

JulienMaille

I have changed the way I interface the COM object, and now with this code: @bool Bridge::winEvent(MSG *message, long *result)
{
if( message->message == WM_USER && message->hwnd == winId() )
{
myQAxObject->dynamicCall("Play()");
return true;
}

// give the event to qt
return false;

}@ I get this error: "QAxBase: Error calling IDispatch member Play: Unknown error"

While if I call myQAxObject->dynamicCall("Play()"); from anywhere else in my code it works.