[Interest] Using Qt as backend for plugin to be loaded on a host software

Nuno Santos nunosantos at imaginando.pt
Wed Sep 16 15:36:36 CEST 2015


I’m developing a plugin to be loaded by a music software host (VST). There are some lighter alternatives to provide graphics, network and other kinds of useful resources but I want to use Qt. I simply Love Qt!

When the plugin is open by the host, it provides a window ref to the plugin. On Window it is a HWND handle, on Mac it is supposedly a WindowRef.

I have a working prototype for Windows. I don’t think it is all correct but its working. When the plugin is first started I check for the existence of a QApplication and start one if not.

Then I create the main controller for my plugin and the QQuickView to draw on the window. To create the QQuickView i’m first creating a QWindow with the ptr to HWND handle as parent:

window = QWindow::fromWinId((WId)ptr);

view = new QQuickView(window);
view->rootContext()->setContextProperty("controller", _controller);

This is actually working quite well on Windows. The same strategy however, doesn’t work on a Mac. When I try to create a QWindow::fromWinId the result is a crash with the following stack trace:

2015-09-16 14:26:18.526 Live[91339:1091079] -[NSHIObject setPostsFrameChangedNotifications:]: unrecognized selector sent to instance 0x19806da0

- The first question is: what is QWindow::fromWinId is expecting to be able to create a window? An NSView?

- The second question is: the stack trace says that NSHIObject didn’t recognized the selector setPostsFrameChangedNotifications. What is a NSHIObject? I can only find references to HIObject from the HIToolbox and that belongs to the Carbon frame work. So far I haven’t been able to call Carbon methods inside my Qt environment even referencing -framework Carbon

- Third question: On window I have an issue with the QApplication args and argv. On debug, there is an ASSERT that says that origArgc is different from the actual argc/argv. On release mode it crashes. Why does QApplication needs to receive the exact same arguments as the initial app? 

- Fourth question: how should I keep my run loop running without depending on the host? On Windows I have created a windows thread to call qApp->exec(). Although a warning that exec should be called on main thread it WORKS! How safe is this? Is there any other recommended option? What about Mac OS X?

I have been google a lot specially about the NSHIObject and I can’t find an answer, not even a simple lead. The closest I have is vstgui, a GUI framework for VST plugins that has open source code and support for Carbon and Cocoa backends but the NSHIObject is intriguing me because I can’t find any explicit reference on how to deal with it.

Any thoughts would be high appreciated.

Thanks in advance,

With my best regards,

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