How do I detect "invalid drawable" in Mac OS X? - macos

Question

I am working on a cross platform application that is over a decade old. The UI is by Qt and backend rendering is done with OpenGL. OpenGL contexts are managed in the back end, not by Qt.
I have recently added error checking and reporting for all OpenGL code in our app. Occasionally a situation arises where the first render initiated by Qt causes an "invalid drawable" error message in the terminal and all subsequent OpenGl calls fail with an "invalid framebuffer" error reported. These invalid drawable error messages have been treated as innocuous in the past, since before the user sees it the drawable eventually becomes valid and the scene is rendered correctly. However, with the new OpenGL error check/report it's not possible since there are large numbers of errors reported.
I would like to test if the drawable is valid. If it is not, it should return before the render starts. How can I verify that the drawable is valid?
MacBook Pro, OS X Mountain Lion (10.8.3), ati graphics card

Answer 1

I don't know at what API level you're working. I'm not sure it's possible to detect the problem after the fact. That is, if all you have is a context (perhaps implicit as the thread's current context) that failed to connect to its drawable.
I presume that Qt is using Cocoa under the hood. I further assume it has created an NSOpenGLContext and is invoking -setView: on it. You get that "invalid drawable" error if, at the time of that call, the view's window doesn't have a window device.
One common technique is to defer setting the context's view until the view has -drawRect: called on it, since at that point you're sure that the view has a window and the window has a device. (Although that ignores the possibility of forced drawing outside of the normal window display mechanism. For example, -cacheDisplayInRect:toBitmapImageRep:.)
If you just want to know at the point of the call to -setView: whether it's safe or not, I think you can rely on checking the value of [[view window] windowNumber]. The docs for -windowNumber say:
If the window doesn’t have a window device, the value returned will be equal to or less than 0.
Another approach is to prevent the problem rather than detect it. The strategy for that is basically to make sure the window has been shown and drawn before the call to -setView:. You may be able to force that by ordering it on-screen and invoking -display on it.

Answer 2

Ken Thomases post gave me the basic info I needed to find a workable solution. An additional conditional was needed. Here's what worked
//----------------------------------------------------------------------------
bool vtkCocoaRenderWindow::IsDrawable()
{
// you must initialize it first
// else it always evaluates false
this->Initialize();
// first check that window is valid
NSView *theView = (NSView*)this->GetWindowId();
bool win =[[theView window] windowNumber]>0;
// then check that the drawable is valid
NSOpenGLContext *context = (NSOpenGLContext *)this->GetContextId();
bool ok = [context view] != nil;
return win && ok;
}