[gst-devel] How to set window ID on Mac OSX

Andreas Schuler andreas at neokast.com
Fri Jun 27 18:29:35 CEST 2008

I have been doing a little bit of research around this issue:
Turns out that Qt is still using the older Carbon windowing framework, which
means that the ID I am getting from Qt is actually a HIViewRef.

The "have-ns-view" message contains a value called "nsview", which can be
cast to a "NSView*", which is a Cocoa based view (the system used by almost
all OSX apps).

According to Apple's dev docs, there are a handful of Carbon functions,
which allow one to attach a Cocoa view to a special wrapper, which makes it
look like a Carbon view. I have tried this using the following code:


CocoaHelper::AttachCocoaViewFromMessage(GstMessage* message,
constWindowHandle& handle)


    WindowHandle outputHandle;

    // Extract the cocoa view from the message:

    NSView* cocoaView = (NSView*)g_value_get_pointer(

        gst_structure_get_value(message->structure, "nsview"));

    if (cocoaView == 0)


        MEDIA_LOG_ERROR("Message does not contain a value with name

        return outputHandle;


    // Create Cocoa wrapper view:

    HIViewRef cocoaWrapperView;

    OSStatus retVal;

    retVal = HICocoaViewCreate(cocoaView, 0, &cocoaWrapperView);

    [cocoaView release];

    if (retVal != 0)


        MEDIA_LOG_ERROR("Unable to create Cocoa wrapper view, return value:
", retVal);

        return outputHandle;


    // Attach view to incoming Carbon view:

    // From Qt sources "qwidget_mac.cpp" confirms that a WId can be

    // safely converted to a HIViewRef on the mac. (see qt_mac_hiview_for
line 254)

    HIViewRef renderView = (HIViewRef)handle.GetHandle();

    if ((retVal = HIViewAddSubview(renderView, cocoaWrapperView)) != 0)


        MEDIA_LOG_ERROR("Unable to attach Cocoa wrapper view to render view.

            " return value: ", retVal);

        return outputHandle;


    outputHandle = WindowHandle((HandleType)cocoaWrapperView);

    MEDIA_LOG_INFO("Successfully attached Cocoa view ", static_cast<void

        " to Carbon view with handle ", handle,

        ". Handle of view wrapper view is ", outputHandle);

    return outputHandle;


The WindowHandle object is just a simple wrapper around the window ID I am
getting from Qt (GetHandle() returns the same value as QWidget::winid()).

I am also calling NSApplicationLoad() during startup to ensure I can use
Cocoa in my Carbon based app.

When I run my app, I don't see any video, instead I get a lot of the
following messages in my debugger:

*2008-06-27 11:05:38.106
com.neokast.stream.player.ui.view_TESTCASES[92020:a80b] invalid drawable*

*2008-06-27 11:05:38.147
com.neokast.stream.player.ui.view_TESTCASES[92020:a80b] ***
_NSAutoreleaseNoPool(): Object 0x1af9cb10 of class NSSurface autoreleased
with no pool in place - just leaking*

*Stack: (0x90c9e12f 0x90baaec2 0x92cdef8b 0x92daa15c 0x190b3bca 0x190b2af7
0x31791f2 0x317acf0 0x317b351 0x317b970 0x31f5d86 0x31f657a 0x31890e3
0x31f5d86 0x31f657a 0x18f13fff 0x3212309 0x32f97ec 0x32f76de 0x94ef3c55

*2008-06-27 11:05:38.147
com.neokast.stream.player.ui.view_TESTCASES[92020:a80b] ***
_NSAutoreleaseNoPool(): Object 0x1afc2dd0 of class NSCFString autoreleased
with no pool in place - just leaking*

*Stack: (0x90c9e12f 0x90baaec2 0x92ddbc92 0x92daa15c 0x190b3bca 0x190b2af7
0x31791f2 0x317acf0 0x317b351 0x317b970 0x31f5d86 0x31f657a 0x31890e3
0x31f5d86 0x31f657a 0x18f13fff 0x3212309 0x32f97ec 0x32f76de 0x94ef3c55

I am less concerned about the apparent leakage (I have been experimenting
with the NSAutoReleasePool to fix those), I think the main problem is the
"invalid drawable" message. I would guess that the frequency of those
messages roughly corresponds to the framerate.

I would greatly appreciate any input from more experienced Mac developers. I
understand that my situation is further complicated by the fact that I am
using Carbon. I will keep digging and post the solution when I find it. It
would be great is someone could then take this code and use it for a proper
implementation of the GstXOverlay interface for the OSX video sink.



On Thu, Jun 26, 2008 at 9:41 AM, Andreas Schuler <andreas at neokast.com>

> Thanks for your detailed response. I have taken a closer look at the source
> code for osxvideosink and found that it supports an "embed" property (which
> is FALSE by default).
> I can tell that when TRUE it will emit a message called "have-ns-view",
> where it passes a GstGLView object to the callback. This object is an NSView
> (part of the Apple Cocoa framework). I have learned that I am supposed to
> attach that to a NSWindow.
> Unfortunately I am pretty new to programming on the Mac. Thus far I have
> been using OS abstraction libraries such as Qt to avoid dealing with
> different windowing systems. Qt gives me back a window handle, which turns
> out to be integer but I have no idea how to retrieve a NSWindow object from
> that.
> So if anyone has done this before on the Mac (with or without Qt) I would
> greatly appreciate your input.
> Thanks,
> Andreas
> On Wed, Jun 25, 2008 at 5:26 PM, Michael Smith <msmith at xiph.org> wrote:
>> On Wed, Jun 25, 2008 at 3:16 PM, Andreas Schuler <andreas at neokast.com>
>> wrote:
>> > Hi all,
>> >
>> > I am trying to get an application to run on a Max OSX 10.5.2. I have
>> > downloaded and built the latest tar balls and everything seems to be
>> running
>> > fine when I use gst-launch.
>> >
>> > >From within my app, which also runs on Windows, I am doing the
>> following to
>> > set the id of the window I want the video to be rendered on:
>> >
>> > - get_bus_set_sync_handler to setup my callback
>> > - The callback function looks as follows:
>> That looks good for Win32 and linux. On linux the 'window handle' will
>> need to be the XID for the window, but that's just a matter of
>> different platform-specific types, not a big problem.
>> On OSX, you're presumably using osxvideosink (well, you say you're
>> using autovideosink, but that'll be selecting osxvideosink
>> internally).
>> osxvideosink doesn't implement GstXOverlay, so you can't do this with
>> it. Because of this it won't ever even emit the prepare-xwindow-id
>> signal, which is what you're seeing.
>> osxvideosink has some other interface to set this up, but I don't know
>> the details of how it works. You may find looking at the output of
>> "gst-inspect-0.10 osxvideosink" to be helpful - it'll tell you what
>> signals exist, etc.
>> If possible, it would be best to make osxvideosink implement the
>> GstXOverlay interface - that would let people use more common code
>> across different platforms. Looking into whether that's possible is on
>> my TODO list, but pretty far down right now - I probably won't get to
>> it for a month or two. If you manage to do that sooner, that'd be
>> great - patches would be very welcome.
>> Mike
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