GstMeta GTypes - API vs implementation?

[Mathieu Duponchelle]

Hey Neil,

The impl vs. API GType situation is very confusing indeed, in my understanding
that design was never used in practice, Wim could probably give more background
about that I reckon.

I know this doesn't directly answer your question, but we've recently merged a
custom meta API that should be enough for 99% of the GstMeta use cases:

<https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/609>.

I would also recommend you take a look at how GstMeta was wrapped in rust,
that might give you some more insight :)

Best,
Mathieu

On 10/6/20 7:03 PM, Neil C Smith wrote:
> Hi All,
>
> Recently I've been looking at one of the missing aspects of the higher
> level Java bindings - mappings for GstMeta.  We've had two pull
> requests in recent times for this, and neither have been suitable for
> merging as is - I'm currently trying to find a way that uses elements
> of both and fixes some key issues.
>
> One concern is understanding the intended relationship between the
> GTypes for API and implementation.  We currently don't expose GType at
> all, but map everything to the Java type system, mirroring the GType
> hierarchies where appropriate (eg. AppSink -> BaseSink -> Element ->
> GstObject ...)  This is the aspect that's getting confusing!
>
> Are there examples of GstMeta with a single API GType and multiple
> implementation GTypes to look at?  If so, is there also anywhere the
> difference in type is relevant in usage?
>
> When you pass a GType in to gst_buffer_get_meta are there any
> guarantees on what implementation types you get back, now or in
> future?
>
> At the moment we only support a 1-to-1 mapping between GTypes and Java
> types.  I'm somewhat inclined to map our types to the API GType - eg.
> VideoCropMeta to GstVideoCropMetaAPI.  That way we can have (Java)
> type guarantees in a call to such as gst_buffer_get_meta.  Any
> thoughts on where that might break down?
>
> The section at https://gstreamer.freedesktop.org/documentation/additional/design/meta.html?gi-language=c#api-examples
> seems to show passing in a GstMetaInfo rather than GType?  Our other
> mapping approach might involve using an opaque MetaInfo wrapper in
> place of API types.  But both those would seem to imply filtering by
> implementation type.
>
> Thanks in advance for any thoughts or pointers you might have here!
>
> Best wishes,
>
> Neil
>
> --
> Neil C Smith
> Codelerity Ltd.
> www.codelerity.com
>
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