After iterating all the providers, all of them returning unsupported
error, gdk_content_provider_union_get_value() returns FALSE without
filing the given GError. Then gdk_clipboard_read_value_internal()
assumes there's a GError when FALSE is returned and
g_task_return_error() fails. We can just chain up to parent
implementation to ensure the GError is filled with unsupported error.
Content providers are meant to be immutable, apart from very special
cases, but in those cases they need to emit
gdk_content_provider_content_changed().
Having a constructor that just uses a get_func invites abuse of this
by not making developers aware of those requirments.
In fact, all users in GTK failed to do this.
Instead, code should use the GtkDragSource::prepare signal to create
content providers when needed.
The same problem exists with gdk_content_provider_new_with_formats(),
but this commit doesn't touch that.
GdkContentFormatsBuilder is currently not introspectable, as it does not
have a GType. We can turn it into a boxed type, but we need to implement
memory management for it.
The current gdk_content_formats_builder_free() function returns a newly
constructed value, so we cannot use it as a GBoxedFreeFunc; additionally
copying a GdkContentFormatsBuilder contents would make it a bit odd, as
you could get multiple identical GdkContentFormats out of the copies.
A simple approach is to model the GdkContentFormatsBuilder API to follow
the GBytes one: use reference counting for memory management, and have
a function to release a reference, return a GdkContentFormats, and reset
the GdkContentFormatsBuilder state.
For language bindings, we can provide a get_formats() function that
returns the GdkContentFormats instance and resets the builder instance,
leaving the reference count untouched.
For C convenience we can keep gdk_content_formats_builder_free(), and
make it a wrapper around gdk_content_formats_builder_get_formats(), with
the guarantee that it'll free the builder instance regardless of its
current reference count.
https://bugzilla.gnome.org/show_bug.cgi?id=793097https://blogs.gnome.org/otte/2018/02/03/builders/
GdkContentProvider is the object that represents local data in the
clipboard.
This patch only introduces the object and adds the clipboard properties,
it does not yet provide a way for the actual implementations to access
it.
The only access that is implemented is the local shortcut GValue access.
