gstreamer: Add DeviceProviderClassExt extension trait for class methods

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1322>

gstreamer/Gir.toml

@@ -56,6 +56,7 @@ manual = [
     "Gst.CapsFeatures",
     "Gst.DebugCategory",
     "Gst.DebugMessage",
+    "Gst.DeviceProviderClass", # for docs only
     "Gst.ElementClass", # for docs only
     "Gst.Segment",
     "Gst.StaticCaps",

gstreamer/src/device_provider.rs

@@ -39,16 +39,9 @@ pub trait DeviceProviderExtManual: sealed::Sealed + IsA<DeviceProvider> + 'stati
     #[doc(alias = "gst_device_provider_class_get_metadata")]
     fn metadata<'a>(&self, key: &str) -> Option<&'a str> {
         unsafe {
-            let klass = (*(self.as_ptr() as *mut glib::gobject_ffi::GTypeInstance)).g_class
-                as *mut ffi::GstDeviceProviderClass;
-
-            let ptr = ffi::gst_device_provider_class_get_metadata(klass, key.to_glib_none().0);
-
-            if ptr.is_null() {
-                None
-            } else {
-                Some(CStr::from_ptr(ptr).to_str().unwrap())
-            }
+            self.unsafe_cast_ref::<DeviceProvider>()
+                .class()
+                .metadata(key)
         }
     }
 
@@ -64,3 +57,29 @@ pub trait DeviceProviderExtManual: sealed::Sealed + IsA<DeviceProvider> + 'stati
 }
 
 impl<O: IsA<DeviceProvider>> DeviceProviderExtManual for O {}
+
+pub unsafe trait DeviceProviderClassExt {
+    #[doc(alias = "get_metadata")]
+    #[doc(alias = "gst_device_provider_class_get_metadata")]
+    fn metadata<'a>(&self, key: &str) -> Option<&'a str> {
+        unsafe {
+            let klass = self as *const _ as *const ffi::GstDeviceProviderClass;
+
+            let ptr = ffi::gst_device_provider_class_get_metadata(
+                mut_override(klass),
+                key.to_glib_none().0,
+            );
+
+            if ptr.is_null() {
+                None
+            } else {
+                Some(CStr::from_ptr(ptr).to_str().unwrap())
+            }
+        }
+    }
+}
+
+unsafe impl<T: IsA<DeviceProvider> + glib::object::IsClass> DeviceProviderClassExt
+    for glib::object::Class<T>
+{
+}

gstreamer/src/lib.rs

@@ -317,7 +317,7 @@ pub mod prelude {
         child_proxy::ChildProxyExtManual,
         clock::ClockExtManual,
         device_monitor::DeviceMonitorExtManual,
-        device_provider::DeviceProviderExtManual,
+        device_provider::{DeviceProviderClassExt, DeviceProviderExtManual},
         element::{ElementClassExt, ElementExtManual},
         format::prelude::*,
         gobject::GObjectExtManualGst,