gstreamer/subclass/

element.rs

// Take a look at the license at the top of the repository in the LICENSE file.

use std::{borrow::Cow, future::Future, sync::atomic};

use glib::{subclass::prelude::*, translate::*};

use super::prelude::*;
use crate::{
    ffi, prelude::*, Element, Event, PadTemplate, QueryRef, StateChange, StateChangeError,
    StateChangeReturn, StateChangeSuccess,
};

#[derive(Debug, Clone)]
pub struct ElementMetadata {
    long_name: Cow<'static, str>,
    classification: Cow<'static, str>,
    description: Cow<'static, str>,
    author: Cow<'static, str>,
    additional: Cow<'static, [(Cow<'static, str>, Cow<'static, str>)]>,
}

impl ElementMetadata {
    pub fn new(long_name: &str, classification: &str, description: &str, author: &str) -> Self {
        Self {
            long_name: Cow::Owned(long_name.into()),
            classification: Cow::Owned(classification.into()),
            description: Cow::Owned(description.into()),
            author: Cow::Owned(author.into()),
            additional: Cow::Borrowed(&[]),
        }
    }

    pub fn with_additional(
        long_name: &str,
        classification: &str,
        description: &str,
        author: &str,
        additional: &[(&str, &str)],
    ) -> Self {
        Self {
            long_name: Cow::Owned(long_name.into()),
            classification: Cow::Owned(classification.into()),
            description: Cow::Owned(description.into()),
            author: Cow::Owned(author.into()),
            additional: additional
                .iter()
                .copied()
                .map(|(key, value)| (Cow::Owned(key.into()), Cow::Owned(value.into())))
                .collect(),
        }
    }

    pub const fn with_cow(
        long_name: Cow<'static, str>,
        classification: Cow<'static, str>,
        description: Cow<'static, str>,
        author: Cow<'static, str>,
        additional: Cow<'static, [(Cow<'static, str>, Cow<'static, str>)]>,
    ) -> Self {
        Self {
            long_name,
            classification,
            description,
            author,
            additional,
        }
    }
}

pub trait ElementImpl: ElementImplExt + GstObjectImpl + Send + Sync {
    fn metadata() -> Option<&'static ElementMetadata> {
        None
    }

    fn pad_templates() -> &'static [PadTemplate] {
        &[]
    }

    /// Perform `transition` on `self`.
    ///
    /// This function must be called with STATE_LOCK held and is mainly used
    /// internally.
    /// ## `transition`
    /// the requested transition
    ///
    /// # Returns
    ///
    /// the [`StateChangeReturn`][crate::StateChangeReturn] of the state transition.
    fn change_state(
        &self,
        transition: StateChange,
    ) -> Result<StateChangeSuccess, StateChangeError> {
        self.parent_change_state(transition)
    }

    /// Retrieves a request pad from the element according to the provided template.
    /// Pad templates can be looked up using
    /// [`ElementFactory::static_pad_templates()`][crate::ElementFactory::static_pad_templates()].
    ///
    /// The pad should be released with [`ElementExt::release_request_pad()`][crate::prelude::ElementExt::release_request_pad()].
    /// ## `templ`
    /// a [`PadTemplate`][crate::PadTemplate] of which we want a pad of.
    /// ## `name`
    /// the name of the request [`Pad`][crate::Pad]
    /// to retrieve. Can be [`None`].
    /// ## `caps`
    /// the caps of the pad we want to
    /// request. Can be [`None`].
    ///
    /// # Returns
    ///
    /// requested [`Pad`][crate::Pad] if found,
    ///  otherwise [`None`]. Release after usage.
    fn request_new_pad(
        &self,
        templ: &crate::PadTemplate,
        name: Option<&str>,
        caps: Option<&crate::Caps>,
    ) -> Option<crate::Pad> {
        self.parent_request_new_pad(templ, name, caps)
    }

    /// called when a request pad is to be released
    fn release_pad(&self, pad: &crate::Pad) {
        self.parent_release_pad(pad)
    }

    /// Sends an event to an element. If the element doesn't implement an
    /// event handler, the event will be pushed on a random linked sink pad for
    /// downstream events or a random linked source pad for upstream events.
    ///
    /// This function takes ownership of the provided event so you should
    /// `gst_event_ref()` it if you want to reuse the event after this call.
    ///
    /// MT safe.
    /// ## `event`
    /// the [`Event`][crate::Event] to send to the element.
    ///
    /// # Returns
    ///
    /// [`true`] if the event was handled. Events that trigger a preroll (such
    /// as flushing seeks and steps) will emit `GST_MESSAGE_ASYNC_DONE`.
    fn send_event(&self, event: Event) -> bool {
        self.parent_send_event(event)
    }

    /// Performs a query on the given element.
    ///
    /// For elements that don't implement a query handler, this function
    /// forwards the query to a random srcpad or to the peer of a
    /// random linked sinkpad of this element.
    ///
    /// Please note that some queries might need a running pipeline to work.
    /// ## `query`
    /// the [`Query`][crate::Query].
    ///
    /// # Returns
    ///
    /// [`true`] if the query could be performed.
    ///
    /// MT safe.
    fn query(&self, query: &mut QueryRef) -> bool {
        self.parent_query(query)
    }

    /// Sets the context of the element. Increases the refcount of the context.
    ///
    /// MT safe.
    /// ## `context`
    /// the [`Context`][crate::Context] to set.
    fn set_context(&self, context: &crate::Context) {
        self.parent_set_context(context)
    }

    /// Sets the clock for the element. This function increases the
    /// refcount on the clock. Any previously set clock on the object
    /// is unreffed.
    /// ## `clock`
    /// the [`Clock`][crate::Clock] to set for the element.
    ///
    /// # Returns
    ///
    /// [`true`] if the element accepted the clock. An element can refuse a
    /// clock when it, for example, is not able to slave its internal clock to the
    /// `clock` or when it requires a specific clock to operate.
    ///
    /// MT safe.
    fn set_clock(&self, clock: Option<&crate::Clock>) -> bool {
        self.parent_set_clock(clock)
    }

    /// Get the clock provided by the given element.
    /// > An element is only required to provide a clock in the PAUSED
    /// > state. Some elements can provide a clock in other states.
    ///
    /// # Returns
    ///
    /// the GstClock provided by the
    /// element or [`None`] if no clock could be provided. Unref after usage.
    ///
    /// MT safe.
    fn provide_clock(&self) -> Option<crate::Clock> {
        self.parent_provide_clock()
    }

    /// Post a message on the element's [`Bus`][crate::Bus]. This function takes ownership of the
    /// message; if you want to access the message after this call, you should add an
    /// additional reference before calling.
    /// ## `message`
    /// a [`Message`][crate::Message] to post
    ///
    /// # Returns
    ///
    /// [`true`] if the message was successfully posted. The function returns
    /// [`false`] if the element did not have a bus.
    ///
    /// MT safe.
    fn post_message(&self, msg: crate::Message) -> bool {
        self.parent_post_message(msg)
    }
}

mod sealed {
    pub trait Sealed {}
    impl<T: super::ElementImplExt> Sealed for T {}
}

pub trait ElementImplExt: sealed::Sealed + ObjectSubclass {
    fn parent_change_state(
        &self,
        transition: StateChange,
    ) -> Result<StateChangeSuccess, StateChangeError> {
        unsafe {
            let data = Self::type_data();
            let parent_class = data.as_ref().parent_class() as *mut ffi::GstElementClass;

            let f = (*parent_class)
                .change_state
                .expect("Missing parent function `change_state`");
            try_from_glib(f(
                self.obj().unsafe_cast_ref::<Element>().to_glib_none().0,
                transition.into_glib(),
            ))
        }
    }

    fn parent_request_new_pad(
        &self,
        templ: &crate::PadTemplate,
        name: Option<&str>,
        caps: Option<&crate::Caps>,
    ) -> Option<crate::Pad> {
        unsafe {
            let data = Self::type_data();
            let parent_class = data.as_ref().parent_class() as *mut ffi::GstElementClass;

            (*parent_class)
                .request_new_pad
                .map(|f| {
                    from_glib_none(f(
                        self.obj().unsafe_cast_ref::<Element>().to_glib_none().0,
                        templ.to_glib_none().0,
                        name.to_glib_full(),
                        caps.to_glib_none().0,
                    ))
                })
                .unwrap_or(None)
        }
    }

    fn parent_release_pad(&self, pad: &crate::Pad) {
        unsafe {
            let data = Self::type_data();
            let parent_class = data.as_ref().parent_class() as *mut ffi::GstElementClass;

            (*parent_class)
                .release_pad
                .map(|f| {
                    f(
                        self.obj().unsafe_cast_ref::<Element>().to_glib_none().0,
                        pad.to_glib_none().0,
                    )
                })
                .unwrap_or(())
        }
    }

    fn parent_send_event(&self, event: Event) -> bool {
        unsafe {
            let data = Self::type_data();
            let parent_class = data.as_ref().parent_class() as *mut ffi::GstElementClass;

            (*parent_class)
                .send_event
                .map(|f| {
                    from_glib(f(
                        self.obj().unsafe_cast_ref::<Element>().to_glib_none().0,
                        event.into_glib_ptr(),
                    ))
                })
                .unwrap_or(false)
        }
    }

    fn parent_query(&self, query: &mut QueryRef) -> bool {
        unsafe {
            let data = Self::type_data();
            let parent_class = data.as_ref().parent_class() as *mut ffi::GstElementClass;

            (*parent_class)
                .query
                .map(|f| {
                    from_glib(f(
                        self.obj().unsafe_cast_ref::<Element>().to_glib_none().0,
                        query.as_mut_ptr(),
                    ))
                })
                .unwrap_or(false)
        }
    }

    fn parent_set_context(&self, context: &crate::Context) {
        unsafe {
            let data = Self::type_data();
            let parent_class = data.as_ref().parent_class() as *mut ffi::GstElementClass;

            (*parent_class)
                .set_context
                .map(|f| {
                    f(
                        self.obj().unsafe_cast_ref::<Element>().to_glib_none().0,
                        context.to_glib_none().0,
                    )
                })
                .unwrap_or(())
        }
    }

    fn parent_set_clock(&self, clock: Option<&crate::Clock>) -> bool {
        unsafe {
            let data = Self::type_data();
            let parent_class = data.as_ref().parent_class() as *mut ffi::GstElementClass;

            (*parent_class)
                .set_clock
                .map(|f| {
                    from_glib(f(
                        self.obj().unsafe_cast_ref::<Element>().to_glib_none().0,
                        clock.to_glib_none().0,
                    ))
                })
                .unwrap_or(false)
        }
    }

    fn parent_provide_clock(&self) -> Option<crate::Clock> {
        unsafe {
            let data = Self::type_data();
            let parent_class = data.as_ref().parent_class() as *mut ffi::GstElementClass;

            (*parent_class)
                .provide_clock
                .map(|f| {
                    from_glib_none(f(self.obj().unsafe_cast_ref::<Element>().to_glib_none().0))
                })
                .unwrap_or(None)
        }
    }

    fn parent_post_message(&self, msg: crate::Message) -> bool {
        unsafe {
            let data = Self::type_data();
            let parent_class = data.as_ref().parent_class() as *mut ffi::GstElementClass;

            if let Some(f) = (*parent_class).post_message {
                from_glib(f(
                    self.obj().unsafe_cast_ref::<Element>().to_glib_none().0,
                    msg.into_glib_ptr(),
                ))
            } else {
                false
            }
        }
    }

    #[inline(never)]
    fn panicked(&self) -> &atomic::AtomicBool {
        #[cfg(panic = "abort")]
        {
            static DUMMY: atomic::AtomicBool = atomic::AtomicBool::new(false);
            &DUMMY
        }
        #[cfg(not(panic = "abort"))]
        {
            self.instance_data::<atomic::AtomicBool>(crate::Element::static_type())
                .expect("instance not initialized correctly")
        }
    }

    fn catch_panic<R, F: FnOnce(&Self) -> R, G: FnOnce() -> R>(&self, fallback: G, f: F) -> R {
        panic_to_error!(self, fallback(), { f(self) })
    }

    fn catch_panic_future<R, F: FnOnce() -> R, G: Future<Output = R>>(
        &self,
        fallback: F,
        fut: G,
    ) -> CatchPanic<Self, F, G> {
        CatchPanic {
            self_: self.ref_counted().downgrade(),
            fallback: Some(fallback),
            fut,
        }
    }

    fn catch_panic_pad_function<R, F: FnOnce(&Self) -> R, G: FnOnce() -> R>(
        parent: Option<&crate::Object>,
        fallback: G,
        f: F,
    ) -> R {
        let element = parent.unwrap().dynamic_cast_ref::<Self::Type>().unwrap();
        let imp = element.imp();

        panic_to_error!(imp, fallback(), { f(imp) })
    }

    fn post_error_message(&self, msg: crate::ErrorMessage) {
        unsafe {
            self.obj()
                .unsafe_cast_ref::<Element>()
                .post_error_message(msg)
        }
    }
}

impl<T: ElementImpl> ElementImplExt for T {}

pin_project_lite::pin_project! {
    #[must_use = "futures do nothing unless you `.await` or poll them"]
    pub struct CatchPanic<T: glib::subclass::types::ObjectSubclass, F, G> {
        self_: glib::subclass::ObjectImplWeakRef<T>,
        fallback: Option<F>,
        #[pin]
        fut: G,
    }
}

impl<R, T: ElementImpl, F: FnOnce() -> R, G: Future<Output = R>> Future for CatchPanic<T, F, G> {
    type Output = R;

    fn poll(
        self: std::pin::Pin<&mut Self>,
        cx: &mut std::task::Context<'_>,
    ) -> std::task::Poll<Self::Output> {
        let this = self.project();

        let Some(self_) = this.self_.upgrade() else {
            return std::task::Poll::Ready((this
                .fallback
                .take()
                .expect("Future polled after resolving"))(
            ));
        };

        panic_to_error!(
            &*self_,
            std::task::Poll::Ready(this.fallback.take().expect("Future polled after resolving")()),
            {
                let fut = this.fut;
                fut.poll(cx)
            }
        )
    }
}

unsafe impl<T: ElementImpl> IsSubclassable<T> for Element {
    fn class_init(klass: &mut glib::Class<Self>) {
        Self::parent_class_init::<T>(klass);
        let klass = klass.as_mut();
        klass.change_state = Some(element_change_state::<T>);
        klass.request_new_pad = Some(element_request_new_pad::<T>);
        klass.release_pad = Some(element_release_pad::<T>);
        klass.send_event = Some(element_send_event::<T>);
        klass.query = Some(element_query::<T>);
        klass.set_context = Some(element_set_context::<T>);
        klass.set_clock = Some(element_set_clock::<T>);
        klass.provide_clock = Some(element_provide_clock::<T>);
        klass.post_message = Some(element_post_message::<T>);

        unsafe {
            for pad_template in T::pad_templates() {
                ffi::gst_element_class_add_pad_template(klass, pad_template.to_glib_none().0);
            }

            if let Some(metadata) = T::metadata() {
                ffi::gst_element_class_set_metadata(
                    klass,
                    metadata.long_name.to_glib_none().0,
                    metadata.classification.to_glib_none().0,
                    metadata.description.to_glib_none().0,
                    metadata.author.to_glib_none().0,
                );

                for (key, value) in &metadata.additional[..] {
                    ffi::gst_element_class_add_metadata(
                        klass,
                        key.to_glib_none().0,
                        value.to_glib_none().0,
                    );
                }
            }
        }
    }

    fn instance_init(instance: &mut glib::subclass::InitializingObject<T>) {
        Self::parent_instance_init::<T>(instance);

        #[cfg(not(panic = "abort"))]
        instance.set_instance_data(Self::static_type(), atomic::AtomicBool::new(false));
    }
}

unsafe extern "C" fn element_change_state<T: ElementImpl>(
    ptr: *mut ffi::GstElement,
    transition: ffi::GstStateChange,
) -> ffi::GstStateChangeReturn {
    let instance = &*(ptr as *mut T::Instance);
    let imp = instance.imp();

    // *Never* fail downwards state changes, this causes bugs in GStreamer
    // and leads to crashes and deadlocks.
    let transition = from_glib(transition);
    let fallback = match transition {
        StateChange::PlayingToPaused | StateChange::PausedToReady | StateChange::ReadyToNull => {
            StateChangeReturn::Success
        }
        _ => StateChangeReturn::Failure,
    };

    panic_to_error!(imp, fallback, {
        StateChangeReturn::from(imp.change_state(transition))
    })
    .into_glib()
}

unsafe extern "C" fn element_request_new_pad<T: ElementImpl>(
    ptr: *mut ffi::GstElement,
    templ: *mut ffi::GstPadTemplate,
    name: *const libc::c_char,
    caps: *const ffi::GstCaps,
) -> *mut ffi::GstPad {
    let instance = &*(ptr as *mut T::Instance);
    let imp = instance.imp();

    let caps = Option::<crate::Caps>::from_glib_borrow(caps);
    let name = Option::<String>::from_glib_none(name);

    // XXX: This is effectively unsafe but the best we can do
    // See https://bugzilla.gnome.org/show_bug.cgi?id=791193
    let pad = panic_to_error!(imp, None, {
        imp.request_new_pad(
            &from_glib_borrow(templ),
            name.as_deref(),
            caps.as_ref().as_ref(),
        )
    });

    // Ensure that the pad is owned by the element now, if a pad was returned
    if let Some(ref pad) = pad {
        assert_eq!(
            pad.parent().as_ref(),
            Some(&*crate::Object::from_glib_borrow(
                ptr as *mut ffi::GstObject
            ))
        );
    }

    pad.to_glib_none().0
}

unsafe extern "C" fn element_release_pad<T: ElementImpl>(
    ptr: *mut ffi::GstElement,
    pad: *mut ffi::GstPad,
) {
    let instance = &*(ptr as *mut T::Instance);
    let imp = instance.imp();

    // If we get a floating reference passed simply return here. It can't be stored inside this
    // element, and if we continued to use it we would take ownership of this floating reference.
    if glib::gobject_ffi::g_object_is_floating(pad as *mut glib::gobject_ffi::GObject)
        != glib::ffi::GFALSE
    {
        return;
    }

    panic_to_error!(imp, (), { imp.release_pad(&from_glib_none(pad)) })
}

unsafe extern "C" fn element_send_event<T: ElementImpl>(
    ptr: *mut ffi::GstElement,
    event: *mut ffi::GstEvent,
) -> glib::ffi::gboolean {
    let instance = &*(ptr as *mut T::Instance);
    let imp = instance.imp();

    panic_to_error!(imp, false, { imp.send_event(from_glib_full(event)) }).into_glib()
}

unsafe extern "C" fn element_query<T: ElementImpl>(
    ptr: *mut ffi::GstElement,
    query: *mut ffi::GstQuery,
) -> glib::ffi::gboolean {
    let instance = &*(ptr as *mut T::Instance);
    let imp = instance.imp();
    let query = QueryRef::from_mut_ptr(query);

    panic_to_error!(imp, false, { imp.query(query) }).into_glib()
}

unsafe extern "C" fn element_set_context<T: ElementImpl>(
    ptr: *mut ffi::GstElement,
    context: *mut ffi::GstContext,
) {
    let instance = &*(ptr as *mut T::Instance);
    let imp = instance.imp();

    panic_to_error!(imp, (), { imp.set_context(&from_glib_borrow(context)) })
}

unsafe extern "C" fn element_set_clock<T: ElementImpl>(
    ptr: *mut ffi::GstElement,
    clock: *mut ffi::GstClock,
) -> glib::ffi::gboolean {
    let instance = &*(ptr as *mut T::Instance);
    let imp = instance.imp();

    let clock = Option::<crate::Clock>::from_glib_borrow(clock);

    panic_to_error!(imp, false, { imp.set_clock(clock.as_ref().as_ref()) }).into_glib()
}

unsafe extern "C" fn element_provide_clock<T: ElementImpl>(
    ptr: *mut ffi::GstElement,
) -> *mut ffi::GstClock {
    let instance = &*(ptr as *mut T::Instance);
    let imp = instance.imp();

    panic_to_error!(imp, None, { imp.provide_clock() }).into_glib_ptr()
}

unsafe extern "C" fn element_post_message<T: ElementImpl>(
    ptr: *mut ffi::GstElement,
    msg: *mut ffi::GstMessage,
) -> glib::ffi::gboolean {
    let instance = &*(ptr as *mut T::Instance);
    let imp = instance.imp();

    // Can't catch panics here as posting the error message would cause
    // this code to be called again recursively forever.
    imp.post_message(from_glib_full(msg)).into_glib()
}

#[cfg(test)]
mod tests {
    use std::sync::{atomic, Arc, Mutex, OnceLock};

    use super::*;
    use crate::ElementFactory;

    pub mod imp {
        use super::*;

        pub struct TestElement {
            pub(super) srcpad: crate::Pad,
            pub(super) sinkpad: crate::Pad,
            pub(super) n_buffers: atomic::AtomicU32,
            pub(super) reached_playing: atomic::AtomicBool,
            pub(super) array: Arc<Mutex<Vec<String>>>,
        }

        impl TestElement {
            fn sink_chain(
                &self,
                _pad: &crate::Pad,
                buffer: crate::Buffer,
            ) -> Result<crate::FlowSuccess, crate::FlowError> {
                self.n_buffers.fetch_add(1, atomic::Ordering::SeqCst);
                self.srcpad.push(buffer)
            }

            fn sink_event(&self, _pad: &crate::Pad, event: crate::Event) -> bool {
                self.srcpad.push_event(event)
            }

            fn sink_query(&self, _pad: &crate::Pad, query: &mut crate::QueryRef) -> bool {
                self.srcpad.peer_query(query)
            }

            fn src_event(&self, _pad: &crate::Pad, event: crate::Event) -> bool {
                self.sinkpad.push_event(event)
            }

            fn src_query(&self, _pad: &crate::Pad, query: &mut crate::QueryRef) -> bool {
                self.sinkpad.peer_query(query)
            }
        }

        #[glib::object_subclass]
        impl ObjectSubclass for TestElement {
            const NAME: &'static str = "TestElement";
            type Type = super::TestElement;
            type ParentType = Element;

            fn with_class(klass: &Self::Class) -> Self {
                let templ = klass.pad_template("sink").unwrap();
                let sinkpad = crate::Pad::builder_from_template(&templ)
                    .chain_function(|pad, parent, buffer| {
                        TestElement::catch_panic_pad_function(
                            parent,
                            || Err(crate::FlowError::Error),
                            |identity| identity.sink_chain(pad, buffer),
                        )
                    })
                    .event_function(|pad, parent, event| {
                        TestElement::catch_panic_pad_function(
                            parent,
                            || false,
                            |identity| identity.sink_event(pad, event),
                        )
                    })
                    .query_function(|pad, parent, query| {
                        TestElement::catch_panic_pad_function(
                            parent,
                            || false,
                            |identity| identity.sink_query(pad, query),
                        )
                    })
                    .build();

                let templ = klass.pad_template("src").unwrap();
                let srcpad = crate::Pad::builder_from_template(&templ)
                    .event_function(|pad, parent, event| {
                        TestElement::catch_panic_pad_function(
                            parent,
                            || false,
                            |identity| identity.src_event(pad, event),
                        )
                    })
                    .query_function(|pad, parent, query| {
                        TestElement::catch_panic_pad_function(
                            parent,
                            || false,
                            |identity| identity.src_query(pad, query),
                        )
                    })
                    .build();

                Self {
                    n_buffers: atomic::AtomicU32::new(0),
                    reached_playing: atomic::AtomicBool::new(false),
                    array: Arc::new(Mutex::new(vec![
                        "default0".to_string(),
                        "default1".to_string(),
                    ])),
                    srcpad,
                    sinkpad,
                }
            }
        }

        impl ObjectImpl for TestElement {
            fn constructed(&self) {
                self.parent_constructed();

                let element = self.obj();
                element.add_pad(&self.sinkpad).unwrap();
                element.add_pad(&self.srcpad).unwrap();
            }

            fn properties() -> &'static [glib::ParamSpec] {
                static PROPERTIES: OnceLock<Vec<glib::ParamSpec>> = OnceLock::new();
                PROPERTIES.get_or_init(|| vec![crate::ParamSpecArray::builder("array").build()])
            }

            fn set_property(&self, _id: usize, value: &glib::Value, pspec: &glib::ParamSpec) {
                match pspec.name() {
                    "array" => {
                        let value = value.get::<crate::Array>().unwrap();
                        let mut array = self.array.lock().unwrap();
                        array.clear();
                        array.extend(value.iter().map(|v| v.get().unwrap()));
                    }
                    _ => unimplemented!(),
                }
            }

            fn property(&self, _id: usize, pspec: &glib::ParamSpec) -> glib::Value {
                match pspec.name() {
                    "array" => crate::Array::new(&*self.array.lock().unwrap()).to_value(),
                    _ => unimplemented!(),
                }
            }
        }

        impl GstObjectImpl for TestElement {}

        impl ElementImpl for TestElement {
            fn metadata() -> Option<&'static ElementMetadata> {
                static ELEMENT_METADATA: std::sync::OnceLock<ElementMetadata> =
                    std::sync::OnceLock::new();

                Some(ELEMENT_METADATA.get_or_init(|| {
                    ElementMetadata::new(
                        "Test Element",
                        "Generic",
                        "Does nothing",
                        "Sebastian Dröge <sebastian@centricular.com>",
                    )
                }))
            }

            fn pad_templates() -> &'static [PadTemplate] {
                static PAD_TEMPLATES: std::sync::OnceLock<Vec<PadTemplate>> =
                    std::sync::OnceLock::new();

                PAD_TEMPLATES.get_or_init(|| {
                    let caps = crate::Caps::new_any();
                    vec![
                        PadTemplate::new(
                            "src",
                            crate::PadDirection::Src,
                            crate::PadPresence::Always,
                            &caps,
                        )
                        .unwrap(),
                        PadTemplate::new(
                            "sink",
                            crate::PadDirection::Sink,
                            crate::PadPresence::Always,
                            &caps,
                        )
                        .unwrap(),
                    ]
                })
            }

            fn change_state(
                &self,
                transition: crate::StateChange,
            ) -> Result<crate::StateChangeSuccess, crate::StateChangeError> {
                let res = self.parent_change_state(transition)?;

                if transition == crate::StateChange::PausedToPlaying {
                    self.reached_playing.store(true, atomic::Ordering::SeqCst);
                }

                Ok(res)
            }
        }
    }

    glib::wrapper! {
        pub struct TestElement(ObjectSubclass<imp::TestElement>) @extends Element, crate::Object;
    }

    impl TestElement {
        pub fn new(name: Option<&str>) -> Self {
            glib::Object::builder().property("name", name).build()
        }
    }

    fn plugin_init(plugin: &crate::Plugin) -> Result<(), glib::BoolError> {
        crate::Element::register(
            Some(plugin),
            "testelement",
            crate::Rank::MARGINAL,
            TestElement::static_type(),
        )
    }

    crate::plugin_define!(
        rssubclasstestelem,
        env!("CARGO_PKG_DESCRIPTION"),
        plugin_init,
        env!("CARGO_PKG_VERSION"),
        "MPL-2.0",
        env!("CARGO_PKG_NAME"),
        env!("CARGO_PKG_NAME"),
        env!("CARGO_PKG_REPOSITORY"),
        "1970-01-01"
    );

    fn init() {
        use std::sync::Once;
        static INIT: Once = Once::new();

        INIT.call_once(|| {
            crate::init().unwrap();
            plugin_register_static().expect("gstreamer subclass element test");
        });
    }

    #[test]
    fn test_element_subclass() {
        init();

        let element = TestElement::new(Some("test"));

        assert_eq!(element.name(), "test");

        assert_eq!(
            element.metadata(crate::ELEMENT_METADATA_LONGNAME),
            Some("Test Element")
        );

        let pipeline = crate::Pipeline::new();
        let src = ElementFactory::make("fakesrc")
            .property("num-buffers", 100i32)
            .build()
            .unwrap();
        let sink = ElementFactory::make("fakesink").build().unwrap();

        pipeline
            .add_many([&src, element.upcast_ref(), &sink])
            .unwrap();
        Element::link_many([&src, element.upcast_ref(), &sink]).unwrap();

        pipeline.set_state(crate::State::Playing).unwrap();
        let bus = pipeline.bus().unwrap();

        let eos = bus.timed_pop_filtered(crate::ClockTime::NONE, &[crate::MessageType::Eos]);
        assert!(eos.is_some());

        pipeline.set_state(crate::State::Null).unwrap();

        let imp = element.imp();
        assert_eq!(imp.n_buffers.load(atomic::Ordering::SeqCst), 100);
        assert!(imp.reached_playing.load(atomic::Ordering::SeqCst));
    }

    #[test]
    fn property_from_iter_if_not_empty() {
        init();

        let elem = crate::ElementFactory::make("testelement").build().unwrap();
        assert!(elem
            .property::<crate::Array>("array")
            .iter()
            .map(|val| val.get::<&str>().unwrap())
            .eq(["default0", "default1"]));

        let elem = crate::ElementFactory::make("testelement")
            .property_from_iter::<crate::Array>("array", ["value0", "value1"])
            .build()
            .unwrap();
        assert!(elem
            .property::<crate::Array>("array")
            .iter()
            .map(|val| val.get::<&str>().unwrap())
            .eq(["value0", "value1"]));

        let array = Vec::<String>::new();
        let elem = crate::ElementFactory::make("testelement")
            .property_if_not_empty::<crate::Array>("array", &array)
            .build()
            .unwrap();
        assert!(elem
            .property::<crate::Array>("array")
            .iter()
            .map(|val| val.get::<&str>().unwrap())
            .eq(["default0", "default1"]));

        let elem = crate::ElementFactory::make("testelement")
            .property_if_not_empty::<crate::Array>("array", ["value0", "value1"])
            .build()
            .unwrap();
        assert!(elem
            .property::<crate::Array>("array")
            .iter()
            .map(|val| val.get::<&str>().unwrap())
            .eq(["value0", "value1"]));
    }
}