gtk4: add custom widget automatically updating the window size

Use it in the example and debug window but let's not make it public yet.
Plan is to have a proper bin on top of gtk4paintablesink at some point.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1680>

video/gtk4/examples/gtksink.rs

@@ -62,23 +62,8 @@ fn create_ui(app: &gtk::Application) {
 
     let vbox = gtk::Box::new(gtk::Orientation::Vertical, 0);
 
-    let picture = gtk::Picture::new();
-    picture.set_paintable(Some(&paintable));
-
-    #[cfg(feature = "gtk_v4_14")]
-    {
-        let offload = gtk::GraphicsOffload::new(Some(&picture));
-        offload.set_enabled(gtk::GraphicsOffloadEnabled::Enabled);
-        #[cfg(feature = "gtk_v4_16")]
-        {
-            offload.set_black_background(true);
-        }
-        vbox.append(&offload);
-    }
-    #[cfg(not(feature = "gtk_v4_14"))]
-    {
-        vbox.append(&picture);
-    }
+    let gst_widget = gstgtk4::RenderWidget::new(&gtksink);
+    vbox.append(&gst_widget);
 
     let label = gtk::Label::new(Some("Position: 00:00:00"));
     vbox.append(&label);

video/gtk4/src/lib.rs

@@ -21,6 +21,10 @@ mod sink;
 mod utils;
 pub use sink::frame::Orientation;
 pub use sink::paintable::Paintable;
+// The widget needs to be public so it can be used by the example and element debug window but
+// we don't want it be part of the official API for now.
+#[doc(hidden)]
+pub use sink::render_widget::RenderWidget;
 pub use sink::PaintableSink;
 
 fn plugin_init(plugin: &gst::Plugin) -> Result<(), glib::BoolError> {

video/gtk4/src/sink/imp.rs

@@ -831,29 +831,11 @@ impl PaintableSink {
                 return;
             }
 
-            let paintable = match &*self_.paintable.lock().unwrap() {
-                Some(paintable) => paintable.get_ref().clone(),
-                None => return,
-            };
-
             let window = gtk::Window::new();
-            let picture = gtk::Picture::new();
-            picture.set_paintable(Some(&paintable));
 
-            #[cfg(feature = "gtk_v4_14")]
-            {
-                let offload = gtk::GraphicsOffload::new(Some(&picture));
-                offload.set_enabled(gtk::GraphicsOffloadEnabled::Enabled);
-                #[cfg(feature = "gtk_v4_16")]
-                {
-                    offload.set_black_background(true);
-                }
-                window.set_child(Some(&offload));
-            }
-            #[cfg(not(feature = "gtk_v4_14"))]
-            {
-                window.set_child(Some(&picture));
-            }
+            let gst_widget = crate::RenderWidget::new(self_.obj().as_ref().upcast_ref());
+            window.set_child(Some(&gst_widget));
+
             window.set_default_size(640, 480);
             if std::env::var("GST_GTK4_WINDOW_FULLSCREEN").as_deref() == Ok("1") {
                 window.set_fullscreened(true);

video/gtk4/src/sink/mod.rs

@@ -41,6 +41,7 @@ use gtk::glib::prelude::*;
 pub(super) mod frame;
 pub(super) mod imp;
 pub(super) mod paintable;
+pub(super) mod render_widget;
 
 enum SinkEvent {
     FrameChanged,

video/gtk4/src/sink/render_widget/imp.rs

@@ -0,0 +1,113 @@
+//
+// Copyright (C) 2024 Guillaume Desmottes <guillaume@desmottes.be>
+//
+// This Source Code Form is subject to the terms of the Mozilla Public License, v2.0.
+// If a copy of the MPL was not distributed with this file, You can obtain one at
+// <https://mozilla.org/MPL/2.0/>.
+//
+// SPDX-License-Identifier: MPL-2.0
+
+use std::cell::{Cell, RefCell};
+
+use gtk::{gdk, glib, prelude::*, subclass::prelude::*};
+use once_cell::sync::Lazy;
+
+#[derive(Default)]
+pub struct RenderWidget {
+    element: RefCell<Option<crate::PaintableSink>>,
+    window_size: Cell<(u32, u32)>,
+}
+
+#[glib::object_subclass]
+impl ObjectSubclass for RenderWidget {
+    const NAME: &'static str = "GstGtk4ExampleRenderWidget";
+    type Type = super::RenderWidget;
+    type ParentType = gtk::Widget;
+
+    fn class_init(klass: &mut Self::Class) {
+        klass.set_layout_manager_type::<gtk::BinLayout>();
+    }
+}
+
+impl ObjectImpl for RenderWidget {
+    fn properties() -> &'static [glib::ParamSpec] {
+        static PROPERTIES: Lazy<Vec<glib::ParamSpec>> = Lazy::new(|| {
+            vec![
+                glib::ParamSpecObject::builder::<crate::PaintableSink>("element")
+                    .nick("Element")
+                    .blurb("The GTK4 Paintable Sink GStreamer element")
+                    .construct_only()
+                    .build(),
+            ]
+        });
+
+        PROPERTIES.as_ref()
+    }
+
+    fn property(&self, _id: usize, pspec: &glib::ParamSpec) -> glib::Value {
+        match pspec.name() {
+            "element" => self.element.borrow().to_value(),
+            _ => unimplemented!(),
+        }
+    }
+
+    fn set_property(&self, _id: usize, value: &glib::Value, pspec: &glib::ParamSpec) {
+        match pspec.name() {
+            "element" => {
+                *self.element.borrow_mut() = value.get::<Option<crate::PaintableSink>>().unwrap();
+            }
+
+            _ => unimplemented!(),
+        }
+    }
+
+    fn constructed(&self) {
+        self.parent_constructed();
+
+        let element = self.element.borrow();
+        let element = element.as_ref().unwrap();
+        let paintable = element.property::<gdk::Paintable>("paintable");
+
+        let picture = gtk::Picture::new();
+        picture.set_paintable(Some(&paintable));
+
+        #[cfg(feature = "gtk_v4_14")]
+        {
+            let offload = gtk::GraphicsOffload::new(Some(&picture));
+            offload.set_enabled(gtk::GraphicsOffloadEnabled::Enabled);
+            #[cfg(feature = "gtk_v4_16")]
+            {
+                offload.set_black_background(true);
+            }
+            offload.set_parent(self.obj().as_ref());
+        }
+        #[cfg(not(feature = "gtk_v4_14"))]
+        {
+            picture.set_parent(self.obj().as_ref());
+        }
+    }
+
+    fn dispose(&self) {
+        while let Some(child) = self.obj().first_child() {
+            child.unparent();
+        }
+    }
+}
+
+impl WidgetImpl for RenderWidget {
+    fn snapshot(&self, snapshot: &gtk::Snapshot) {
+        let window_width = self.obj().width() as u32;
+        let window_height = self.obj().height() as u32;
+        let new_size = (window_width, window_height);
+        let updated = self.window_size.replace(new_size) != new_size;
+
+        if updated {
+            let element = self.element.borrow();
+            let element = element.as_ref().unwrap();
+            element.set_property("window-width", window_width);
+            element.set_property("window-height", window_height);
+        }
+
+        self.parent_snapshot(snapshot)
+    }
+}

video/gtk4/src/sink/render_widget/mod.rs

@@ -0,0 +1,23 @@
+//
+// Copyright (C) 2024 Guillaume Desmottes <guillaume@desmottes.be>
+//
+// This Source Code Form is subject to the terms of the Mozilla Public License, v2.0.
+// If a copy of the MPL was not distributed with this file, You can obtain one at
+// <https://mozilla.org/MPL/2.0/>.
+//
+// SPDX-License-Identifier: MPL-2.0
+
+use gtk::glib;
+
+mod imp;
+
+/// Use a simple container widget to automatically pass the window size to gtk4paintablesink.
+glib::wrapper! {
+    pub struct RenderWidget(ObjectSubclass<imp::RenderWidget>) @extends gtk::Widget;
+}
+
+impl RenderWidget {
+    pub fn new(element: &gst::Element) -> Self {
+        glib::Object::builder().property("element", element).build()
+    }
+}