window: Use custom overlay visual for drop target

Closes #59

Part-of: <https://gitlab.gnome.org/GNOME/loupe/-/merge_requests/258>

src/widgets/drag_overlay.rs

@@ -0,0 +1,121 @@
+// Copyright (c) 2022  Maximiliano Sandoval R <msandova@gnome.org>
+// Copyright (c) 2023 FineFindus
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+//
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+use gtk::{glib, prelude::*};
+
+mod imp {
+    use std::cell::RefCell;
+
+    use adw::subclass::prelude::*;
+    use glib::{ParamSpec, Properties, Value};
+
+    use super::*;
+
+    #[derive(Debug, Default, Properties)]
+    #[properties(wrapper_type = super::LpDragOverlay)]
+    pub struct LpDragOverlay {
+        /// Usual content
+        #[property(set = Self::set_child)]
+        pub child: Option<gtk::Widget>,
+        /// Widget overplayed when dragging over child
+        #[property(set = Self::set_overlayed)]
+        pub overlayed: Option<gtk::Widget>,
+        pub overlay: gtk::Overlay,
+        pub revealer: gtk::Revealer,
+        #[property(set = Self::set_drop_target, get, explicit_notify)]
+        pub drop_target: RefCell<Option<gtk::DropTarget>>,
+        pub handler_id: RefCell<Option<glib::SignalHandlerId>>,
+    }
+
+    #[glib::object_subclass]
+    impl ObjectSubclass for LpDragOverlay {
+        const NAME: &'static str = "LpDragOverlay";
+        type Type = super::LpDragOverlay;
+        type ParentType = adw::Bin;
+    }
+
+    impl ObjectImpl for LpDragOverlay {
+        fn properties() -> &'static [ParamSpec] {
+            Self::derived_properties()
+        }
+
+        fn set_property(&self, id: usize, value: &Value, pspec: &ParamSpec) {
+            self.derived_set_property(id, value, pspec)
+        }
+
+        fn property(&self, id: usize, pspec: &ParamSpec) -> Value {
+            self.derived_property(id, pspec)
+        }
+
+        fn constructed(&self) {
+            self.overlay.set_parent(&*self.obj());
+            self.overlay.add_overlay(&self.revealer);
+
+            self.revealer.set_can_target(false);
+            self.revealer
+                .set_transition_type(gtk::RevealerTransitionType::Crossfade);
+            self.revealer.set_reveal_child(false);
+        }
+
+        fn dispose(&self) {
+            self.overlay.unparent();
+        }
+    }
+    impl WidgetImpl for LpDragOverlay {}
+    impl BinImpl for LpDragOverlay {}
+
+    impl LpDragOverlay {
+        pub fn set_child(&self, child: Option<gtk::Widget>) {
+            self.overlay.set_child(child.as_ref());
+        }
+
+        pub fn set_overlayed(&self, overlayed: Option<gtk::Widget>) {
+            self.revealer.set_child(overlayed.as_ref());
+        }
+
+        pub fn set_drop_target(&self, drop_target: &gtk::DropTarget) {
+            if let Some(target) = self.drop_target.borrow_mut().take() {
+                self.obj().remove_controller(&target);
+
+                if let Some(handler_id) = self.handler_id.borrow_mut().take() {
+                    target.disconnect(handler_id);
+                }
+            }
+
+            let handler_id = drop_target.connect_current_drop_notify(
+                glib::clone!(@weak self.revealer as revealer => move |target| {
+                    let reveal = target.current_drop().is_some();
+                    revealer.set_reveal_child(reveal);
+                }),
+            );
+            self.handler_id.replace(Some(handler_id));
+
+            // avoid `gtk_event_controller_get_widget (controller) == NULL` failing
+            if drop_target.widget().is_visible() {
+                self.obj().add_controller(drop_target.clone());
+                self.drop_target.replace(Some(drop_target.clone()));
+                self.obj().notify("drop-target");
+            }
+        }
+    }
+}
+
+glib::wrapper! {
+    pub struct LpDragOverlay(ObjectSubclass<imp::LpDragOverlay>)
+        @extends gtk::Widget, adw::Bin;
+}

src/widgets/mod.rs

@@ -16,6 +16,7 @@
 //
 // SPDX-License-Identifier: GPL-3.0-or-later
 
+mod drag_overlay;
 mod image;
 mod image_page;
 mod image_view;
@@ -24,6 +25,7 @@ mod print_preview;
 mod properties_view;
 mod sliding_view;
 
+pub use drag_overlay::LpDragOverlay;
 pub use image::LpImage;
 pub use image_page::LpImagePage;
 pub use image_view::LpImageView;