feat(gui): slideshow in the photo viewer

A new top-bar button, or F5, plays the loaded photos as a slideshow: it goes
fullscreen, hides the controls and moves on every five seconds. A video plays
to its end first, and the slideshow waits for each photo to load before it
starts counting. Stepping by hand restarts the count. Escape stops the
slideshow instead of closing the viewer, and fullscreen is left only if the
slideshow entered it. The slideshow ends on the last photo of the timeline.

crates/pdfs-gui/src/app/pages/photo_viewer.rs

@@ -9,6 +9,12 @@ const ZOOM_STEP: f64 = 1.25;
 /// The furthest the lightbox zooms in, as a multiple of the image's own pixels.
 const MAX_ZOOM: f64 = 8.0;
 
+/// How long a slideshow shows each photo. A video plays to its end instead.
+const SLIDESHOW_INTERVAL: Duration = Duration::from_secs(5);
+
+/// How often a slideshow checks whether the video on screen has finished.
+const SLIDESHOW_VIDEO_POLL: Duration = Duration::from_millis(500);
+
 /// How close to the end of the loaded photos stepping forward starts paging the
 /// next batch in, so reaching the end rarely means waiting for it.
 const PAGE_AHEAD: u32 = 10;
@@ -75,6 +81,12 @@ pub(crate) struct Viewer {
     pub(crate) chrome: Vec<gtk4::Revealer>,
     pub(crate) chrome_hover: Cell<bool>,
     pub(crate) chrome_source: RefCell<Option<glib::SourceId>>,
+    /// The slideshow toggle; the timer that moves it on; and whether it put
+    /// the window in fullscreen, so stopping it only leaves a fullscreen it
+    /// entered itself.
+    pub(crate) slideshow: gtk4::ToggleButton,
+    pub(crate) slide_source: RefCell<Option<glib::SourceId>>,
+    pub(crate) slide_fullscreened: Cell<bool>,
 }
 
 /// Camera/exposure/location facts pulled from a photo's own EXIF tags, as
@@ -220,6 +232,8 @@ pub(crate) fn load_photo(ui: &Rc<Ui>, viewer: &Rc<Viewer>, uid: String) {
     stop_video(viewer);
     viewer.media.set_visible_child_name("photo");
     reset_zoom(viewer);
+    // A slideshow waits for this photo to land before it starts counting.
+    disarm_slide(viewer);
 
     // Looked up before the group is refetched, which empties it.
     let item = viewer_item(ui, viewer, &uid);
@@ -244,6 +258,7 @@ pub(crate) fn load_photo(ui: &Rc<Ui>, viewer: &Rc<Viewer>, uid: String) {
         Request::OpenPhoto { uid: uid.clone() },
     );
     let viewer = viewer.clone();
+    let ui = ui.clone();
     glib::spawn_future_local(async move {
         let result = rx.recv().await;
         // The user may have moved on while this was in flight; that photo's own
@@ -254,6 +269,9 @@ pub(crate) fn load_photo(ui: &Rc<Ui>, viewer: &Rc<Viewer>, uid: String) {
         viewer.spinner.stop();
         viewer.spinner.set_visible(false);
         viewer.loading.set(false);
+        // Armed whether or not the photo rendered: a slideshow steps past a
+        // photo it could not show rather than stalling on it.
+        arm_slide(&ui, &viewer);
 
         let fail = |message: &str| {
             viewer.status.set_label(message);
@@ -295,6 +313,92 @@ pub(crate) fn load_photo(ui: &Rc<Ui>, viewer: &Rc<Viewer>, uid: String) {
     });
 }
 
+/// Start the slideshow: fullscreen, controls out of the way, and the photo on
+/// screen counted down once it has loaded.
+fn start_slideshow(ui: &Rc<Ui>, viewer: &Rc<Viewer>) {
+    if !viewer.window.is_fullscreen() {
+        viewer.window.fullscreen();
+        viewer.slide_fullscreened.set(true);
+    }
+    viewer
+        .slideshow
+        .set_icon_name("media-playback-pause-symbolic");
+    hide_chrome(viewer);
+    if !viewer.loading.get() {
+        arm_slide(ui, viewer);
+    }
+}
+
+/// Stop the slideshow, leaving fullscreen only if the slideshow entered it.
+fn stop_slideshow(viewer: &Rc<Viewer>) {
+    disarm_slide(viewer);
+    if viewer.slide_fullscreened.replace(false) {
+        viewer.window.unfullscreen();
+    }
+    viewer
+        .slideshow
+        .set_icon_name("media-playback-start-symbolic");
+    if viewer.slideshow.is_active() {
+        viewer.slideshow.set_active(false);
+    }
+}
+
+/// Move a running slideshow on from the photo on screen: after
+/// [`SLIDESHOW_INTERVAL`] for a still, once it has played through for a video.
+fn arm_slide(ui: &Rc<Ui>, viewer: &Rc<Viewer>) {
+    disarm_slide(viewer);
+    if !viewer.slideshow.is_active() {
+        return;
+    }
+    let playing_video = viewer.media.visible_child_name().as_deref() == Some("video");
+    let (ui, weak) = (ui.clone(), Rc::downgrade(viewer));
+    let source = if playing_video {
+        glib::timeout_add_local(SLIDESHOW_VIDEO_POLL, move || {
+            let Some(viewer) = weak.upgrade() else {
+                return glib::ControlFlow::Break;
+            };
+            let ended = viewer
+                .video
+                .media_stream()
+                .is_some_and(|stream| stream.is_ended());
+            if !ended {
+                return glib::ControlFlow::Continue;
+            }
+            // This timer is finishing, so its id must not be removed again.
+            viewer.slide_source.borrow_mut().take();
+            next_slide(&ui, &viewer);
+            glib::ControlFlow::Break
+        })
+    } else {
+        glib::timeout_add_local_once(SLIDESHOW_INTERVAL, move || {
+            let Some(viewer) = weak.upgrade() else {
+                return;
+            };
+            viewer.slide_source.borrow_mut().take();
+            next_slide(&ui, &viewer);
+        })
+    };
+    *viewer.slide_source.borrow_mut() = Some(source);
+}
+
+fn disarm_slide(viewer: &Rc<Viewer>) {
+    if let Some(source) = viewer.slide_source.borrow_mut().take() {
+        source.remove();
+    }
+}
+
+/// Step the slideshow forward, or end it at the last photo there is.
+fn next_slide(ui: &Rc<Ui>, viewer: &Rc<Viewer>) {
+    let model = &ui.gallery.model;
+    let last = find_photo_index(model, &viewer.uid.borrow())
+        .is_some_and(|index| index + 1 >= model.n_items());
+    if last && !ui.gallery.has_more.get() {
+        stop_slideshow(viewer);
+        return;
+    }
+    navigate_photo(ui, viewer, 1);
+}
+
 /// Stop whatever the player was playing, so stepping off a video (or closing
 /// the lightbox) doesn't leave its sound running.
 fn stop_video(viewer: &Rc<Viewer>) {
@@ -988,6 +1092,14 @@ pub(crate) fn open_photo_viewer(ui: &Rc<Ui>, initial_uid: String) {
     favorite_btn.add_css_class("flat");
     favorite_btn.add_css_class("viewer-action-btn");
 
+    let slideshow_btn = gtk4::ToggleButton::builder()
+        .icon_name("media-playback-start-symbolic")
+        .tooltip_text(gettext("Slideshow (F5)"))
+        .valign(gtk4::Align::Center)
+        .build();
+    slideshow_btn.add_css_class("flat");
+    slideshow_btn.add_css_class("viewer-action-btn");
+
     // Only shown for a shot stored as more than one file; `load_photo_group`
     // decides that per photo.
     let group_btn = action("view-paged-symbolic", &gettext("Other files of this photo"));
@@ -1005,6 +1117,7 @@ pub(crate) fn open_photo_viewer(ui: &Rc<Ui>, initial_uid: String) {
     top_bar.append(&group_btn);
     top_bar.append(&favorite_btn);
     top_bar.append(&info_toggle);
+    top_bar.append(&slideshow_btn);
     top_bar.append(&download_btn);
     top_bar.append(&delete_btn);
     top_bar.append(&open_ext_btn);
@@ -1131,6 +1244,9 @@ pub(crate) fn open_photo_viewer(ui: &Rc<Ui>, initial_uid: String) {
         chrome: vec![top_reveal.clone(), prev_reveal.clone(), next_reveal.clone()],
         chrome_hover: Cell::new(false),
         chrome_source: RefCell::new(None),
+        slideshow: slideshow_btn.clone(),
+        slide_source: RefCell::new(None),
+        slide_fullscreened: Cell::new(false),
     });
 
     let initial_idx = find_photo_index(&ui.gallery.model, &initial_uid).unwrap_or(0);
@@ -1244,6 +1360,7 @@ pub(crate) fn open_photo_viewer(ui: &Rc<Ui>, initial_uid: String) {
     let viewer_closing = viewer.clone();
     window.connect_close_request(move |_| {
         stop_video(&viewer_closing);
+        disarm_slide(&viewer_closing);
         if let Some(source) = viewer_closing.chrome_source.borrow_mut().take() {
             source.remove();
         }
@@ -1257,6 +1374,16 @@ pub(crate) fn open_photo_viewer(ui: &Rc<Ui>, initial_uid: String) {
             .set_reveal_child(toggle.is_active());
     });
 
+    let ui_slides = ui.clone();
+    let viewer_slides = viewer.clone();
+    slideshow_btn.connect_toggled(move |toggle| {
+        if toggle.is_active() {
+            start_slideshow(&ui_slides, &viewer_slides);
+        } else {
+            stop_slideshow(&viewer_slides);
+        }
+    });
+
     let toggle_off = info_toggle.clone();
     info_close.connect_clicked(move |_| toggle_off.set_active(false));
 
@@ -1426,6 +1553,13 @@ pub(crate) fn open_photo_viewer(ui: &Rc<Ui>, initial_uid: String) {
             Some("minus" | "KP_Subtract") => zoom_by(&viewer_key, 1.0 / ZOOM_STEP),
             Some("0" | "KP_0") => reset_zoom(&viewer_key),
             Some("1" | "KP_1") => zoom_to(&viewer_key, Some(1.0), None),
+            Some("F5") => viewer_key
+                .slideshow
+                .set_active(!viewer_key.slideshow.is_active()),
+            // Escape ends a slideshow before it closes anything.
+            Some("Escape") if viewer_key.slideshow.is_active() => {
+                viewer_key.slideshow.set_active(false)
+            }
             Some("i") => viewer_key
                 .info_toggle
                 .set_active(!viewer_key.info_toggle.is_active()),