application: Add shortcuts for paning the view

Closes #54

data/gtk/help_overlay.ui

@@ -156,6 +156,35 @@
             </child>
           </object>
         </child>
+        <child>
+          <object class="GtkShortcutsGroup">
+            <property name="title" translatable="yes" context="shortcut window">Pan View</property>
+            <child>
+              <object class="GtkShortcutsShortcut">
+                <property name="title" translatable="yes" context="shortcut window">Pan Left</property>
+                <property name="action-name">win.pan-left</property>
+              </object>
+            </child>
+            <child>
+              <object class="GtkShortcutsShortcut">
+                <property name="title" translatable="yes" context="shortcut window">Pan Right</property>
+                <property name="action-name">win.pan-right</property>
+              </object>
+            </child>
+            <child>
+              <object class="GtkShortcutsShortcut">
+                <property name="title" translatable="yes" context="shortcut window">Pan Up</property>
+                <property name="action-name">win.pan-up</property>
+              </object>
+            </child>
+            <child>
+              <object class="GtkShortcutsShortcut">
+                <property name="title" translatable="yes" context="shortcut window">Pan Down</property>
+                <property name="action-name">win.pan-down</property>
+              </object>
+            </child>
+          </object>
+        </child>
         <child>
           <object class="GtkShortcutsGroup">
             <property name="title" translatable="yes" context="shortcut window">Gestures</property>

src/application.rs

@@ -195,6 +195,12 @@ impl LpApplication {
             "win.zoom-out",
             &["<Ctrl>minus", "minus", "<Ctrl>KP_Subtract", "KP_Subtract"],
         );
+
+        self.set_accels_for_action("win.pan-left", &["<Ctrl>Left"]);
+        self.set_accels_for_action("win.pan-right", &["<Ctrl>Right"]);
+        self.set_accels_for_action("win.pan-up", &["<Ctrl>Up"]);
+        self.set_accels_for_action("win.pan-down", &["<Ctrl>Down"]);
+
         self.set_accels_for_action("win.rotate(90.0)", &["<Ctrl>R"]);
         self.set_accels_for_action("win.rotate(-90.0)", &["<Ctrl><Shift>R"]);
 

src/widgets/image.rs

@@ -1053,6 +1053,35 @@ impl LpImage {
         }
     }
 
+    /// Stepwise scrolls inside an image when zoomed in
+    pub fn pan(&self, direction: &gtk::PanDirection) {
+        let sign = match direction {
+            gtk::PanDirection::Left | gtk::PanDirection::Up => -1.,
+            gtk::PanDirection::Right | gtk::PanDirection::Down => 1.,
+            _ => {
+                log::error!("Unkown pan direction {direction:?}");
+                return;
+            }
+        };
+
+        let (adjustment, max) = match direction {
+            gtk::PanDirection::Left | gtk::PanDirection::Right => {
+                (self.hadjustment(), self.max_hadjustment_value())
+            }
+            gtk::PanDirection::Up | gtk::PanDirection::Down => {
+                (self.vadjustment(), self.max_vadjustment_value())
+            }
+            _ => {
+                log::error!("Unkown pan direction {direction:?}");
+                return;
+            }
+        };
+
+        let value = (adjustment.value() + sign * adjustment.step_increment()).clamp(0., max);
+
+        adjustment.set_value(value);
+    }
+
     fn hadjustment(&self) -> gtk::Adjustment {
         if let Some(adj) = self.imp().hadjustment.borrow().as_ref() {
             adj.clone()
@@ -1109,7 +1138,7 @@ impl LpImage {
             0.,
             // upper
             content_width,
-            // arrow button step (probably irrelevant)
+            // arrow button and shortcut step
             widget_width * 0.1,
             // page up/down step
             widget_width * 0.9,
@@ -1128,7 +1157,7 @@ impl LpImage {
             0.,
             // upper
             content_height,
-            // arrow button step (probably irrelevant)
+            // arrow button and shortcut step
             widget_height * 0.1,
             // page up/down step
             widget_height * 0.9,

src/widgets/image_view.rs

@@ -408,6 +408,10 @@ impl LpImageView {
         }
     }
 
+    pub fn current_image(&self) -> Option<LpImage> {
+        self.imp().sliding_view.current_page().map(|x| x.image())
+    }
+
     pub fn current_page(&self) -> Option<LpImagePage> {
         self.imp().sliding_view.current_page()
     }

src/window.rs

@@ -125,6 +125,22 @@ mod imp {
                 win.zoom_best_fit();
             });
 
+            klass.install_action("win.pan-up", None, move |win, _, _| {
+                win.pan(&gtk::PanDirection::Up);
+            });
+
+            klass.install_action("win.pan-down", None, move |win, _, _| {
+                win.pan(&gtk::PanDirection::Down);
+            });
+
+            klass.install_action("win.pan-left", None, move |win, _, _| {
+                win.pan(&gtk::PanDirection::Left);
+            });
+
+            klass.install_action("win.pan-right", None, move |win, _, _| {
+                win.pan(&gtk::PanDirection::Right);
+            });
+
             klass.install_action("win.leave-fullscreen", None, move |win, _, _| {
                 win.toggle_fullscreen(false);
             });
@@ -367,6 +383,12 @@ impl LpWindow {
         }
     }
 
+    fn pan(&self, direction: &gtk::PanDirection) {
+        if let Some(image) = self.imp().image_view.current_image() {
+            image.pan(direction);
+        }
+    }
+
     async fn pick_file(&self) {
         let filter_store = gio::ListStore::new(gtk::FileFilter::static_type());