Implement drag source on images

Using a 128x128 thumbnail.

src/widgets/image.rs

@@ -210,4 +210,22 @@ impl LpImage {
         let imp = self.imp();
         imp.texture.borrow().clone()
     }
+
+    pub fn content_provider(&self) -> anyhow::Result<gdk::ContentProvider> {
+        let imp = self.imp();
+        let mut contents = vec![];
+
+        if let Some(file) = imp.file.borrow().as_ref() {
+            let content = gdk::ContentProvider::for_value(&file.to_value());
+            contents.push(content);
+        }
+
+        if let Some(texture) = imp.texture.borrow().as_ref() {
+            let bytes = texture.save_to_png_bytes();
+            let content = gdk::ContentProvider::for_bytes("image/png", &bytes);
+            contents.push(content);
+        }
+
+        Ok(gdk::ContentProvider::new_union(contents.as_slice()))
+    }
 }

src/widgets/image_view.rs

@@ -31,6 +31,7 @@ use ashpd::WindowIdentifier;
 use once_cell::sync::Lazy;
 use std::cell::{Cell, RefCell};
 
+use crate::thumbnail::Thumbnail;
 use crate::util;
 use crate::widgets::LpImage;
 
@@ -131,6 +132,30 @@ mod imp {
                 .connect_pressed(clone!(@weak obj => move |_, x, y| {
                     obj.show_popover_at(x, y);
                 }));
+
+            let source = gtk::DragSource::new();
+
+            source.connect_prepare(
+                glib::clone!(@weak obj => @default-return None, move |_, _, _| {
+                    match obj.imp().picture.content_provider() {
+                        Ok(content) => Some(content),
+                        Err(e) => {
+                            log::error!("Could not get content provider: {:?}", e);
+                            None
+                        }
+                    }
+                }),
+            );
+
+            source.connect_drag_begin(glib::clone!(@weak obj => move |source, _| {
+                let imp = obj.imp();
+                if let Some(texture) = imp.picture.texture() {
+                    let thumbnail = Thumbnail::new(&texture);
+                    source.set_icon(Some(&thumbnail), 0, 0);
+                };
+            }));
+
+            obj.add_controller(&source);
         }
     }