Very simple click and dragging

src/bin/trees/main.rs

@@ -679,7 +679,8 @@ fn populate_tree(
     commands
         .entity(trigger.target())
         .insert((mesh, material, transform))
-        .observe(delete_tree);
+        .observe(delete_tree)
+        .observe(drag_tree);
 }
 
 fn hide_monologue_preview(
@@ -700,3 +701,23 @@ fn hide_monologue_preview(
         commands.entity(*preview).despawn_related::<Children>();
     }
 }
+
+fn drag_tree(
+    trigger: Trigger<Pointer<Drag>>,
+    mut query: Query<&mut Transform, With<Tree>>,
+    camera: Single<(&Camera, &GlobalTransform), With<Camera>>,
+    window: Single<&Window>
+) {
+    if let Ok(mut t) = query.get_mut(trigger.target()) {
+        let world_position = window
+            .cursor_position()
+            .and_then(|cursor| {
+                camera.0.viewport_to_world(camera.1, cursor).ok()
+            }).map(|ray| {
+                // Compute ray's distance to entity
+                let distance = ray.intersect_plane(t.translation, InfinitePlane3d::new(t.up())).unwrap();
+                ray.get_point(distance)
+            });
+        t.translation = world_position.unwrap();
+    }
+}