refactor and clean up app library

examples/app_library/group_grid/imp.rs

@@ -0,0 +1,41 @@
+use glib::subclass::Signal;
+use gtk4::subclass::prelude::*;
+use gtk4::{gio, glib, GridView, ScrolledWindow};
+use gtk4::{prelude::*, CustomFilter};
+use once_cell::sync::{Lazy, OnceCell};
+
+#[derive(Default)]
+pub struct GroupGrid {
+    pub group_grid_view: OnceCell<GridView>,
+    pub group_scroll_window: OnceCell<ScrolledWindow>,
+    pub group_model: OnceCell<gio::ListStore>,
+}
+
+#[glib::object_subclass]
+impl ObjectSubclass for GroupGrid {
+    // `NAME` needs to match `class` attribute of template
+    const NAME: &'static str = "GroupGrid";
+    type Type = super::GroupGrid;
+    type ParentType = gtk4::Box;
+}
+
+impl ObjectImpl for GroupGrid {
+    fn signals() -> &'static [Signal] {
+        static SIGNALS: Lazy<Vec<Signal>> = Lazy::new(|| {
+            vec![Signal::builder(
+                // Signal name
+                "group-changed",
+                // Types of the values which will be sent to the signal handler
+                &[CustomFilter::static_type().into()],
+                // Type of the value the signal handler sends back
+                <()>::static_type().into(),
+            )
+            .build()]
+        });
+        SIGNALS.as_ref()
+    }
+}
+
+impl WidgetImpl for GroupGrid {}
+
+impl BoxImpl for GroupGrid {}