Create the window adapter lazily in C++

api/cpp/include/slint_testing.h

@@ -21,21 +21,21 @@ template<typename Component>
 inline void send_mouse_click(const Component *component, float x, float y)
 {
     auto crc = *component->self_weak.into_dyn().lock();
-    cbindgen_private::slint_send_mouse_click(&crc, x, y, &component->m_window.window_handle());
+    cbindgen_private::slint_send_mouse_click(&crc, x, y, &component->window().window_handle());
 }
 
 template<typename Component>
 inline void send_keyboard_char(const Component *component, const slint::SharedString &str,
                                bool pressed)
 {
-    cbindgen_private::slint_send_keyboard_char(&str, pressed, &component->m_window.window_handle());
+    cbindgen_private::slint_send_keyboard_char(&str, pressed, &component->window().window_handle());
 }
 
 template<typename Component>
 inline void send_keyboard_string_sequence(const Component *component,
                                           const slint::SharedString &str)
 {
-    cbindgen_private::send_keyboard_string_sequence(&str, &component->m_window.window_handle());
+    cbindgen_private::send_keyboard_string_sequence(&str, &component->window().window_handle());
 }
 
 #define assert_eq(A, B)                                                                            \

api/cpp/lib.rs

@@ -29,6 +29,15 @@ pub unsafe extern "C" fn slint_windowrc_init(out: *mut WindowAdapterRcOpaque) {
     core::ptr::write(out as *mut Rc<dyn WindowAdapter>, win);
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn slint_ensure_backend() {
+    i_slint_backend_selector::with_platform(|_b| {
+        // Nothing to do, just make sure a backend was created
+        Ok(())
+    })
+    .unwrap()
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn slint_run_event_loop() {
     i_slint_backend_selector::with_platform(|b| b.run_event_loop()).unwrap();

internal/compiler/generator/cpp.rs

@@ -732,9 +732,8 @@ fn generate_public_component(file: &mut File, component: &llr::PublicComponent)
         // FIXME: many of the different component bindings need to access this
         Access::Public,
         Declaration::Var(Var {
-            ty: "slint::Window".into(),
+            ty: "std::optional<slint::Window>".into(),
             name: "m_window".into(),
-            init: Some("slint::Window{slint::private_api::WindowAdapterRc()}".into()),
             ..Default::default()
         }),
     ));
@@ -780,7 +779,7 @@ fn generate_public_component(file: &mut File, component: &llr::PublicComponent)
         Declaration::Function(Function {
             name: "show".into(),
             signature: "()".into(),
-            statements: Some(vec!["m_window.show();".into()]),
+            statements: Some(vec!["window().show();".into()]),
             ..Default::default()
         }),
     ));
@@ -790,7 +789,7 @@ fn generate_public_component(file: &mut File, component: &llr::PublicComponent)
         Declaration::Function(Function {
             name: "hide".into(),
             signature: "()".into(),
-            statements: Some(vec!["m_window.hide();".into()]),
+            statements: Some(vec!["window().hide();".into()]),
             ..Default::default()
         }),
     ));
@@ -800,10 +799,15 @@ fn generate_public_component(file: &mut File, component: &llr::PublicComponent)
         Declaration::Function(Function {
             name: "window".into(),
             signature: "() const -> slint::Window&".into(),
-            statements: Some(vec![format!(
+            statements: Some(vec![
-                "return const_cast<{} *>(this)->m_window;",
+                format!("auto self = const_cast<{} *>(this);", component_struct.name),
-                component_struct.name
+                "if (!m_window.has_value()) {".into(),
-            )]),
+                "   auto &window = self->m_window.emplace(slint::private_api::WindowAdapterRc());"
+                    .into(),
+                "   window.window_handle().set_component(*self);".into(),
+                "}".into(),
+                "return self->m_window.value();".into(),
+            ]),
             ..Default::default()
         }),
     ));
@@ -1225,15 +1229,12 @@ fn generate_item_tree(
     ];
 
     if parent_ctx.is_none() {
-        create_code.extend([format!(
+        create_code.push("slint::cbindgen_private::slint_ensure_backend();".into());
-            "{}->m_window.window_handle().set_component(*self_rc);",
-            root_access
-        )]);
     }
 
     create_code.extend([
         format!(
-            "{}->m_window.window_handle().register_component(self, self->item_array());",
+            "if (auto &window = {}->m_window) window->window_handle().register_component(self, self->item_array());",
             root_access
         ),
         format!("self->init({}, self->self_weak, 0, 1 {});", root_access, init_parent_parameters),
@@ -1266,7 +1267,7 @@ fn generate_item_tree(
     let mut destructor = vec!["auto self = this;".to_owned()];
 
     destructor.push(format!(
-        "{}->m_window.window_handle().unregister_component(self, item_array());",
+        "if (auto &window = {}->m_window) window->window_handle().unregister_component(self, item_array());",
         root_access
     ));
 

tests/cases/types/length.slint

@@ -47,7 +47,7 @@ assert(instance.get_test_zero());
 assert(instance.get_test());
 
 ratio = 2.;
-instance.m_window.window_handle().set_scale_factor(ratio);
+instance.window().window_handle().set_scale_factor(ratio);
 assert_eq(instance.get_l1(), 12.);
 assert_eq(instance.get_l2(), 12. * ratio);
 assert_eq(instance.get_l3(), 100. + 12. * ratio);

tests/cases/types/rem.slint

@@ -32,7 +32,7 @@ assert(instance.get_test());
 
 assert_eq(instance.get_phx_to_rem(), 2.);
 assert_eq(instance.get_px_to_rem(), 2.);
-instance.m_window.window_handle().set_scale_factor(2.0);
+instance.window().window_handle().set_scale_factor(2.0);
 assert_eq(instance.get_phys_pixel_size(), 20.);
 assert_eq(instance.get_phx_to_rem(), 1.);
 assert_eq(instance.get_px_to_rem(), 2.);