Reduce the amount of re-creation of cells in repeaters when the model changes (#1954)

Re-use the cells but mark them as dirty, instead of re-creating them every time.
In the included test-case that provides behavior that's
more intuitive.

api/cpp/include/slint.h

@@ -1299,7 +1299,14 @@ public:
     void ensure_updated(const Parent *parent) const
     {
         if (model.is_dirty()) {
+            auto preserved_data = inner ? std::make_optional(std::move(inner->data)) : std::nullopt;
             inner = std::make_shared<RepeaterInner>();
+            if (auto data = preserved_data) {
+                inner->data = std::move(*data);
+                for (auto &&compo_with_state : inner->data) {
+                    compo_with_state.state = RepeaterInner::State::Dirty;
+                }
+            }
             if (auto m = model.get()) {
                 m->attach_peer(inner);
             }

internal/core/model.rs

@@ -773,7 +773,13 @@ impl<C: RepeatedComponent + 'static> Repeater<C> {
         let model = self.data().project_ref().model;
 
         if model.is_dirty() {
-            *self.data().inner.borrow_mut() = RepeaterInner::default();
+            self.data()
+                .inner
+                .borrow_mut()
+                .components
+                .iter_mut()
+                .for_each(|c| c.0 = RepeatedComponentState::Dirty);
+
             self.data().is_dirty.set(true);
             let m = model.get();
             let peer = self.project_ref().0.model_peer();

tests/cases/models/model.slint

@@ -72,7 +72,8 @@ assert_eq!(instance.get_clicked_index(), 2);
 slint_testing::send_mouse_click(&instance, 15., 5.);
 assert_eq!(instance.get_clicked_score(), 222000);
 assert_eq!(instance.get_clicked_name(), slint::SharedString::from("cruel"));
-assert_eq!(instance.get_clicked_internal_state(), 1);
+// Rectangle was re-used!
+assert_eq!(instance.get_clicked_internal_state(), 2);
 
 another_model.push(("a4".into(), "!".into(), 444.));
 slint_testing::send_mouse_click(&instance, 35., 5.);
@@ -85,7 +86,8 @@ another_model.set_row_data(1, ("a2".into(), "idyllic".into(), 555.));
 slint_testing::send_mouse_click(&instance, 15., 5.);
 assert_eq!(instance.get_clicked_score(), 555000);
 assert_eq!(instance.get_clicked_name(), slint::SharedString::from("idyllic"));
-assert_eq!(instance.get_clicked_internal_state(), 2);
+// Rectangle was re-used!
+assert_eq!(instance.get_clicked_internal_state(), 3);
 assert_eq!(instance.get_clicked_index(), 1);
 
 another_model.remove(1);
@@ -126,7 +128,8 @@ assert_eq(instance.get_clicked_index(), 2);
 slint_testing::send_mouse_click(&instance, 15., 5.);
 assert_eq(instance.get_clicked_score(), 222000);
 assert_eq(instance.get_clicked_name(), "cruel");
-assert_eq(instance.get_clicked_internal_state(), 1);
+// Rectangle was re-used!
+assert_eq(instance.get_clicked_internal_state(), 2);
 
 another_model->push_back({"a4", "!", 444.});
 slint_testing::send_mouse_click(&instance, 35., 5.);
@@ -138,7 +141,8 @@ another_model->set_row_data(1, {"a2", "idyllic", 555.});
 slint_testing::send_mouse_click(&instance, 15., 5.);
 assert_eq(instance.get_clicked_score(), 555000);
 assert_eq(instance.get_clicked_name(), "idyllic");
-assert_eq(instance.get_clicked_internal_state(), 2);
+// Rectangle was re-used!
+assert_eq(instance.get_clicked_internal_state(), 3);
 assert_eq(instance.get_clicked_index(), 1);
 
 another_model->erase(1);
@@ -179,7 +183,8 @@ assert.equal(instance.clicked_index, 2);
 instance.send_mouse_click(15., 5.);
 assert.equal(instance.clicked_score, 222000);
 assert.equal(instance.clicked_name, "cruel");
-assert.equal(instance.clicked_internal_state, 1);
+// Rectangle was re-used!
+assert.equal(instance.clicked_internal_state, 2);
 
 another_model.push({account: "a4", name: "!", score: 444.});
 instance.send_mouse_click(35., 5.);
@@ -191,7 +196,8 @@ another_model.setRowData(1, {account: "a2", name: "idyllic", score: 555.});
 instance.send_mouse_click(15., 5.);
 assert.equal(instance.clicked_score, 555000);
 assert.equal(instance.clicked_name, "idyllic");
-assert.equal(instance.clicked_internal_state, 2);
+// Rectangle was re-used!
+assert.equal(instance.clicked_internal_state, 3);
 assert.equal(instance.clicked_index, 1);
 
 another_model.remove(1, 1);

tests/cases/models/repeater_model_change.slint

@@ -0,0 +1,100 @@
+// Copyright © SixtyFPS GmbH <info@slint-ui.com>
+// SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-commercial
+
+
+
+TestCase := Rectangle {
+    width: 100phx;
+    height: 100phx;
+
+    property <int> creation-count: 0;
+    property <bool> condition;
+    property <length> test-width: preferred-width;
+
+    HorizontalLayout {
+        if condition: Rectangle {
+            preferred-width: 10px;
+            init => {
+                creation-count += 1;
+            }
+        }
+    }
+
+}
+
+
+/*
+```cpp
+auto handle = TestCase::create();
+const TestCase &instance = *handle;
+
+assert_eq(instance.get_test_width(), 0.);
+assert_eq(instance.get_creation_count(), 0);
+
+instance.set_condition(true);
+assert_eq(instance.get_test_width(), 10.);
+assert_eq(instance.get_creation_count(), 1);
+
+instance.set_condition(false);
+assert_eq(instance.get_test_width(), 0.);
+assert_eq(instance.get_creation_count(), 1);
+
+instance.set_condition(true);
+assert_eq(instance.get_test_width(), 10.);
+assert_eq(instance.get_creation_count(), 2);
+
+instance.set_condition(false);
+instance.set_condition(true);
+assert_eq(instance.get_test_width(), 10.);
+assert_eq(instance.get_creation_count(), 2);
+```
+
+
+```rust
+let instance = TestCase::new();
+
+assert_eq!(instance.get_test_width(), 0.);
+assert_eq!(instance.get_creation_count(), 0);
+
+instance.set_condition(true);
+assert_eq!(instance.get_test_width(), 10.);
+assert_eq!(instance.get_creation_count(), 1);
+
+instance.set_condition(false);
+assert_eq!(instance.get_test_width(), 0.);
+assert_eq!(instance.get_creation_count(), 1);
+
+instance.set_condition(true);
+assert_eq!(instance.get_test_width(), 10.);
+assert_eq!(instance.get_creation_count(), 2);
+
+instance.set_condition(false);
+instance.set_condition(true);
+assert_eq!(instance.get_test_width(), 10.);
+assert_eq!(instance.get_creation_count(), 2);
+```
+
+```js
+var instance = new slint.TestCase();
+
+assert.equal(instance.test_width, 0.);
+assert.equal(instance.creation_count, 0);
+
+instance.condition = true;
+assert.equal(instance.test_width, 10.);
+assert.equal(instance.creation_count, 1);
+
+instance.condition = false;
+assert.equal(instance.test_width, 0.);
+assert.equal(instance.creation_count, 1);
+
+instance.condition = true;
+assert.equal(instance.test_width, 10.);
+assert.equal(instance.creation_count, 2);
+
+instance.condition = false;
+instance.condition = true;
+assert.equal(instance.test_width, 10.);
+assert.equal(instance.creation_count, 2);
+```
+*/