Add support for tracking the length of a model in C++

Similar to the parent commit, the model tracks changes to the rows and
marks an internal property dirty. Since we have a base class this is a
little less intrusive.

cc #98

api/sixtyfps-cpp/include/sixtyfps.h

@@ -456,6 +456,11 @@ public:
     /// Internal function called by the view to register itself
     void attach_peer(private_api::ModelPeer p) { peers.push_back(std::move(p)); }
 
+    /// \private
+    /// Internal function called from within bindings to register with the currently
+    /// evaluating dependency and get notified when this model's row count changes.
+    void track_row_count_changes() { model_dirty_property.get(); }
+
 protected:
     /// Notify the views that a specific row was changed
     void row_changed(int row)
@@ -465,11 +470,13 @@ protected:
     /// Notify the views that rows were added
     void row_added(int index, int count)
     {
+        model_dirty_property.set_dirty();
         for_each_peers([=](auto peer) { peer->row_added(index, count); });
     }
     /// Notify the views that rows were removed
     void row_removed(int index, int count)
     {
+        model_dirty_property.set_dirty();
         for_each_peers([=](auto peer) { peer->row_removed(index, count); });
     }
 
@@ -489,6 +496,7 @@ private:
                     peers.end());
     }
     std::vector<private_api::ModelPeer> peers;
+    private_api::Property<bool> model_dirty_property;
 };
 
 namespace private_api {

api/sixtyfps-cpp/include/sixtyfps_properties.h

@@ -142,6 +142,7 @@ struct Property
     }
 
     bool is_dirty() const { return cbindgen_private::sixtyfps_property_is_dirty(&inner); }
+    void set_dirty() const { cbindgen_private::sixtyfps_property_set_dirty(&inner); }
 
     static void link_two_way(const Property<T> *p1, const Property<T> *p2)
     {

api/sixtyfps-cpp/tests/datastructures.cpp

@@ -73,6 +73,35 @@ TEST_CASE("Property Tracker")
     REQUIRE(!tracker1.is_dirty());
 }
 
+TEST_CASE("Model row changes")
+{
+    using namespace sixtyfps::private_api;
+
+    auto model = std::make_shared<sixtyfps::VectorModel<int>>();
+
+    PropertyTracker tracker;
+
+    REQUIRE(tracker.evaluate([&]() {
+        model->track_row_count_changes();
+        return model->row_count();
+    }) == 0);
+    REQUIRE(!tracker.is_dirty());
+    model->push_back(1);
+    model->push_back(2);
+    REQUIRE(tracker.is_dirty());
+    REQUIRE(tracker.evaluate([&]() {
+        model->track_row_count_changes();
+        return model->row_count();
+    }) == 2);
+    REQUIRE(!tracker.is_dirty());
+    model->erase(0);
+    REQUIRE(tracker.is_dirty());
+    REQUIRE(tracker.evaluate([&]() {
+        model->track_row_count_changes();
+        return model->row_count();
+    }) == 1);
+}
+
 TEST_CASE("Image")
 {
     using namespace sixtyfps;

sixtyfps_compiler/generator/cpp.rs

@@ -1628,7 +1628,7 @@ fn compile_expression(
                 "[](const sixtyfps::Image &img) { return img.size(); }".into()
             }
             BuiltinFunction::ArrayLength => {
-                "[](const auto &model) { return (*model).row_count(); }".into()
+                "[](const auto &model) { (*model).track_row_count_changes(); return (*model).row_count(); }".into()
             }
             BuiltinFunction::Rgb => {
                 "[](int r, int g, int b, float a) {{ return sixtyfps::Color::from_argb_uint8(std::clamp(a * 255., 0., 255.), std::clamp(r, 0, 255), std::clamp(g, 0, 255), std::clamp(b, 0, 255)); }}".into()

sixtyfps_runtime/corelib/properties.rs

@@ -1815,6 +1815,12 @@ pub(crate) mod ffi {
         handle.0.access(|binding| binding.map_or(false, |b| b.dirty.get()))
     }
 
+    /// Marks the property as dirty and notifies dependencies.
+    #[no_mangle]
+    pub extern "C" fn sixtyfps_property_set_dirty(handle: &PropertyHandleOpaque) {
+        handle.0.mark_dirty()
+    }
+
     /// Destroy handle
     #[no_mangle]
     pub unsafe extern "C" fn sixtyfps_property_drop(handle: *mut PropertyHandleOpaque) {

tests/cases/models/array.60

@@ -20,8 +20,11 @@ const TestCase &instance = *handle;
 
 assert_eq(instance.get_num_ints(), 5);
 
-instance.set_ints(std::make_shared<sixtyfps::VectorModel<int>>(std::vector<int>{1, 2, 3, 4, 5, 6, 7}));
+auto model = std::make_shared<sixtyfps::VectorModel<int>>(std::vector<int>{1, 2, 3, 4, 5, 6, 7});
+instance.set_ints(model);
 assert_eq(instance.get_num_ints(), 7);
+model->push_back(8);
+assert_eq(instance.get_num_ints(), 8);
 ```