Add support for recursive dependency tracking across PropertyTracker instances

By default PropertyTracker::evaluate() registers the currently
evaluating binding/tracker as a dependency. This should help with
repeaters and other scenarios where in the run-time we use property
trackers but want to track the overall "dirtyness" in the window with
regards to whether a redraw is needed or not.

The new evaluate_as_dependency_root() function allows skipping this
mechanism and is used for the two trackers in the window.

api/sixtyfps-cpp/include/sixtyfps_properties.h

@@ -228,6 +228,22 @@ struct PropertyTracker
         return result;
     }
 
+    template<typename F>
+    auto evaluate_as_dependency_root(const F &f) const -> std::enable_if_t<std::is_same_v<decltype(f()), void>>
+    {
+        cbindgen_private::sixtyfps_property_tracker_evaluate_as_dependency_root(
+                &inner, [](void *f) { (*reinterpret_cast<const F *>(f))(); }, const_cast<F *>(&f));
+    }
+
+    template<typename F>
+    auto evaluate_as_dependency_root(const F &f) const
+            -> std::enable_if_t<!std::is_same_v<decltype(f()), void>, decltype(f())>
+    {
+        decltype(f()) result;
+        this->evaluate_as_dependency_root([&] { result = f(); });
+        return result;
+    }
+
 private:
     cbindgen_private::PropertyTrackerOpaque inner;
 };

api/sixtyfps-cpp/tests/datastructures.cpp

@@ -42,3 +42,25 @@ TEST_CASE("Basic SharedVector API", "[vector]")
         REQUIRE(vec[2] == 10);
     }
 }
+
+TEST_CASE("Property Tracker")
+{
+    using namespace sixtyfps;
+    PropertyTracker tracker1;
+    PropertyTracker tracker2;
+    Property<int> prop(42);
+
+    auto r = tracker1.evaluate([&]() { return tracker2.evaluate([&]() { return prop.get(); }); });
+    REQUIRE(r == 42);
+
+    prop.set(1);
+    REQUIRE(tracker2.is_dirty());
+    REQUIRE(tracker1.is_dirty());
+
+    r = tracker1.evaluate(
+            [&]() { return tracker2.evaluate_as_dependency_root([&]() { return prop.get(); }); });
+    REQUIRE(r == 1);
+    prop.set(100);
+    REQUIRE(tracker2.is_dirty());
+    REQUIRE(!tracker1.is_dirty());
+}