C++: Add slint::Window::scale_factor() getter (#3004)

Fixes #3003

CHANGELOG.md

@@ -13,6 +13,8 @@ All notable changes to this project are documented in this file.
 
 ### C++
 
+ - Added `slint::Window::scale_factor()` as getter to read the system device pixel ratio.
+
 ### LSP
 
  - Correctly use the CARGO_MANIFEST_DIR as the base for import and image in `slint!` macro

api/cpp/include/slint.h

@@ -496,6 +496,10 @@ class Window
     /// a window frame (if present).
     void set_size(const slint::PhysicalSize &size) { inner.set_physical_size(size); }
 
+    /// This function returns the scale factor that allows converting between logical and
+    /// physical pixels.
+    float scale_factor() const { return inner.scale_factor(); }
+
     /// Dispatch a key press event to the scene.
     ///
     /// Use this when you're implementing your own backend and want to forward user input events.

api/cpp/tests/window.cpp

@@ -28,3 +28,19 @@ TEST_CASE("Basic Window Visibility")
     instance->hide();
     REQUIRE(instance->window().is_visible() == false);
 }
+
+TEST_CASE("Window Scale Factory Existence")
+{
+    using namespace slint::interpreter;
+    using namespace slint;
+
+    ComponentCompiler compiler;
+    auto comp_def = compiler.build_from_source(R"(
+        export App := Window {
+        }
+    )",
+                                               "");
+    REQUIRE(comp_def.has_value());
+    auto instance = comp_def->create();
+    REQUIRE(instance->window().scale_factor() > 0);
+}