cxx-qt: add a CxxQtSignalHandler struct and closure trait

This allows for FnMut to be used for signals later.

Related to #595

crates/cxx-qt/src/cxxqtsignalhandler.rs

@@ -0,0 +1,52 @@
+// SPDX-FileCopyrightText: 2023 Klarälvdalens Datakonsult AB, a KDAB Group company <info@kdab.com>
+// SPDX-FileContributor: Andrew Hayzen <andrew.hayzen@kdab.com>
+// SPDX-FileContributor: Leon Matthes <leon.matthes@kdab.com>
+//
+// SPDX-License-Identifier: MIT OR Apache-2.0
+
+use cxx::ExternType;
+
+/// A trait which describes the closure to be used with [CxxQtSignalHandler].
+pub trait CxxQtSignalHandlerClosure {
+    /// The Id of the CXX type
+    type Id;
+    /// The type of the closure
+    type FnType: ?Sized;
+}
+
+/// A signal handler helper which is used to move a FnMut closure into C++
+#[repr(transparent)]
+pub struct CxxQtSignalHandler<T>
+where
+    T: CxxQtSignalHandlerClosure,
+{
+    closure: Box<T::FnType>,
+}
+
+impl<T> CxxQtSignalHandler<T>
+where
+    T: CxxQtSignalHandlerClosure,
+{
+    /// Create a new signal handler with the given closure
+    //
+    // Note that we cannot use From as we cannot infer the type in the caller
+    pub fn new(closure: Box<T::FnType>) -> Self {
+        Self { closure }
+    }
+
+    /// A mutable reference to the inner closure
+    pub fn closure(&mut self) -> &mut Box<T::FnType> {
+        &mut self.closure
+    }
+}
+
+// Safety:
+//
+// Static checks on the C++ and Rust side to ensure the size is the same.
+unsafe impl<T> ExternType for CxxQtSignalHandler<T>
+where
+    T: CxxQtSignalHandlerClosure,
+{
+    type Kind = cxx::kind::Trivial;
+    type Id = T::Id;
+}

crates/cxx-qt/src/lib.rs

@@ -9,11 +9,13 @@
 //!
 //! See the [book](https://kdab.github.io/cxx-qt/book/) for more information.
 
+mod cxxqtsignalhandler;
 mod cxxqtthread;
 
 pub use cxx_qt_macro::bridge;
 pub use cxx_qt_macro::qobject;
 
+pub use cxxqtsignalhandler::{CxxQtSignalHandler, CxxQtSignalHandlerClosure};
 pub use cxxqtthread::CxxQtThread;
 
 /// This trait is automatically implemented for all types which are marked as `#[qobject]`.