Allow type casting in QML

Using the TypeCast object allows casting objects as other reference
types. TypeCast defines one cast function per generated reference type.

Change-Id: Id99fe82e9f94ab63c237bbc68caef72867a15e2f
Reviewed-by: Karsten Heimrich <karsten.heimrich@qt.io>

3 files changed, 75 insertions, 2 deletions

diff --git a/src/Qt.DotNet.Adapter/Qt/DotNet/TypeCast.cs b/src/Qt.DotNet.Adapter/Qt/DotNet/TypeCast.cs
new file mode 100644
index 0000000..6e4cc32
--- /dev/null
+++ b/src/Qt.DotNet.Adapter/Qt/DotNet/TypeCast.cs
@@ -0,0 +1,18 @@
+/***************************************************************************************************
+ Copyright (C) 2025 The Qt Company Ltd.
+ SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
+***************************************************************************************************/
+
+using System;
+using Qt.Quick;
+
+namespace Qt.DotNet
+{
+    [Include]
+    [QmlElement(Singleton = true)]
+    public class TypeCast
+    {
+        [Include]
+        public TypeCast() { }
+    }
+}
diff --git a/src/Qt.DotNet.GenerationRules/Qt/DotNet/CodeGeneration/Rules/TypeCasting/GenerateTypeCast.cs b/src/Qt.DotNet.GenerationRules/Qt/DotNet/CodeGeneration/Rules/TypeCasting/GenerateTypeCast.cs
new file mode 100644
index 0000000..af59fdd
--- /dev/null
+++ b/src/Qt.DotNet.GenerationRules/Qt/DotNet/CodeGeneration/Rules/TypeCasting/GenerateTypeCast.cs
@@ -0,0 +1,53 @@
+/***************************************************************************************************
+ Copyright (C) 2025 The Qt Company Ltd.
+ SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
+***************************************************************************************************/
+
+using System.Reflection;
+
+namespace Qt.DotNet.CodeGeneration.Rules.TypeCasting
+{
+    using static Placeholders;
+    using static Traits;
+
+    public class GenerateTypeCast : Class.GenerateClass
+    {
+        public override bool Matches(MemberInfo src)
+            => base.Matches(src) && src != TypeOf<TypeCast>();
+        public override IEnumerable<MemberInfo> DependsOn => [TypeOf<TypeCast>()];
+        public override Result Execute(MemberInfo src)
+        {
+            if (src is not Type type)
+                return Error();
+
+            var typeCast = TypeOf<TypeCast>();
+
+            // TO-DO: Generate unique names for cast functions
+
+            ////////////////////////////////////////////////////////////////////////////////////////
+            //
+            if (typeCast.GetPlaceholder(MethodDeclarations) is not { } methods)
+                return Error();
+            methods += $@"
+Q_INVOKABLE {type.MFn(Ns | Name)} *as{type.MFn(Name)}(QObject *obj);
+{Blank}";
+            ////////////////////////////////////////////////////////////////////////////////////////
+            //
+            if (typeCast.GetPlaceholder(Includes) is not { } includes)
+                return Error();
+            includes += $"#include <{type.MFn(Ns | Dir)}{type.MFn(File)}.h>";
+
+            ////////////////////////////////////////////////////////////////////////////////////////
+            //
+            if (typeCast.GetPlaceholder(MethodsImplementation) is not { } implementation)
+                return Error();
+            implementation += $@"
+{type.MFn(Ns | Name)} *{typeCast.MFn(Ns | Name)}::as{type.MFn(Name)}(QObject *qObj)
+{{
+    return Convert::as<{type.MFn(Ns | Name)}>(qObj);
+}}
+{Blank}";
+            return Ok;
+        }
+    }
+}
diff --git a/src/Qt.DotNet.Generator/Qt/DotNet/CodeGeneration/DependencyGraph.cs b/src/Qt.DotNet.Generator/Qt/DotNet/CodeGeneration/DependencyGraph.cs
index 85b1ee5..97788b6 100644
--- a/src/Qt.DotNet.Generator/Qt/DotNet/CodeGeneration/DependencyGraph.cs
+++ b/src/Qt.DotNet.Generator/Qt/DotNet/CodeGeneration/DependencyGraph.cs
@@ -257,9 +257,11 @@ namespace Qt.DotNet.CodeGeneration
                 }
             }
 
-            await Task.WhenAll(
-                assembly.ExportedTypes
+            var exportedTypes = assembly.ExportedTypes
                 .Where(x => x.DeclaringType == null)
+                .Append(TypeOf<TypeCast>());
+
+            await Task.WhenAll(exportedTypes
                 .Select(x => Task.Run(async () => await AddEdgeAsync(Root, x))));
 
             if (!Edges.Any())