Garbage collection capable wrappers for skia interop

kotlin-native/Interop/Skia/SKIA_PLUGIN.md

@@ -1,3 +1,4 @@
+
 Skia interop support
 ====================
 
@@ -15,6 +16,33 @@ plugin = org.jetbrains.kotlin.native.cinterop.plugin.skia
 language = C++
 ```
 
+Interop
+-------
+
+There are two kinds of C++ classes used in Skia.
+The ones with ref()/unref() reference counting are abstracted with SkiaRefCnt interface.
+The ones without are abstracted with CPlusPlusClass interface.
+
+So, for example, for SkPaint one has
+
+class SkPaint(rawPtr) : CStructVar(rawPtr), CPlusPlusClass {
+    // A C interop-like function having methods,
+    // as well as low level __init__ and __destroy__
+}
+
+class Paint(cpp: SkPaint, managed: Boolean) : ManagedType(cpp) {
+    // A wrapper capable of garbage collection delegating all calls and fields to SkPaint
+}
+
+
+    * pointer returned from C++ -> Wrapper(cpp, managed = false)
+    * sk_sp returned from C++ -> calls .release() ; Wrapper(cpp, managed = true)
+    * pointer passed to C++ -> kotlin.cpp.ptr
+    * sk_sp passed to C++ -> sk_ref_sp(kotlin.cpp.ptr)
+    * constructor call -> allocate Cpp; Wrapper(cpp, managed = true)
+    * garbage collection -> if (managed==true): calls __destroy__() for CPluaPlusClass or unref() for SkiaRefCnt
+
+
 Implementation details
 ======================
 

kotlin-native/Interop/Skia/src/main/kotlin/org/jetbrains/kotlin/native/interop/skia/SkiaStubsBuildingContextImpl.kt

@@ -9,6 +9,7 @@ import org.jetbrains.kotlin.native.interop.gen.Classifier
 import org.jetbrains.kotlin.native.interop.gen.StubIrContext
 import org.jetbrains.kotlin.native.interop.gen.StubsBuildingContextImpl
 import org.jetbrains.kotlin.native.interop.indexer.StructDecl
+import org.jetbrains.kotlin.native.interop.indexer.StructDef
 
 class SkiaStubsBuildingContextImpl(stubIrContext: StubIrContext) : StubsBuildingContextImpl(stubIrContext) {
     override val declarationMapper = SkiaDeclarationMapperImpl()
@@ -23,4 +24,21 @@ class SkiaStubsBuildingContextImpl(stubIrContext: StubIrContext) : StubsBuilding
             return getKotlinClassForPointed(structArgument)
         }
     }
+
+    override fun isCppClass(spelling: String): Boolean {
+        val decl = nativeIndex.structs.firstOrNull { it.spelling == spelling } ?: return false
+        return decl.def?.kind == StructDef.Kind.CLASS
+    }
+
+    override fun managedWrapperClassifier(cppClassifier: Classifier): Classifier? {
+        if (cppClassifier.pkg != "org.jetbrains.skiko.skia.native") return null
+        // TODO: it'd be nice to check inheritance from SkiaRefCnt or CPlusPlusClass,
+        // but unable to do that at StubType level.
+        if (!(cppClassifier.topLevelName.startsWith("Sk") || cppClassifier.topLevelName.startsWith("Gr"))) return null
+        if (cppClassifier.topLevelName == "SkString") return null
+        // TODO: We only managed C++ classes, not structs for now.
+        if (!isCppClass(cppClassifier.topLevelName)) return null
+
+        return Classifier.topLevel(cppClassifier.pkg, cppClassifier.topLevelName.drop(2))
+    }
 }