Use dart:_wasm constructs to avoid dependence on WebAssembly.Function

lib/web_ui/dev/test_dart2wasm.js

@@ -64,7 +64,8 @@ window.onload = async function () {
         const skwasmInstance = await skwasm();
         window._flutter_skwasmInstance = skwasmInstance;
         resolve({
-          "skwasm": skwasmInstance.asm ?? skwasmInstance.wasmExports,
+          "skwasm": skwasmInstance.wasmExports,
+          "skwasmWrapper": skwasmInstance,
           "ffi": {
             "memory": skwasmInstance.wasmMemory,
           }

lib/web_ui/lib/src/engine/skwasm/skwasm_impl/raw/raw_image.dart

@@ -5,6 +5,7 @@
 @DefaultAsset('skwasm')
 library skwasm_impl;
 
+import 'dart:_wasm';
 import 'dart:ffi';
 import 'dart:js_interop';
 
@@ -39,45 +40,27 @@ external ImageHandle imageCreateFromPixels(
   int rowByteCount,
 );
 
-// We actually want this function to look something like this:
-//
-// @Native<ImageHandle Function(
-//   WasmExternRef,
-//   Int,
-//   Int,
-//   SurfaceHandle,
-// )>(symbol: 'image_createFromTextureSource', isLeaf: true)
-// external ImageHandle imageCreateFromTextureSource(
-//   JSAny textureSource,
-//   int width,
-//   int height,
-//   SurfaceHandle handle,
-// );
-//
-// However, this doesn't work because we cannot use extern refs as part of @Native
-// annotations currently. For now, we can use JS interop to expose this function
-// instead.
-extension SkwasmImageExtension on SkwasmInstance {
-  @JS('wasmExports.image_createFromTextureSource')
-  external JSNumber imageCreateFromTextureSource(
-    JSAny textureSource,
-    JSNumber width,
-    JSNumber height,
-    JSNumber surfaceHandle,
-  );
-}
+// We use a wasm import directly here instead of @Native since this uses an externref
+// in the function signature.
 ImageHandle imageCreateFromTextureSource(
   JSAny frame,
   int width,
   int height,
   SurfaceHandle handle
 ) => ImageHandle.fromAddress(
-  skwasmInstance.imageCreateFromTextureSource(
-    frame,
-    width.toJS,
-    height.toJS,
-    handle.address.toJS,
-  ).toDartInt
+  imageCreateFromTextureSourceImpl(
+    externRefForJSAny(frame),
+    width.toWasmI32(),
+    height.toWasmI32(),
+    handle.address.toWasmI32(),
+  ).toIntUnsigned()
+);
+@pragma('wasm:import', 'skwasm.image_createFromTextureSource')
+external WasmI32 imageCreateFromTextureSourceImpl(
+  WasmExternRef? frame,
+  WasmI32 width,
+  WasmI32 height,
+  WasmI32 surfaceHandle,
 );
 
 @Native<Void Function(ImageHandle)>(symbol:'image_ref', isLeaf: true)

lib/web_ui/lib/src/engine/skwasm/skwasm_impl/raw/skwasm_module.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:_wasm';
 import 'dart:js_interop';
 
 @JS()
@@ -17,37 +18,11 @@ extension WebAssemblyMemoryExtension on WebAssemblyMemory {
 class SkwasmInstance {}
 
 extension SkwasmInstanceExtension on SkwasmInstance {
-  external JSNumber getEmptyTableSlot();
-
-  // The function here *must* be a directly exported wasm function, not a
-  // JavaScript function. If you actually need to add a JavaScript function,
-  // use `addFunction` instead.
-  external void setWasmTableEntry(JSNumber index, JSAny function);
-
-  external JSNumber addFunction(WebAssemblyFunction function);
-  external void removeFunction(JSNumber functionPointer);
-
   external WebAssemblyMemory get wasmMemory;
 }
 
 @JS('window._flutter_skwasmInstance')
 external SkwasmInstance get skwasmInstance;
 
-@JS()
-@staticInterop
-@anonymous
-class WebAssemblyFunctionType {
-  external factory WebAssemblyFunctionType({
-    required JSArray parameters,
-    required JSArray results,
-  });
-}
-
-@JS('WebAssembly.Function')
-@staticInterop
-class WebAssemblyFunction {
-  external factory WebAssemblyFunction(
-    WebAssemblyFunctionType functionType,
-    JSFunction function
-  );
-}
+@pragma('wasm:import', 'skwasmWrapper.addFunction')
+external WasmI32 addFunction(WasmFuncRef function);

lib/web_ui/lib/src/engine/skwasm/skwasm_impl/surface.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:_wasm';
 import 'dart:async';
 import 'dart:ffi';
 import 'dart:js_interop';
@@ -11,7 +12,52 @@ import 'package:ui/src/engine.dart';
 import 'package:ui/src/engine/skwasm/skwasm_impl.dart';
 import 'package:ui/ui.dart' as ui;
 
+@pragma('wasm:export')
+WasmVoid callbackHandler(WasmI32 callbackId, WasmI32 context, WasmExternRef? jsContext) {
+  // Actually hide this call behind whether skwasm is enabled. Otherwise, the SkwasmCallbackHandler
+  // won't actually be tree-shaken, and we end up with skwasm imports in non-skwasm builds.
+  if (FlutterConfiguration.flutterWebUseSkwasm) {
+    SkwasmCallbackHandler.instance.handleCallback(callbackId, context, jsContext);
+  }
+  return WasmVoid();
+}
+
+// This class handles callbacks coming from Skwasm by keeping a map of callback IDs to Completers
+class SkwasmCallbackHandler {
+  SkwasmCallbackHandler._withCallbackPointer(this.callbackPointer);
+
+  factory SkwasmCallbackHandler._() {
+    final WasmFuncRef wasmFunction = WasmFunction<WasmVoid Function(WasmI32, WasmI32, WasmExternRef?)>.fromFunction(callbackHandler);
+    final int functionIndex = addFunction(wasmFunction).toIntUnsigned();
+    return SkwasmCallbackHandler._withCallbackPointer(
+      OnRenderCallbackHandle.fromAddress(functionIndex)
+    );
+  }
+  static SkwasmCallbackHandler instance = SkwasmCallbackHandler._();
 
+  final OnRenderCallbackHandle callbackPointer;
+  final Map<CallbackId, Completer<JSAny>> _pendingCallbacks = <int, Completer<JSAny>>{};
+
+  // Returns a future that will resolve when Skwasm calls back with the given callbackID
+  Future<JSAny> registerCallback(int callbackId) {
+    final Completer<JSAny> completer = Completer<JSAny>();
+    _pendingCallbacks[callbackId] = completer;
+    return completer.future;
+  }
+
+  void handleCallback(WasmI32 callbackId, WasmI32 context, WasmExternRef? jsContext) {
+    // Skwasm can either callback with a JS object (an externref) or it can call back
+    // with a simple integer, which usually refers to a pointer on its heap. In order
+    // to coerce these into a single type, we just make the completers take a JSAny
+    // that either contains the JS object or a JSNumber that contains the integer value.
+    final Completer<JSAny> completer = _pendingCallbacks.remove(callbackId.toIntUnsigned())!;
+    if (!jsContext.isNull) {
+      completer.complete(jsContext!.toJS);
+    } else {
+      completer.complete(context.toIntUnsigned().toJS);
+    }
+  }
+}
 
 class SkwasmSurface {
   factory SkwasmSurface() {
@@ -25,32 +71,16 @@ class SkwasmSurface {
 
   SkwasmSurface._fromHandle(this.handle) : threadId = surfaceGetThreadId(handle);
   final SurfaceHandle handle;
-  OnRenderCallbackHandle _callbackHandle = nullptr;
-  final Map<CallbackId, Completer<JSAny>> _pendingCallbacks = <int, Completer<JSAny>>{};
 
   final int threadId;
 
   void _initialize() {
-    final WebAssemblyFunction wasmFunction = WebAssemblyFunction(
-      WebAssemblyFunctionType(
-        parameters: <JSString>[
-          'i32'.toJS,
-          'i32'.toJS,
-          'externref'.toJS
-        ].toJS,
-        results: <JSString>[].toJS
-      ),
-      _callbackHandler.toJS,
-    );
-    _callbackHandle = OnRenderCallbackHandle.fromAddress(
-      skwasmInstance.addFunction(wasmFunction).toDartInt,
-    );
-    surfaceSetCallbackHandler(handle, _callbackHandle);
+    surfaceSetCallbackHandler(handle, SkwasmCallbackHandler.instance.callbackPointer);
   }
 
   Future<DomImageBitmap> renderPicture(SkwasmPicture picture) async {
     final int callbackId = surfaceRenderPicture(handle, picture.handle);
-    final DomImageBitmap bitmap = (await _registerCallback(callbackId)) as DomImageBitmap;
+    final DomImageBitmap bitmap = (await SkwasmCallbackHandler.instance.registerCallback(callbackId)) as DomImageBitmap;
     return bitmap;
   }
 
@@ -60,7 +90,7 @@ class SkwasmSurface {
       image.handle,
       format.index,
     );
-    final int context = (await _registerCallback(callbackId) as JSNumber).toDartInt;
+    final int context = (await SkwasmCallbackHandler.instance.registerCallback(callbackId) as JSNumber).toDartInt;
     final SkDataHandle dataHandle = SkDataHandle.fromAddress(context);
     final int byteCount = skDataGetSize(dataHandle);
     final Pointer<Uint8> dataPointer = skDataGetConstPointer(dataHandle).cast<Uint8>();
@@ -72,23 +102,7 @@ class SkwasmSurface {
     return ByteData.sublistView(output);
   }
 
-  Future<JSAny> _registerCallback(int callbackId) {
-    final Completer<JSAny> completer = Completer<JSAny>();
-    _pendingCallbacks[callbackId] = completer;
-    return completer.future;
-  }
-
-  void _callbackHandler(JSNumber callbackId, JSNumber context, JSAny? jsContext) {
-    final Completer<JSAny> completer = _pendingCallbacks.remove(callbackId.toDartInt)!;
-    if (jsContext.isUndefinedOrNull) {
-      completer.complete(context);
-    } else {
-      completer.complete(jsContext);
-    }
-  }
-
   void dispose() {
     surfaceDestroy(handle);
-    skwasmInstance.removeFunction(_callbackHandle.address.toJS);
   }
 }

web_sdk/sdk_rewriter.dart

@@ -178,6 +178,9 @@ List<String> getExtraImportsForLibrary(String libraryName) {
       extraImports.add(entry.value);
     }
   }
+  if (libraryName == 'skwasm_impl') {
+    extraImports.add("import 'dart:_wasm';");
+  }
   return extraImports;
 }
 

web_sdk/test/sdk_rewriter_test.dart

@@ -184,6 +184,7 @@ void printSomething() {
       "import 'dart:_web_unicode';",
       "import 'dart:_web_test_fonts';",
       "import 'dart:_web_locale_keymap' as locale_keymap;",
+      "import 'dart:_wasm';",
     ]);
 
     // Other libraries (should not have extra imports).