fixes: #345 InvokeMethodWithReply segfault

embedder/embedder.go

@@ -46,6 +46,10 @@ type FlutterOpenGLTexture struct {
 type FlutterTask = C.FlutterTask
 
 // FlutterEngine corresponds to the C.FlutterEngine with his associated callback's method.
+//
+// Trick the panic error:
+// cgo argument has Go pointer to Go pointer
+// GoType(KeepAlive) -> unsafe.Pointer -> uintptr(KeepAlive) -> unsafe.Pointer(Pass to the C function) <- uintptr <- unsafe.Pointer <- GoType
 type FlutterEngine struct {
 	// Flutter Engine.
 	Engine C.FlutterEngine
@@ -346,17 +350,24 @@ func (flu *FlutterEngine) MarkExternalTextureFrameAvailable(textureID int64) Res
 
 // DataCallback is a function called when a PlatformMessage response send back
 // to the embedder.
-type DataCallback func(binaryReply []byte)
+//
+// Trick the panic error:
+// cgo argument has Go pointer to Go pointer
+// GoType(KeepAlive) -> unsafe.Pointer -> uintptr(KeepAlive) -> unsafe.Pointer(Pass to the C function) <- uintptr <- unsafe.Pointer <- GoType
+type DataCallback struct {
+	// Handle func
+	Handle func(binaryReply []byte)
+}
 
 // CreatePlatformMessageResponseHandle creates a platform message response
 // handle that allows the embedder to set a native callback for a response to a
 // message.
 // Must be collected via `ReleasePlatformMessageResponseHandle` after the call
 // to `SendPlatformMessage`.
-func (flu *FlutterEngine) CreatePlatformMessageResponseHandle(callback DataCallback) (PlatformMessageResponseHandle, error) {
+func (flu *FlutterEngine) CreatePlatformMessageResponseHandle(callback *DataCallback) (PlatformMessageResponseHandle, error) {
 	var responseHandle *C.FlutterPlatformMessageResponseHandle
 
-	callbackPointer := uintptr(unsafe.Pointer(&callback))
+	callbackPointer := uintptr(unsafe.Pointer(callback))
 	defer func() {
 		runtime.KeepAlive(callbackPointer)
 	}()

embedder/embedder_proxy.go

@@ -97,6 +97,6 @@ func proxy_post_task_callback(task C.FlutterTask, targetTimeNanos C.uint64_t, us
 //export proxy_desktop_binary_reply
 func proxy_desktop_binary_reply(data *C.uint8_t, dataSize C.size_t, userData unsafe.Pointer) {
 	callbackPointer := *(*uintptr)(userData)
-	handler := *(*DataCallback)(unsafe.Pointer(callbackPointer))
-	handler(C.GoBytes(unsafe.Pointer(data), C.int(dataSize)))
+	callback := (*DataCallback)(unsafe.Pointer(callbackPointer))
+	callback.Handle(C.GoBytes(unsafe.Pointer(data), C.int(dataSize)))
 }

messenger.go

@@ -3,6 +3,7 @@ package flutter
 import (
 	"errors"
 	"fmt"
+	"runtime"
 	"sync"
 
 	"github.com/go-flutter-desktop/go-flutter/embedder"
@@ -37,9 +38,13 @@ func newMessenger(engine *embedder.FlutterEngine) *messenger {
 func (m *messenger) SendWithReply(channel string, binaryMessage []byte) (binaryReply []byte, err error) {
 	reply := make(chan []byte)
 	defer close(reply)
-	responseHandle, err := m.engine.CreatePlatformMessageResponseHandle(func(binaryMessage []byte) {
-		reply <- binaryMessage
-	})
+	callbackHandle := &embedder.DataCallback{
+		Handle: func(binaryMessage []byte) {
+			reply <- binaryMessage
+		},
+	}
+	defer runtime.KeepAlive(callbackHandle)
+	responseHandle, err := m.engine.CreatePlatformMessageResponseHandle(callbackHandle)
 	if err != nil {
 		return nil, err
 	}