ffi_native_only_classes_extending_nativefieldwrapperclass1_can_be_pointer

Only classes extending NativeFieldWrapperClass1 can be passed as Pointer.

The analyzer produces this diagnostic when a function or method annotated with @Native has a parameter in its FFI signature that is a Pointer, but the corresponding Dart parameter type is a class instance that doesn't extend NativeFieldWrapperClass1 (or is a Pointer or TypedData).

The following code produces this diagnostic because MyService doesn't extend NativeFieldWrapperClass1, but the @Native signature for its process method indicates the receiver should be passed as a Pointer<Void>:

import 'dart:ffi';
​
class MyService { // MyService does not extend NativeFieldWrapperClass1
  @Native<Void Function(Pointer<Void>, Int8)>(symbol: 'MyService_process')
  external void process(int data);
}

1. If the Dart class is intended to wrap a native object: Make the Dart class extend NativeFieldWrapperClass1. This is the correct approach if the Dart class instance has a corresponding native object whose pointer should be passed.

   dart

   import 'dart:ffi';
   ​
   class MyService extends NativeFieldWrapperClass1 {
     @Native<Void Function(Pointer<Void>, Int8)>(symbol: 'MyService_process')
     external void process(int data);
   }
   

2. If you intend to pass an opaque handle to the Dart object: Change the FFI signature in the @Native annotation to use Handle instead of Pointer for the parameter. This allows passing a reference to the Dart object itself, which native code can interact with using the Dart C API.

   dart

   import 'dart:ffi';
   ​
   class MyService {
     @Native<Void Function(Handle, Int8)>(symbol: 'MyService_process')
     external void process(int data);
   }