Dart:ffi - 'must be a subtype' for 'lookupFunction' issue for native code in dart

Question

The type 'Uint32 Function(IntPtr, Pointer<Uint32>)' must be a subtype of 'int Function(IntPtr, Pointer<Uint32>)' for 'lookupFunction'.

class Psapi {
  static final ffi.DynamicLibrary psapi = ffi.DynamicLibrary.open('psapi.dll');

  static int GetWindowThreadProcessId(
      ffi.IntPtr hwnd, ffi.Pointer<ffi.Uint32> lpdwProcessId) {
    return psapi.lookupFunction<ffi.Uint32 Function(ffi.IntPtr hwnd, ffi.Pointer<ffi.Uint32> lpdwProcessId),
        int Function(ffi.IntPtr hwnd, ffi.Pointer<ffi.Uint32> lpdwProcessId)>('GetWindowThreadProcessId')(
      hwnd,
      lpdwProcessId,
    );
  }
}

In this code int Function(ffi.IntPtr hwnd, ffi.Pointer<ffi.Uint32> lpdwProcessId) this part shows error and says The type 'Uint32 Function(IntPtr, Pointer<Uint32>)' must be a subtype of 'int Function(IntPtr, Pointer<Uint32>)' for 'lookupFunction' Can someone explain where it gone wrong and what's the solution. Thanks in advance.

Answer 1

The lookupFunction's second type argument represent the Dart function signature for the corresponding Win32 function. IntPtr's Dart signature would be int. But implementing the code directly can cause error.