must_return_void

The return type of the function passed to 'NativeCallable.listener' must be 'void' rather than '{0}'.

The analyzer produces this diagnostic when you pass a function that doesn't return void to the NativeCallable.listener constructor.

NativeCallable.listener creates a native callable that can be invoked from any thread. The native code that invokes the callable sends a message back to the isolate that created the callable, and doesn't wait for a response. So it isn't possible to return a result from the callable.

For more information about FFI, see C interop using dart:ffi.

The following code produces this diagnostic because the function f returns int rather than void.

import 'dart:ffi';
​
int f(int i) => i * 2;
​
void g() {
  NativeCallable<Int32 Function(Int32)>.listener(f);
}

Change the return type of the function to void.

import 'dart:ffi';
​
void f(int i) => print(i * 2);
​
void g() {
  NativeCallable<Void Function(Int32)>.listener(f);
}