问如何创建一个指向接受dart:ffi中字符串的dart函数的指针？

EN

import 'dart:ffi';

import 'package:ffi/ffi.dart';

// void* magickSetProgressMonitor(uint32_t dummy, const void* progress_monitor,...);
// we need a pair of typedefs to describe the signature on both sides
// the two typedefs will look the same if there are just pointers etc
// the differences come in parameters like integers
// this is the C signature
typedef magick_set_progress_monitor = Pointer<Void> Function(
  Uint32, // this is just here as an example to show a 32 bit unsigned
  Pointer<NativeFunction<magick_progress_monitor>>,
);
// this is the Dart signature
typedef magickSetProgressMonitor = Pointer<Void> Function(
  int, // ffi has turned the C uint type to a Dart int
  Pointer<NativeFunction<magick_progress_monitor>>,
);

void main() {
  final nativeLib = DynamicLibrary.open('mydll');
  // lookupFunction is the improved syntax for looking up functions in the lib
  // its generics are the C-style signature followed by the Dart-style
  // it returns a Function that matches the Dart-style typedef
  final mspm = nativeLib
      .lookupFunction<magick_set_progress_monitor, magickSetProgressMonitor>(
    'magickSetProgressMonitor',
  );
  
  // before calling mspm it needs the callback function, so look that up
  // this converts the Dart-style function to a pointer to a native function
  // matching the C-style typedef
  final callbackFunction = Pointer.fromFunction<magick_progress_monitor>(
    callback,
    true,
  );
  
  // and call mspm passing the (dummy, example) int and callback function
  final result = mspm(123, callbackFunction);
}

//typedef bool(*MagickProgressMonitor)(const char *,const long long,
//     const long long,void *);
// callback functions need a C-style typedef (so that FFI knows the int sizes)
typedef magick_progress_monitor = Bool Function(
  Pointer<Utf8>, // the documentation says this is a string, so use Utf8
  LongLong,
  LongLong,
  Pointer<Void>,
);

bool callback(
  Pointer<Utf8> t,
  int offset,
  int extent,
  Pointer<Void> client_data,
) {
  final text = t.toDartString(); // convert the Pointer<Utf8> to Dart string
  // do something here with the values - probably call a Dart function provided by the user
  return true;
}