NativeCallable<T extends Function>.listener 构造函数

构建一个可以从任何线程调用的原生可调用(NativeCallable)。

当原生代码调用函数 nativeFunction 时，参数将通过 SendPort 发送到创建了原生可调用的 Isolate，然后回调将被触发。

原生代码不会等待回调的响应，因此只支持返回 void 的函数。

回调将在未来的某个时刻被调用。原生调用者不能假定回调会立即运行。传递给回调的资源（如 malloc'd 内存指针、输出参数）必须在调用完成前保持有效。

当不再需要时，必须关闭此回调。创建了回调的 Isolate 将保持活动状态直到调用 close。调用 NativeCallable.close 后，从原生代码调用 nativeFunction 将导致未定义的行为。

例如

import 'dart:async';
import 'dart:ffi';
import 'package:ffi/ffi.dart';

// Processes a simple HTTP GET request using a native HTTP library that
// processes the request on a background thread.
Future<String> httpGet(String uri) async {
  final uriPointer = uri.toNativeUtf8();

  // Create the NativeCallable.listener.
  final completer = Completer<String>();
  late final NativeCallable<NativeHttpCallback> callback;
  void onResponse(Pointer<Utf8> responsePointer) {
    completer.complete(responsePointer.toDartString());
    calloc.free(responsePointer);
    calloc.free(uriPointer);

    // Remember to close the NativeCallable once the native API is
    // finished with it, otherwise this isolate will stay alive
    // indefinitely.
    callback.close();
  }
  callback = NativeCallable<NativeHttpCallback>.listener(onResponse);

  // Invoke the native HTTP API. Our example HTTP library processes our
  // request on a background thread, and calls the callback on that same
  // thread when it receives the response.
  nativeHttpGet(uriPointer, callback.nativeFunction);

  return completer.future;
}

// Load the native functions from a DynamicLibrary.
final DynamicLibrary dylib = DynamicLibrary.process();
typedef NativeHttpCallback = Void Function(Pointer<Utf8>);

typedef HttpGetFunction = void Function(
    Pointer<Utf8>, Pointer<NativeFunction<NativeHttpCallback>>);
typedef HttpGetNativeFunction = Void Function(
    Pointer<Utf8>, Pointer<NativeFunction<NativeHttpCallback>>);
final nativeHttpGet =
    dylib.lookupFunction<HttpGetNativeFunction, HttpGetFunction>(
        'http_get');