如何使用Flutter Dart的Win32包读取Boca打印机状态

问题背景 #

需要实现打印机与应用的双向通信，包含写入、读取两个功能。目前已通过Winspool API的WritePrinter实现向打印机写入数据，但调用ReadPrinter读取打印机状态时出现异常：函数既未执行，也未抛出错误。

现有代码 #

打开打印机 #

final alloc = Arena();
final pPrinterName = printerName.toNativeUtf16(allocator: alloc);
final pWritePrinterHandle = alloc<HANDLE>();
final pDefault = alloc<PRINTER_DEFAULTS>()
  ..ref.pDatatype = nullptr
  ..ref.pDevMode = nullptr
  ..ref.DesiredAccess = PRINTER_ALL_ACCESS;

// Open Printer
var fSuccess = OpenPrinter(pPrinterName, pWritePrinterHandle, pDefault);
if (fSuccess == 0) {
  final error = GetLastError();
  throw Exception('OpenPrint error, status: $fSuccess, error: $error');
}

向打印机发送命令 #

final pDocInfo = alloc<DOC_INFO_1>()
  ..ref.pDocName =
      'Boca Systems Communicator'.toNativeUtf16(allocator: alloc)
  ..ref.pDatatype =
      'RAW'.toNativeUtf16(allocator: alloc) // RAW, TEXT or XPS_PASS
  ..ref.pOutputFile = nullptr;

// 启动打印作业
var fStartDocJobId =
    StartDocPrinter(pWritePrinterHandle.value, 1, pDocInfo);
if (fStartDocJobId == 0) { // 原代码错误：误用了打开打印机的fSuccess变量，应判断作业ID是否为0
  final error = GetLastError();
  throw Exception(
      'StartDocPrinter error, status: $fStartDocJobId, error: $error');
}

var fStartPrintSuccess = StartPagePrinter(pWritePrinterHandle.value);

// 发送打印命令
String dataToPrint = '<S92>';
final cWritten = alloc<DWORD>();
final data = dataToPrint.toNativeUtf8(allocator: alloc);

final writeSuccess = WritePrinter(
    pWritePrinterHandle.value, data, dataToPrint.length, cWritten);

if (dataToPrint.length != cWritten.value) {
  final error = GetLastError();
  throw Exception(
      'WritePrinter error, status: $writeSuccess, error: $error');
}

读取打印机的代码（存在问题） #

final printerJobName = '$printerName, Job $fStartDocJobId';
final Completer<void> completer = Completer<void>();

final pReadPrinterHandle = alloc<HANDLE>();
final pReadDefault = alloc<PRINTER_DEFAULTS>()
  ..ref.pDatatype = nullptr
  ..ref.pDevMode = nullptr
  ..ref.DesiredAccess = PRINTER_USE_ACCESS;

final openPrinterDetailName = printerJobName.toNativeUtf16(allocator: alloc);

// 尝试打开作业句柄读取
var fReadOpenSuccess =
    OpenPrinter(openPrinterDetailName, pReadPrinterHandle, pReadDefault);
if (fReadOpenSuccess == 0) {
  final error = GetLastError();
  throw Exception(
      'Read OpenPrint error, status: $fReadOpenSuccess, error: $error, openPrinterDetailName $printerJobName');
}

final readBufferSize = 1024;
final pReadData = alloc<Uint8>(readBufferSize);
final bytesRead = alloc<Uint32>();

final readSuccess = ReadPrinter(
    pReadPrinterHandle.value, pReadData, readBufferSize, bytesRead);

if (readSuccess == 0) {
  final error = GetLastError();
  throw Exception('ReadPrinter error, status: $readSuccess, error: $error');
}
// 处理读取数据
final readData = pReadData.cast<Utf8>().toDartString();

问题分析与修正方案 #

核心问题点 #

1. 错误的句柄打开参数：OpenPrinter的第一个参数应为打印机名称，而非打印机名, Job 作业ID格式的作业标识，作业名称无法用于打开可读取的打印机句柄。
2. 权限不足：读取打印机响应需要PRINTER_READ_ACCESS或更高权限（如已有的PRINTER_ALL_ACCESS），PRINTER_USE_ACCESS权限不足以读取数据。
3. 未完成打印作业：原写入代码缺少EndPagePrinter和EndDocPrinter调用，打印机可能未处理完命令就被尝试读取，导致无响应。
4. ReadPrinter使用限制：该函数仅适用于支持双向通信的打印机（如部分POS热敏打印机），且需确保打印机收到命令后会返回响应数据。

修正后的读取代码 #

复用已打开的打印机句柄（无需重新打开），确保作业完成后再读取：

// 必须先结束打印作业，让打印机处理命令并生成响应
EndPagePrinter(pWritePrinterHandle.value);
EndDocPrinter(pWritePrinterHandle.value);

final readBufferSize = 1024;
final pReadData = alloc<Uint8>(readBufferSize);
final bytesRead = alloc<Uint32>();

// 使用已有的高权限打印机句柄读取
final readSuccess = ReadPrinter(
    pWritePrinterHandle.value, pReadData, readBufferSize, bytesRead);

if (readSuccess == 0) {
  final error = GetLastError();
  // ERROR_NO_DATA表示打印机无响应数据返回，属于正常情况
  if (error == ERROR_NO_DATA) {
    print('打印机无返回数据');
  } else {
    throw Exception('ReadPrinter error, status: $readSuccess, error: $error');
  }
} else {
  // 处理读取到的数据
  final readData = pReadData.cast<Utf8>().toDartString();
  print('读取到打印机数据：$readData');
}

额外注意事项 #

• 确认打印机硬件支持双向通信，且驱动已启用双向支持。
• 需确保发送的命令（如示例中的<S92>）是打印机支持的查询类命令，能触发响应。
• 若读取仍无响应，可尝试增加读取前的延迟，或通过GetPrinter函数先确认打印机处于就绪状态。

内容的提问来源于stack exchange，提问作者Savin Sharma