為什么 Dart FFI 會從此 C 結構生成不透明的 class？

Question

I want to use the MAVLink C library to parse MAVLink packets in dart, but the Dart FFI generated mavlink_message_t is an opaque class, while other structs like mavlink_system_t are generated normally with all their attributes. 這種行為的原因是什么，我該如何解決？

mavlink_message_t C 結構

MAVPACKED(
typedef struct __mavlink_message {
    uint16_t checksum;      ///< sent at end of packet
    uint8_t magic;          ///< protocol magic marker
    uint8_t len;            ///< Length of payload
    uint8_t incompat_flags; ///< flags that must be understood
    uint8_t compat_flags;   ///< flags that can be ignored if not understood
    uint8_t seq;            ///< Sequence of packet
    uint8_t sysid;          ///< ID of message sender system/aircraft
    uint8_t compid;         ///< ID of the message sender component
    uint32_t msgid:24;      ///< ID of message in payload
    uint64_t payload64[(MAVLINK_MAX_PAYLOAD_LEN+MAVLINK_NUM_CHECKSUM_BYTES+7)/8];
    uint8_t ck[2];          ///< incoming checksum bytes
    uint8_t signature[MAVLINK_SIGNATURE_BLOCK_LEN];
}) mavlink_message_t;

mavlink_system_t C 結構

MAVPACKED(
typedef struct __mavlink_system {
    uint8_t sysid;   ///< Used by the MAVLink message_xx_send() convenience function
    uint8_t compid;  ///< Used by the MAVLink message_xx_send() convenience function
}) mavlink_system_t;

FFI 生成 Dart 類

class mavlink_message_t extends ffi.Opaque {}

@ffi.Packed(1)
class mavlink_system_t extends ffi.Struct {
  @ffi.Uint8()
  external int sysid;

  @ffi.Uint8()
  external int compid;
}

Answer 1

感謝@Richard Heap 的評論，我發現這個問題的根源是msgid:24字段定義。 由於它是一個位字段，目前 Dart FFI 不支持，所以 ffigen 會從中生成一個不透明的 class 定義。 刪除msgid:24聲明后，生成了 ffi.Struct 而不是 ffi.Opaque

@ffi.Packed(1)
class mavlink_message_t extends ffi.Struct {
  @ffi.Uint16()
  external int checksum;

  @ffi.Uint8()
  external int magic;

  @ffi.Uint8()
  external int len;

  @ffi.Uint8()
  external int incompat_flags;

  @ffi.Uint8()
  external int compat_flags;

  @ffi.Uint8()
  external int seq;

  @ffi.Uint8()
  external int sysid;

  @ffi.Uint8()
  external int compid;

  @ffi.Array.multi([33])
  external ffi.Array<ffi.Uint64> payload64;

  @ffi.Array.multi([2])
  external ffi.Array<ffi.Uint8> ck;

  @ffi.Array.multi([13])
  external ffi.Array<ffi.Uint8> signature;
}

這當然是不夠的，因為我需要所有字段，但是我將測試一個解決方法。