Flutter typedef 函数类型

 typedef 函数 在flutter中的应用：

实际上在使用 flutter 的时候经常会用到这个东西：比如 IconButton 组件里面  onPressed

IconButton(
  onPressed: () {},
  icon: Icon(Icons.add),
),

到 onPressed 定义的地方:  VoidCallback

/// The callback that is called when the button is tapped or otherwise activated.
///
/// If this is set to null, the button will be disabled.
final VoidCallback? onPressed;

 到 VoidCallback 定义的地方:   发现是一个  typedef 类型

/// Signature of callbacks that have no arguments and return no data.
typedef VoidCallback = void Function();

其实 flutter 还有很多类型的东西：

typedef VoidCallback        = void Function();
typedef ValueChanged     = void Function(T value);
typedef ValueSetter      = void Function(T value);
typedef ValueGetter      = T Function();
typedef IterableFilter   = Iterable Function(Iterable input);
typedef AsyncCallback       = Future Function();
typedef AsyncValueSetter = Future Function(T value);
 

上面都是 Flutter 自带的一些 typedef 函数，当然你也可以自定义。不过使用他自带的其实也都够用了！

typedef Compare = int Function(T a, T b);

因为 typedef 是一个类型， 所以他可以使用 is 方法判断函数是不是你自己定义的：

typedef Compare = int Function(T a, T b);

int sort(Object a, Object b) => 0;


class SortedCollection {
  Compare compare;

  SortedCollection(this.compare);
}

void main() {
  SortedCollection coll = SortedCollection(sort);
  printf(coll.compare is Function); //false
  printf(coll.compare is Compare);  //TRUE
}

typedef 自定义函数

自定义其他 多参数的情况：

typedef VoidCallback = void Function();
typedef ErrorCallback = void Function(int code, String error);
typedef V2TimUserFullInfoCallback = void Function(V2TimUserFullInfo info);
typedef OnTotalUnreadMessageCountChanged = void Function(int totalUnreadCount);
typedef OnUserStatusChanged = void Function(
    List userStatusList);
typedef OnLog = void Function(int logLevel, String logContent);
typedef OnRecvC2CTextMessageCallback = void Function(
  String msgID,
  V2TimUserInfo userInfo,
  String text,
);
typedef OnRecvMessageExtensionsChanged = void Function(
  String msgID,
  List extensions,
);
typedef OnRecvMessageExtensionsDeleted = void Function(
  String msgID,
  List extensionKeys,
);
typedef OnMessageDownloadProgressCallback = void Function(
  V2TimMessageDownloadProgress messageProgress,
);
typedef OnRecvC2CCustomMessageCallback = void Function(
  String msgID,
  V2TimUserInfo sender,
  String customData,
);
typedef OnRecvGroupTextMessageCallback = void Function(
  String msgID,
  String groupID,
  V2TimGroupMemberInfo sender,
  String text,
);

typedef OnRecvGroupCustomMessageCallback = void Function(
  String msgID,
  String groupID,
  V2TimGroupMemberInfo sender,
  String customData,
);
typedef OnMemberEnterCallback = void Function(
  String groupID,
  List memberList,
);
// void 	onMemberEnter (String groupID, List< V2TIMGroupMemberInfo > memberList)
typedef OnMemberLeaveCallback = void Function(
  String groupID,
  V2TimGroupMemberInfo member,
);
// void 	onMemberLeave (String groupID, V2TIMGroupMemberInfo member)
typedef OnMemberInvitedCallback = void Function(
  String groupID,
  V2TimGroupMemberInfo opUser,
  List memberList,
);
// void 	onMemberInvited (String groupID, V2TIMGroupMemberInfo opUser, List< V2TIMGroupMemberInfo > memberList)
typedef OnMemberKickedCallback = void Function(
  String groupID,
  V2TimGroupMemberInfo opUser,
  List memberList,
);
// void 	onMemberKicked (String groupID, V2TIMGroupMemberInfo opUser, List< V2TIMGroupMemberInfo > memberList)
typedef OnMemberInfoChangedCallback = void Function(
  String groupID,
  List v2TIMGroupMemberChangeInfoList,
);
// void 	onMemberInfoChanged (String groupID, List< V2TIMGroupMemberChangeInfo > v2TIMGroupMemberChangeInfoList)
typedef OnGroupCreatedCallback = void Function(
  String groupID,
);
// void 	onGroupCreated (String groupID)
typedef OnGroupDismissedCallback = void Function(
  String groupID,
  V2TimGroupMemberInfo opUser,
);
// void 	onGroupDismissed (String groupID, V2TIMGroupMemberInfo opUser)
typedef OnGroupRecycledCallback = void Function(
  String groupID,
  V2TimGroupMemberInfo opUser,
);
// void 	onGroupRecycled (String groupID, V2TIMGroupMemberInfo opUser)
typedef OnGroupInfoChangedCallback = void Function(
  String groupID,
  List changeInfos,
);
// void 	onGroupInfoChanged (String groupID, List< V2TIMGroupChangeInfo > changeInfos)
typedef OnReceiveJoinApplicationCallback = void Function(
  String groupID,
  V2TimGroupMemberInfo member,
  String opReason,
);
// void 	onReceiveJoinApplication (String groupID, V2TIMGroupMemberInfo member, String opReason)
typedef OnApplicationProcessedCallback = void Function(
  String groupID,
  V2TimGroupMemberInfo opUser,
  bool isAgreeJoin,
  String opReason,
);
// void 	onApplicationProcessed (String groupID, V2TIMGroupMemberInfo opUser, boolean isAgreeJoin, String opReason)
typedef OnGrantAdministratorCallback = void Function(
  String groupID,
  V2TimGroupMemberInfo opUser,
  List memberList,
);
// void 	onGrantAdministrator (String groupID, V2TIMGroupMemberInfo opUser, List< V2TIMGroupMemberInfo > memberList)
typedef OnRevokeAdministratorCallback = void Function(
  String groupID,
  V2TimGroupMemberInfo opUser,
  List memberList,
);
// void 	onRevokeAdministrator (String groupID, V2TIMGroupMemberInfo opUser, List< V2TIMGroupMemberInfo > memberList)
typedef OnQuitFromGroupCallback = void Function(
  String groupID,
);
// void 	onQuitFromGroup (String groupID)
typedef OnReceiveRESTCustomDataCallback = void Function(
  String groupID,
  String customData,
);
// void 	onReceiveRESTCustomData (String groupID, byte[] customData)
typedef OnGroupAttributeChangedCallback = void Function(
  String groupID,
  Map groupAttributeMap,
);
// void 	onGroupAttributeChanged (String groupID, Map< String, String > groupAttributeMap)
typedef OnRecvNewMessageCallback = void Function(
  V2TimMessage msg,
);

// void 	onGroupAttributeChanged (String groupID, Map< String, String > groupAttributeMap)
typedef OnRecvMessageModified = void Function(
  V2TimMessage msg,
);

// void 	onRecvNewMessage (V2TIMMessage msg)
typedef OnRecvC2CReadReceiptCallback = void Function(
  List receiptList,
);
// void 	onRecvC2CReadReceipt (List< V2TIMMessageReceipt > receiptList)
typedef OnRecvMessageRevokedCallback = void Function(
  String msgID,
);
// void 	onRecvMessageRevoked (String msgID)
//
typedef OnFriendApplicationListAddedCallback = void Function(
  List applicationList,
);
// void 	onFriendApplicationListAdded (List< V2TIMFriendApplication > applicationList)
typedef OnFriendApplicationListDeletedCallback = void Function(
  List userIDList,
);
// void 	onFriendApplicationListDeleted (List< String > userIDList)
typedef OnFriendApplicationListReadCallback = void Function();
// void 	onFriendApplicationListRead ()
typedef OnFriendListAddedCallback = void Function(List users);
// void 	onFriendListAdded (List< V2TIMFriendInfo > users)
typedef OnFriendListDeletedCallback = void Function(List userList);
// void 	onFriendListDeleted (List< String > userList)
typedef OnBlackListAddCallback = void Function(List infoList);
// void 	onBlackListAdd (List< V2TIMFriendInfo > infoList)
typedef OnBlackListDeletedCallback = void Function(List userList);
// void 	onBlackListDeleted (List< String > userList)
typedef OnFriendInfoChangedCallback = void Function(
  List infoList,
);
// void 	onFriendInfoChanged (List< V2TIMFriendInfo > infoList)
typedef OnConversationChangedCallback = void Function(
  List conversationList,
);
// void 	onNewConversation (List< V2TIMConversation > conversationList)
typedef OnNewConversation = void Function(
  List conversationList,
);

// void 	onConversationChanged (List< V2TIMConversation > conversationList)
typedef OnReceiveNewInvitationCallback = void Function(
  String inviteID,
  String inviter,
  String groupID,
  List inviteeList,
  String data,
);
// void 	onReceiveNewInvitation (String inviteID, String inviter, String groupID, List< String > inviteeList, String data)
typedef OnInviteeAcceptedCallback = void Function(
  String inviteID,
  String invitee,
  String data,
);
// void 	onInviteeAccepted (String inviteID, String invitee, String data)
typedef OnInviteeRejectedCallback = void Function(
  String inviteID,
  String invitee,
  String data,
);
// void 	onInviteeRejected (String inviteID, String invitee, String data)
typedef OnInvitationCancelledCallback = void Function(
  String inviteID,
  String inviter,
  String data,
);
// void 	onInvitationCancelled (String inviteID, String inviter, String data)
typedef OnInvitationTimeoutCallback = void Function(
  String inviteID,
  List inviteeList,
);
// void 	onInvitationTimeout (String inviteID, List< String > inviteeList)
//
typedef OnSendMessageProgressCallback = void Function(
  V2TimMessage message,
  int progress,
);
typedef OnRecvMessageReadReceipts = void Function(
  List receiptList,
);
typedef OnConversationGroupCreated = void Function(
    String groupName, List conversationList);

typedef OnConversationGroupDeleted = void Function(String groupName);

typedef OnConversationGroupNameChanged = void Function(
    String oldName, String newName);

typedef OnConversationsAddedToGroup = void Function(
    String groupName, List conversationList);

typedef OnConversationsDeletedFromGroup = void Function(
    String groupName, List conversationList);

typedef OnTopicCreated = void Function(String groupID, String topicID);

typedef OnTopicDeleted = void Function(
    String groupID, List topicIDList);

typedef OnTopicInfoChanged = void Function(
    String groupID, V2TimTopicInfo topicInfo);

typedef OnGroupCounterChanged = void Function(
  String groupID,
  String key,
  int newValue,
);

 typedef 函数 实践：

按照使用的场景，最多出现的地方就是我们的 回调函数，不管是按钮的，还是其他的地方，和其他语言的回调函数类似：

• javascript    Function,  箭头函数
• c#  事件和委托

都是可以 把函数 当做参数传递，然后在另外一个地方等待调用。

typedef 函数 高阶用法示例：

► 腾讯 im中的 使用 typedef 实例

  @override
  Future init(
      {
      /// Callback from TUIKit invoke, includes IM SDK API error, notify information, Flutter error.
      ValueChanged? onTUIKitCallbackListener,
      required int sdkAppID,
      required LogLevelEnum loglevel,
      required V2TimSDKListener listener,
      LanguageEnum? language,
      String? extraLanguage,
      TIMUIKitConfig? config,

      /// Specify the current device platform, mobile or desktop, based on your needs.
      /// TUIKit will automatically determine the platform if no specification is provided. DeviceType? platform,
      DeviceType? platform,
      VoidCallback? onWebLoginSuccess}) async {
    省略
}

 V2TimSDKListener  listener  定义如下  

class V2TimSDKListener {
  VoidCallback onConnecting = () {};
  VoidCallback onConnectSuccess = () {};
  ErrorCallback onConnectFailed = (int code, String error) {};
  VoidCallback onKickedOffline = () {};
  VoidCallback onUserSigExpired = () {};
  V2TimUserFullInfoCallback onSelfInfoUpdated = (
    V2TimUserFullInfo info,
  ) {};
  OnUserStatusChanged onUserStatusChanged = (List userStatusList){};
  OnLog onLog = (int logLevel,String logContent){
    
  };


  V2TimSDKListener({
    ErrorCallback? onConnectFailed,
    VoidCallback? onConnectSuccess,
    VoidCallback? onConnecting,
    VoidCallback? onKickedOffline,
    V2TimUserFullInfoCallback? onSelfInfoUpdated,
    VoidCallback? onUserSigExpired,
    OnUserStatusChanged? onUserStatusChanged,
    OnLog? onLog,
  }) {
    if (onConnectFailed != null) {
      this.onConnectFailed = onConnectFailed;
    }
    if (onConnectSuccess != null) {
      this.onConnectSuccess = onConnectSuccess;
    }
    if (onConnecting != null) {
      this.onConnecting = onConnecting;
    }
    if (onKickedOffline != null) {
      this.onKickedOffline = onKickedOffline;
    }
    if (onSelfInfoUpdated != null) {
      this.onSelfInfoUpdated = onSelfInfoUpdated;
    }
    if (onUserSigExpired != null) {
      this.onUserSigExpired = onUserSigExpired;
    }
    if(onUserStatusChanged!=null){
      this.onUserStatusChanged = onUserStatusChanged;
    }
    if(onLog!=null){
      this.onLog = onLog;
    }
  }
}

他的嵌套使用很普遍，甚至直接 定义一个类型来接收 回调函数：合理使用的情况下，会给我们代码提供很大便捷