how to calculate the byte sum of struct in swift - swift

Question

this is the code in OC
It is not easy for me to convert it to swift, especially the calculate of check sum part
If anyone knows how to write, please let me know. Thank you.
OC Code:
MessageToOnboard {
unsigned char header[2];
ushort length;
uint8_t msgID;
uint missionID;
ushort wayPointID;
double lon;
double lat;
float height;
unsigned char wayPointAction;
// unsigned long reserve;
unsigned char checkSum;
}
MessageToOnboard message;
message.header[0] = 'Q';
message.header[1] = 'Z';
message.msgID = 0x21;
message.missionID = 12;
message.wayPointID = 10;
message.lon = 34.2323242;
message.lat = 68.2121221;
message.height = 30;
message.wayPointAction = '2';
message.length = sizeof(message) -5;
uint8_t *msgBIn = (uint8_t *)&message;
int iLength = sizeof(message);
uint8_t icheckSum = 0;
for (int i = 0 ; i < iLength - 3; i++)
{
icheckSum += *(msgBIn+2+i);
}
message.checkSum = icheckSum;
NSData *data = [[NSData alloc] initWithBytes:&message length:sizeof(message)];
Swift Code:
I tried this, but it is not right, I don't know how to use pointer to calculate the sum bytes of struct in swift
let msgBIn = withUnsafePointer(to: &message) {$0}
print(msgBIn.pointee)
let iLength: Int = MemoryLayout<MessageToOnboard>.size
let icheckSum: UInt8 = 0
for i in 0..<iLength - 1 {
let pointer = msgBIn.advanced(by: 2 + i)
icheckSum += UInt8(pointer)
}

Answer 1

finally, solved this problem
Message is a Struct
func sumByte(msg: inout Message) -> UInt8 {
let pointer = withUnsafePointer(to: msg) {UnsafeRawPointer($0)}
var sum: UInt8 = 0;
for i in 0..<(MemoryLayout<Message>.size) {
let nextP = pointer.advanced(by: i)
sum += nextP.load(as: UInt8.self)
}
return sum;
}