如何将字符串转换为NSData以将其传递给BLE设备?

NSString * str = @"Hello World";
NSString * hexStr = [NSString stringWithFormat:@"%@",
                     [NSData dataWithBytes:[str cStringUsingEncoding:NSUTF8StringEncoding] 
                                    length:strlen([str cStringUsingEncoding:NSUTF8StringEncoding])]];

for(NSString * toRemove in [NSArray arrayWithObjects:@"<", @">", @" ", nil]) 
    hexStr = [hexStr stringByReplacingOccurrencesOfString:toRemove withString:@""];

NSLog(@"%@", hexStr); 48656c6c6f20776f726c64

但困难的部分就是从那开始的。如何将每两个十六进制分隔为uint8\u t。因为我不知道长度,但用户可以写任何长度的字符串,这将是巨大的十六进制。如果我知道长度,我可以这样做

NSMutableData *concatenate = [NSMutableData data];
uint8_t first=0x48;
uint8_t second = 0x56;
uint8_t third = 0x6c;
uint8_t fourth = 0x6f;
uint8_t fifth=0x20;
uint8_t sixth=0x77;
uint8_t seventh=0x6f;
uint8_t eighth=0x72;
uint8_t ninth=0x6c;
uint8_t tenth=0x64;

NSData* one = [NSData dataWithBytes:(void*)&first length:sizeof(first)];
NSData* two = [NSData dataWithBytes:(void*)&second length:sizeof(second)];
NSData* three = [NSData dataWithBytes:(void*)&third length:sizeof(third)];
NSData* four = [NSData dataWithBytes:(void*)&fourth length:sizeof(fourth)];
NSData* five = [NSData dataWithBytes:(void*)&fifth length:sizeof(fifth)];
NSData* six = [NSData dataWithBytes:(void*)&sixth length:sizeof(sixth)];
NSData* seven = [NSData dataWithBytes:(void*)&seventh length:sizeof(seventh)];
NSData* eight = [NSData dataWithBytes:(void*)&eighth length:sizeof(eighth)];
NSData* nine = [NSData dataWithBytes:(void*)&nineth length:sizeof(nineth)];
NSData* ten = [NSData dataWithBytes:(void*)&tenth length:sizeof(tenth)];

[concatenate appendData:one];
[concatenate appendData:two];
[concatenate appendData:three];
[concatenate appendData:four];
[concatenate appendData:five];
[concatenate appendData:six];
[concatenate appendData:seven];
[concatenate appendData:eight];
[concatenate appendData:nine];
[concatenate appendData:ten];

但当我不知道长度时,我就不知道了。如有任何帮助或线索,我们将不胜感激。