在處理藍(lán)牙數(shù)據(jù)的時(shí)候，經(jīng)常遇到進(jìn)制之間的轉(zhuǎn)換，藍(lán)牙處理的是16進(jìn)制（NSData），而我們習(xí)慣的計(jì)數(shù)方式是10進(jìn)制，為了節(jié)省空間，藍(lán)牙也會把16進(jìn)制（NSData）拆成2進(jìn)制記錄。這里我們研究下如何在他們之間進(jìn)行轉(zhuǎn)換。
假設(shè)我們要向藍(lán)牙發(fā)送0x1B9901這條數(shù)據(jù)
Byte轉(zhuǎn)NSData
Byte value[3]={0};value[0]=0x1B;value[1]=0x99;value[2]=0x01;NSData * data = [NSData dataWithBytes:&value length:sizeof(value)];//發(fā)送數(shù)據(jù)[self.peripheral writeValue:data forCharacteristic:self.write type:CBCharacteristicWriteWithoutResponse];
優(yōu)點(diǎn)：這種方法比較簡單，沒有進(jìn)行轉(zhuǎn)換，直接一個(gè)字節(jié)一個(gè)字節(jié)的拼裝好發(fā)送出去。
缺點(diǎn)：當(dāng)發(fā)送數(shù)據(jù)比較長時(shí)會很麻煩，而且不易更改。
NSString轉(zhuǎn)NSData
- (NSData )hexToBytes:(NSString )str{NSMutableData data = [NSMutableData data];int idx;for (idx = 0; idx+2 <= str.length; idx+=2) { NSRange range = NSMakeRange(idx, 2); NSString hexStr = [str substringWithRange:range]; NSScanner* scanner = [NSScanner scannerWithString:hexStr]; unsigned int intValue; [scanner scanHexInt:&intValue]; [data appendBytes:&intValue length:1];}return data;}//發(fā)送數(shù)據(jù)[self.peripheral writeValue:[self hexToBytes:@"1B9901"] forCharacteristic:self.write type:CBCharacteristicWriteWithoutResponse];
優(yōu)點(diǎn)：比較直觀，可以一次轉(zhuǎn)換一長條數(shù)據(jù)，對于一些功能簡單的藍(lán)牙程序，這種轉(zhuǎn)換能處理大部分情況。
缺點(diǎn)：只能發(fā)送一些固定的指令，不能參與計(jì)算。
求校驗(yàn)和
接下來探討下發(fā)送的數(shù)據(jù)需要計(jì)算的情況。最常用的發(fā)送數(shù)據(jù)需要計(jì)算的場景是求校驗(yàn)和（CHECKSUM）。這個(gè)根據(jù)硬件廠商來定，常見的求校驗(yàn)和的規(guī)則有：
如果發(fā)送數(shù)據(jù)長度為n字節(jié)，則CHECKSUM為前n-1字節(jié)之和的低字節(jié)
CHECKSUM=0x100-CHECKSUM（上一步的校驗(yàn)和）
如果我要發(fā)送帶上校驗(yàn)和的0x1B9901，方法就是：
- (NSData *)getCheckSum:(NSString *)byteStr{int length = (int)byteStr.length/2;NSData *data = [self hexToBytes:byteStr];Byte *bytes = (unsigned char *)[data bytes];Byte sum = 0;for (int i = 0; i<length; i++) { sum += bytes[i];}int sumT = sum;int at = 256 - sumT;printf("校驗(yàn)和：%d\n",at);if (at == 256) { at = 0;}NSString *str = [NSString stringWithFormat:@"%@%@",byteStr,[self ToHex:at]];return [self hexToBytes:str];}//將十進(jìn)制轉(zhuǎn)化為十六進(jìn)制- (NSString *)ToHex:(int)tmpid{NSString *nLetterValue;NSString *str =@"";int ttmpig;for (int i = 0; i<9; i++) { ttmpig=tmpid%16; tmpid=tmpid/16; switch (ttmpig) { case 10: nLetterValue =@"A";break; case 11: nLetterValue =@"B";break; case 12: nLetterValue =@"C";break; case 13: nLetterValue =@"D";break; case 14: nLetterValue =@"E";break; case 15: nLetterValue =@"F";break; default: nLetterValue = [NSString stringWithFormat:@"%u",ttmpig]; } str = [nLetterValue stringByAppendingString:str]; if (tmpid == 0) { break; }}//不夠一個(gè)字節(jié)湊0if(str.length == 1){ return [NSString stringWithFormat:@"0%@",str];}else{ return str;}}//發(fā)送數(shù)據(jù)NSData *data = [self getCheckSum:@"1B9901"];//data=<1b99014b>[self.peripheral writeValue:data forCharacteristic:self.write type:CBCharacteristicWriteWithoutResponse];
拆分?jǐn)?shù)據(jù)
這種是比較麻煩的，舉個(gè)栗子：在傳輸某條信息時(shí)，我想把時(shí)間放進(jìn)去，不能用時(shí)間戳，還要節(jié)省空間，這樣就出現(xiàn)了一種新的方式存儲時(shí)間。這里再補(bǔ)充一些C語言知識：
一個(gè)字節(jié)8位（bit）
char 1字節(jié) int 4字節(jié) unsigned 2字節(jié) float 4字節(jié)
存儲時(shí)間的條件是：
只用四個(gè)字節(jié)（32位）
前5位表示年（從2000年算起），接著4位表示月，接著5位表示日，接著5位表示時(shí)，接著6位表示分，接著3位表示星期，剩余4位保留。
這樣直觀的解決辦法就是分別取出現(xiàn)在時(shí)間的年月日時(shí)分星期，先轉(zhuǎn)成2進(jìn)制，再轉(zhuǎn)成16進(jìn)制發(fā)出去。當(dāng)然你這么寫進(jìn)去，讀的時(shí)候就要把16進(jìn)制數(shù)據(jù)先轉(zhuǎn)成2進(jìn)制再轉(zhuǎn)成10進(jìn)制顯示。我們就按這個(gè)簡單粗暴的思路來，準(zhǔn)備工作如下：
10進(jìn)制轉(zhuǎn)2進(jìn)制
// 十進(jìn)制轉(zhuǎn)二進(jìn)制- (NSString *)toBinarySystemWithDecimalSystem:(int)num length:(int)length{int remainder = 0; //余數(shù)int divisor = 0; //除數(shù)NSString * prepare = @"";while (true){ remainder = num%2; divisor = num/2; num = divisor; prepare = [prepare stringByAppendingFormat:@"%d",remainder]; if (divisor == 0) { break; }}//倒序輸出NSString * result = @"";for (int i = length -1; i >= 0; i --){ if (i <= prepare.length - 1) { result = [result stringByAppendingFormat:@"%@", [prepare substringWithRange:NSMakeRange(i , 1)]]; }else{ result = [result stringByAppendingString:@"0"]; }}return result;}
2進(jìn)制轉(zhuǎn)10進(jìn)制
// 二進(jìn)制轉(zhuǎn)十進(jìn)制- (NSString *)toDecimalWithBinary:(NSString *)binary{int ll = 0 ;int temp = 0 ;for (int i = 0; i < binary.length; i ++){ temp = [[binary substringWithRange:NSMakeRange(i, 1)] intValue]; temp = temp * powf(2, binary.length - i - 1); ll += temp;}NSString * result = [NSString stringWithFormat:@"%d",ll];return result;}
16進(jìn)制和2進(jìn)制互轉(zhuǎn)
- (NSString *)getBinaryByhex:(NSString *)hex binary:(NSString *)binary{NSMutableDictionary *hexDic = [[NSMutableDictionary alloc] init];hexDic = [[NSMutableDictionary alloc] initWithCapacity:16];[hexDic setObject:@"0000" forKey:@"0"];[hexDic setObject:@"0001" forKey:@"1"];[hexDic setObject:@"0010" forKey:@"2"];[hexDic setObject:@"0011" forKey:@"3"];[hexDic setObject:@"0100" forKey:@"4"];[hexDic setObject:@"0101" forKey:@"5"];[hexDic setObject:@"0110" forKey:@"6"];[hexDic setObject:@"0111" forKey:@"7"];[hexDic setObject:@"1000" forKey:@"8"];[hexDic setObject:@"1001" forKey:@"9"];[hexDic setObject:@"1010" forKey:@"a"];[hexDic setObject:@"1011" forKey:@"b"];[hexDic setObject:@"1100" forKey:@"c"];[hexDic setObject:@"1101" forKey:@"d"];[hexDic setObject:@"1110" forKey:@"e"];[hexDic setObject:@"1111" forKey:@"f"];NSMutableString *binaryString=[[NSMutableString alloc] init];if (hex.length) { for (int i=0; i<[hex length]; i++) { NSRange rage; rage.length = 1; rage.location = i; NSString *key = [hex substringWithRange:rage]; [binaryString appendString:hexDic[key]]; }}else{ for (int i=0; i<binary.length; i+=4) { NSString *subStr = [binary substringWithRange:NSMakeRange(i, 4)]; int index = 0; for (NSString *str in hexDic.allValues) { index ++; if ([subStr isEqualToString:str]) { [binaryString appendString:hexDic.allKeys[index-1]]; break; } } }}return binaryString;}
有了這幾種轉(zhuǎn)換函數(shù)，完成上面的功能就容易多了，具體怎么操作這里就不寫一一出來了。但總感覺怪怪的，這么一個(gè)小功能怎么要寫這么一大堆代碼，當(dāng)然還可以用c語言的方法去解決。這里主要是為了展示iOS中數(shù)據(jù)如何轉(zhuǎn)換，C語言的實(shí)現(xiàn)方法這里就不寫了，有興趣的同學(xué)可以研究下。
附帶兩個(gè)函數(shù)
int轉(zhuǎn)NSData
- (NSData *) setId:(int)Id {//用4個(gè)字節(jié)接收Byte bytes[4];bytes[0] = (Byte)(Id>>24);bytes[1] = (Byte)(Id>>16);bytes[2] = (Byte)(Id>>8);bytes[3] = (Byte)(Id);NSData *data = [NSData dataWithBytes:bytes length:4];}
NSData轉(zhuǎn)int接受到的數(shù)據(jù)0x00000a0122
//4字節(jié)表示的intNSData intData = [data subdataWithRange:NSMakeRange(2, 4)];int value = CFSwapInt32BigToHost((int)([intData bytes]));//655650//2字節(jié)表示的intNSData intData = [data subdataWithRange:NSMakeRange(4, 2)];int value = CFSwapInt16BigToHost((int)([intData bytes]));//290//1字節(jié)表示的intchar *bs = (unsigned char *)[[data subdataWithRange:NSMakeRange(5, 1) ] bytes];int value = *bs;//34
這兩個(gè)轉(zhuǎn)換在某些場景下使用頻率也是挺高的，藍(lán)牙里面的數(shù)據(jù)轉(zhuǎn)換基本也就這么多了，希望能夠幫助大家。更多關(guān)于字節(jié)編碼的問題，大家可以點(diǎn)這里：傳送門
