以NSString形式寫入NSData

我要寫入Ble時，會先用文字格式給出命令，再轉為NSData寫入device

-(void) setGSensorParameter:(NSString*)parameter completion:(WriteValueResponse)completion{

    if (![_bleManager isConnected]) {

        return;

    }

    NSUInteger len = parameter.length;

    NSString* dataString = [[NSString alloc]initWithString:parameter];

    

    NSMutableData* data = [[NSMutableData alloc]init];

    if (dataString.length % 2 != 0) {

        dataString = [NSString stringWithFormat:@"0%@",dataString];

    }

    //逐字轉成Bytes
    for (int i = 0; i < len; i = i + 2)
    {
        NSRange range = NSMakeRange(i, 1);
        NSString *firstString = [dataString substringWithRange:range];
        range = NSMakeRange(i + 1, 1);
        NSString *secondString = [dataString substringWithRange:range];
        NSNumber* firstDigit = [NSNumber numberWithInt:[[DataManager unitDigitFromHexString:firstString] intValue] * 16];
        NSNumber* secondDigit = [DataManager unitDigitFromHexString:secondString];
        NSNumber* number = [NSNumber numberWithInt:[firstDigit intValue] + [secondDigit intValue]];

        

        unsigned char charNumber = [number charValue];
        [data appendBytes:&charNumber length:1];

        

    }
    //NSLog(@"data:%@",data);

    [_bleManager writeValueWithWithData:data

                         forServiceUUID:@"XXXX"

                     characteristicUUID:@"YYYY"

                           withResponse:completion];

}

+ (NSNumber *) unitDigitFromHexString:(NSString *)unitDigit{

    int number = 0;

    if ([unitDigit isEqualToString:@"a"] || [unitDigit isEqualToString:@"A"]) {

        number = 10;

    }

    else if ([unitDigit isEqualToString:@"b"] || [unitDigit isEqualToString:@"B"]) {

        number = 11;

    }

    else if ([unitDigit isEqualToString:@"c"] || [unitDigit isEqualToString:@"C"]) {

        number = 12;

    }

    else if ([unitDigit isEqualToString:@"d"] || [unitDigit isEqualToString:@"D"]) {

        number = 13;

    }

    else if ([unitDigit isEqualToString:@"e"] || [unitDigit isEqualToString:@"E"]) {

        number = 14;

    }

    else if ([unitDigit isEqualToString:@"f"] || [unitDigit isEqualToString:@"F"]) {

        number = 15;

    }

    else if (0 <= [unitDigit intValue] < 10)

    {

        NSNumber *result = [NSNumber numberWithInt:[unitDigit intValue]];

        return result;

    }

    else

    {

        return nil;

    }

    

    NSNumber *result = [NSNumber numberWithInt:number];

    return result;

}