Question

我想将MIDI信息打包到NSData对象中。

int messageType = 3; // 0-15
int channel = 5;      // 0-15
int data1 = 56;       // 0-127
int data2 = 78;       // 0-127

int packed = data2;
packed += data1 * 127;
packed += channel * 16129; // 127^2
packed += messageType * 258064; // 127^2 * 16

NSLog(@"packed %d", packed);

NSData *packedData = [NSData dataWithBytes:&packed length:sizeof(packed)];

int recovered;
[packedData getBytes:&recovered];

NSLog(@"recovered %d", recovered);

这非常有效，虽然我为自己感到自豪，但我知道转换为字节并没有正确完成：它应该是一个没有大量加法和乘法的直接转换。怎么办呢？

编辑：我现在知道我可以这样做

char theBytes[] = {messageType, channel, data1, data2};
NSData *packedData = [NSData dataWithBytes:&theBytes length:sizeof(theBytes)];

并在Java方面

byte[] byteBuffer = new byte[4]; // Receive buffer
while (in.read(byteBuffer) != -1) {  
    System.out.println("data2="  + byteBuffer[3]);
}

它会起作用，但我想要一个只有3个字节的NSData的解决方案。

Answer 1

就个人而言，我会选择NSString：

NSString *dataString = [NSString stringWithFormat:@"%i+%i+%i+%i", messageType, channel, data1, data2];
NSData *packedData = [dataString dataUsingEncoding:NSUTF8StringEncoding];

易于使用，易于转移。拆包有点复杂，但也不困难。

NSScanner *scanner = [NSScanner scannerWithString:[[[NSString alloc] initWithData:packedData encoding:NSUTF8StringEncoding] autorelease]];
int messageType, channel, data1, data2;
[scanner scanInt:&messageType];
[scanner scanInt:&channel];
[scanner scanInt:&data1];
[scanner scanInt:&data2];

Answer 2

这是我放在一起的3字节解决方案。

char theBytes[] = {message_type  * 16 + channel, data1, data2};
NSData *packedData = [NSData dataWithBytes:&theBytes length:sizeof(theBytes)];

char theBytesRecovered[3];
[packedData getBytes:theBytesRecovered];

int messageTypeAgain = (int)theBytesRecovered[0]/16;
int channelAgain = (int)theBytesRecovered[0] % 16;
int data1Again = (int)theBytesRecovered[1];
int data2Again = (int)theBytesRecovered[2];

NSLog(@"packed %d %d %d %d", messageTypeAgain, channelAgain, data1Again, data2Again);

并且在导线的另一侧，这很容易拾取，因为每个字节都是一个字节。我刚刚在iOS端和Java端完成了这个尝试，并且两者都没有问题。 endian-ness没有问题，因为每个整数都适合一个字节（或者一个字节中的两个，在一种情况下）。

Answer 3

你有几种选择。

因为看起来你想要在NSData表示中连续的数据集......

您将要创建一个压缩结构，并将数据作为预定义的字节顺序传递给NSData调用（因此两端都知道如何取消归档数据glob）。

/* pack this struct's ivars and and enable -Wreorder to sanity check that the compiler does not reorder members -- i see no reason for the compiler to do this since the fields are equal size/type */
struct t_midi_message {
    UInt8 message_type; /* 0-15 */
    UInt8 channel; /* 0-15 */
    UInt8 data1; /* 0-127 */
    UInt8 data2; /* 0-127 */
};

union t_midi_message_archive {
/* members - as a union for easy endian swapping */
    SInt32 glob;
    t_midi_message message;
    enum { ValidateSize = 1 / (4 == sizeof(t_midi_message)) };
/* nothing unusual here, although you may want a ctor which takes NSData as an argument */
    t_midi_message_archive();
    t_midi_message_archive(const t_midi_message&);
    t_midi_message_archive(const t_midi_message_archive&);
    t_midi_message_archive& operator=(const t_midi_message_archive&);

/* swap routines -- just pass @member glob to the system's endian routines */
    void swapToNativeEndianFromTransferEndian();
    void swapToTransferEndianFromNativeEndian();

};

void a(const t_midi_message_archive& msg) {

    t_midi_message_archive copy(msg);
    copy.swapToTransferEndianFromNativeEndian();

    NSData * packedData([NSData dataWithBytes:&copy.glob length:sizeof(copy.glob)]);
    assert(packedData);

    t_midi_message_archive recovered;
    [packedData getBytes:&recovered.glob];

    recovered.swapToNativeEndianFromTransferEndian();
    /* recovered may now be used safely */
}

打包到字节（NSData）

3 个答案: