Dispatch I/O和Dispatch Data可以實(shí)現(xiàn)輸入/輸出做到多個(gè)線程并列讀取谷炸。
Dispatch I/O讀寫(xiě)文件時(shí)睹欲，使用Global Dispatch Queue將一個(gè)文件按某個(gè)大小read/write锣险。
分割讀取的數(shù)據(jù)使用Dispatch Data可以更為簡(jiǎn)單的進(jìn)行結(jié)合和分割膀息。

// Apple System Log API用的源代碼
static int
_asl_auxiliary(aslmsg msg, const char *title, const char *uti, const char *url, int *out_fd)
{
    asl_msg_t *merged_msg;
    asl_msg_aux_t aux;
    asl_msg_aux_0_t aux0;
    fileport_t fileport;
    kern_return_t kstatus;
    uint32_t outlen, newurllen, len, where;
    int status, fd, fdpair[2];
    caddr_t out, newurl;
    dispatch_queue_t pipe_q;
    dispatch_io_t pipe_channel;
    dispatch_semaphore_t sem;
    /* ..... 此處省略若干代碼.....*/
    
    // 創(chuàng)建串行隊(duì)列
    pipe_q = dispatch_queue_create("PipeQ", NULL);
    // 生成Dispatch I/O勉痴，指定發(fā)生錯(cuò)誤時(shí)執(zhí)行處理的Block秒际，以及執(zhí)行該Block的Dispatch Queue悬赏。 
    pipe_channel = dispatch_io_create(DISPATCH_IO_STREAM, fd, pipe_q, ^(int err){
        close(fd);
    });
    
    *out_fd = fdpair[1];
    
    // 該函數(shù)設(shè)定一次讀取的大小（分割大新病）
    dispatch_io_set_low_water(pipe_channel, SIZE_MAX);
    // 使用Global Dispatch Queue并列讀取闽颇，當(dāng)每個(gè)分割的文件塊讀取結(jié)束，將Dispatch Data傳遞給回調(diào)的Block.
    dispatch_io_read(pipe_channel, 0, SIZE_MAX, pipe_q, ^(bool done, dispatch_data_t pipedata, int err){
        if (err == 0) // err等于0 說(shuō)明讀取無(wú)誤
        {
            // 讀取完“單個(gè)文件塊”的大小
            size_t len = dispatch_data_get_size(pipedata);
            if (len > 0)
            {
                // 定義一個(gè)字節(jié)數(shù)組bytes
                const char *bytes = NULL;
                char *encoded;
                
                dispatch_data_t md = dispatch_data_create_map(pipedata, (const void **)&bytes, &len);
                encoded = asl_core_encode_buffer(bytes, len);
                asl_set((aslmsg)merged_msg, ASL_KEY_AUX_DATA, encoded);
                free(encoded);
                _asl_send_message(NULL, merged_msg, -1, NULL);
                asl_msg_release(merged_msg);
                dispatch_release(md);
            }
        }
        
        if (done)
        {
            dispatch_semaphore_signal(sem);
            dispatch_release(pipe_channel);
            dispatch_release(pipe_q);
        }
    });

// 異步串行讀取文件
NSString *desktop = @"/Users/xxxx/Desktop";
NSString *path = [desktop stringByAppendingPathComponent:@"整理.md"];
dispatch_queue_t queue = dispatch_queue_create("queue", NULL);

dispatch_fd_t fd = open(path.UTF8String, O_RDONLY, 0);
dispatch_io_t io = dispatch_io_create(DISPATCH_IO_STREAM, fd, queue, ^(int error) {
    close(fd);
});

size_t water = 1024 * 1024;
dispatch_io_set_low_water(io, water);
dispatch_io_set_high_water(io, water);

long long fileSize = [[NSFileManager defaultManager] attributesOfItemAtPath:path error:nil].fileSize;
NSMutableData *totalData = [[NSMutableData alloc] init];

dispatch_io_read(io, 0, fileSize, queue, ^(bool done, dispatch_data_t  _Nullable data, int error) {
    
    if (error == 0) {
        size_t len = dispatch_data_get_size(data);
        if (len > 0) {
            [totalData appendData:(NSData *)data];
        }
    }
    
    if (done) {
        NSString *str = [[NSString alloc] initWithData:totalData encoding:NSUTF8StringEncoding];
        NSLog(@"%@", str);
    }
});

// 異步并行讀取文件
NSString *desktop = @"/Users/xxx/Desktop";
NSString *path = [desktop stringByAppendingPathComponent:@"整理.md"];

dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT);
dispatch_fd_t fd = open(path.UTF8String, O_RDONLY);
dispatch_io_t io = dispatch_io_create(DISPATCH_IO_RANDOM, fd, queue, ^(int error) {
    close(fd);
});

off_t currentSize = 0;
long long fileSize = [[NSFileManager defaultManager] attributesOfItemAtPath:path error:nil].fileSize;

size_t offset = 1024 * 1024;
dispatch_group_t group = dispatch_group_create();
NSMutableData *totalData = [[NSMutableData alloc] initWithLength:fileSize];

for (; currentSize <= fileSize; currentSize += offset) {
    dispatch_group_enter(group);
    dispatch_io_read(io, currentSize, offset, queue, ^(bool done, dispatch_data_t  _Nullable data, int error) {
        
        if (error == 0) {
            size_t len = dispatch_data_get_size(data);
            if (len > 0) {
                const void *bytes = NULL;
                (void)dispatch_data_create_map(data, (const void **)&bytes, &len);
                [totalData replaceBytesInRange:NSMakeRange(currentSize, len) withBytes:bytes length:len];
            }
        }
        
        if (done) {
            dispatch_group_leave(group);
            
            NSString *str = [[NSString alloc] initWithData:totalData encoding:NSUTF8StringEncoding];
            NSLog(@"%@", str);
        }
    });
}