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第三章 3.2.13 Dispatch I/O

第三章 3.2.13 Dispatch I/O

作者: MrSYLong | 来源:发表于2018-07-26 23:53 被阅读3次

    Dispatch I/O和Dispatch Data可以实现输入/输出做到多个线程并列读取。
    Dispatch I/O读写文件时,使用Global Dispatch Queue将一个文件按某个大小read/write。
    分割读取的数据使用Dispatch Data可以更为简单的进行结合和分割。

    // Apple System Log API用的源代码
    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;
        /* ..... 此处省略若干代码.....*/
        
        // 创建串行队列
        pipe_q = dispatch_queue_create("PipeQ", NULL);
        // 生成Dispatch I/O,指定发生错误时执行处理的Block,以及执行该Block的Dispatch Queue。 
        pipe_channel = dispatch_io_create(DISPATCH_IO_STREAM, fd, pipe_q, ^(int err){
            close(fd);
        });
        
        *out_fd = fdpair[1];
        
        // 该函数设定一次读取的大小(分割大小)
        dispatch_io_set_low_water(pipe_channel, SIZE_MAX);
        // 使用Global Dispatch Queue并列读取,当每个分割的文件块读取结束,将Dispatch Data传递给回调的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 说明读取无误
            {
                // 读取完“单个文件块”的大小
                size_t len = dispatch_data_get_size(pipedata);
                if (len > 0)
                {
                    // 定义一个字节数组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);
            }
        });
    
    
    // 异步串行读取文件
    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);
        }
    });
    
    // 异步并行读取文件
    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);
            }
        });
    }
    

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