1. + (UIColor *)boringColor; 
2. { 
3.     static UIColor *color; 
4.     static dispatch_once_t onceToken; 
5.     dispatch_once(&onceToken, ^{ 
6.         color = [UIColor colorWithRed:0.380f green:0.376f blue:0.376f alpha:1.000f]; 
7.     }); 
8.     return color; 
9. } 

1. - (void)foo 
2. { 
3.     double delayInSeconds = 2.0; 
4.     dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t) (delayInSeconds * NSEC_PER_SEC)); 
5.     dispatch_after(popTime, dispatch_get_main_queue(), ^(void){ 
6.         [self bar]; 
7.     }); 
8. } 

1. - (id)init; 
2. { 
3.     self = [super init]; 
4.     if (self != nil) { 
5.         NSString *label = [NSString stringWithFormat:@"%@.isolation.%p", [self class], self]; 
6.         self.isolationQueue = dispatch_queue_create([label UTF8String], 0); 
7.          
8.         label = [NSString stringWithFormat:@"%@.work.%p", [self class], self]; 
9.         self.workQueue = dispatch_queue_create([label UTF8String], 0); 
10.     } 
11.     return self; 
12. } 

1. - (void)setCount:(NSUInteger)count forKey:(NSString *)key 
2. { 
3.     key = [key copy]; 
4.     dispatch_async(self.isolationQueue, ^(){ 
5.         if (count == 0) { 
6.             [self.counts removeObjectForKey:key]; 
7.         } else { 
8.             self.counts[key] = @(count); 
9.         } 
10.     }); 
11. } 
12.  
13. - (NSUInteger)countForKey:(NSString *)key; 
14. { 
15.     __block NSUInteger count; 
16.     dispatch_sync(self.isolationQueue, ^(){ 
17.         NSNumber *n = self.counts[key]; 
18.         count = [n unsignedIntegerValue]; 
19.     }); 
20.     return count; 
21. } 

1. self.isolationQueue = dispatch_queue_create([label UTF8String], DISPATCH_QUEUE_CONCURRENT); 

1. - (void)setCount:(NSUInteger)count forKey:(NSString *)key 
2. { 
3.     key = [key copy]; 
4.     dispatch_barrier_async(self.isolationQueue, ^(){ 
5.         if (count == 0) { 
6.             [self.counts removeObjectForKey:key]; 
7.         } else { 
8.             self.counts[key] = @(count); 
9.         } 
10.     }); 
11. } 

1. dispatch_queue_t queueA; // assume we have this 
2. dispatch_sync(queueA, ^(){ 
3.     dispatch_sync(queueA, ^(){ 
4.         foo(); 
5.     }); 
6. }); 

1. dispatch_queue_t queueA; // assume we have this 
2. dispatch_queue_t queueB; // assume we have this 
3.  
4. dispatch_sync(queueA, ^(){ 
5.     foo(); 
6. }); 
7.  
8. void foo(void) 
9. { 
10.     dispatch_sync(queueB, ^(){ 
11.         bar(); 
12.     }); 
13. } 
14.  
15. void bar(void) 
16. { 
17.     dispatch_sync(queueA, ^(){ 
18.         baz(); 
19.     }); 
20. } 

1. dispatch_queue_t queueA; // assume we have this 
2. dispatch_async(queueA, ^(){ 
3.     dispatch_async(queueA, ^(){ 
4.         foo(); 
5.     }); 
6. }); 

1. - (void)processImage:(UIImage *)image completionHandler:(void(^)(BOOL success))handler; 
2. { 
3.     dispatch_async(self.isolationQueue, ^(void){ 
4.         // do actual processing here 
5.         dispatch_async(self.resultQueue, ^(void){ 
6.             handler(YES); 
7.         }); 
8.     }); 
9. } 

1. for (size_t y = 0; y < height; ++y) { 
2.     for (size_t x = 0; x < width; ++x) { 
3.         // Do something with x and y here 
4.     } 
5. } 

1. dispatch_apply(height, dispatch_get_global_queue(0, 0), ^(size_t y) { 
2.     for (size_t x = 0; x < width; x += 2) { 
3.         // Do something with x and y here 
4.     } 
5. }); 

1. dispatch_group_t group = dispatch_group_create(); 
2.  
3. dispatch_queue_t queue = dispatch_get_global_queue(0, 0); 
4. dispatch_group_async(group, queue, ^(){ 
5.     // Do something that takes a while 
6.     [self doSomeFoo]; 
7.     dispatch_group_async(group, dispatch_get_main_queue(), ^(){ 
8.         self.foo = 42; 
9.     }); 
10. }); 
11. dispatch_group_async(group, ^(){ 
12.     // Do something else that takes a while 
13.     [self doSomeBar]; 
14.     dispatch_group_async(group, dispatch_get_main_queue(), ^(){ 
15.         self.bar = 1; 
16.     }); 
17. }); 
18.  
19. // This block will run once everything above is done: 
20. dispatch_group_notify(group, dispatch_get_main_queue(), ^(){ 
21.     NSLog(@"foo: %d", self.foo); 
22.     NSLog(@"bar: %d", self.bar); 
23. }); 

1. - (void)withGroup:(dispatch_group_t)group performBlock:(dispatch_block_t)block 
2. { 
3.     if (group == NULL) { 
4.         [self performBlock:block]; 
5.     } else { 
6.         dispatch_group_enter(group); 
7.         [self performBlock:^(){ 
8.             block(); 
9.             dispatch_group_leave(group); 
10.         }]; 
11.     } 
12. } 

1. + (void)withGroup:(dispatch_group_t)group  
2.         sendAsynchronousRequest:(NSURLRequest *)request  
3.         queue:(NSOperationQueue *)queue  
4.         completionHandler:(void (^)(NSURLResponse*, NSData*, NSError*))handler 
5. { 
6.     if (group == NULL) { 
7.         [self sendAsynchronousRequest:request  
8.                                 queue:queue  
9.                     completionHandler:handler]; 
10.     } else { 
11.         dispatch_group_enter(group); 
12.         [self sendAsynchronousRequest:request  
13.                                 queue:queue  
14.                     completionHandler:^(NSURLResponse *response, NSData *data, NSError *error){ 
15.             handler(response, data, error); 
16.             dispatch_group_leave(group); 
17.         }]; 
18.     } 
19. } 

1. NSRunningApplication *mail = [NSRunningApplication  
2.   runningApplicationsWithBundleIdentifier:@"com.apple.mail"]; 
3. if (mail == nil) { 
4.     return; 
5. } 
6. pid_t const pid = mail.processIdentifier; 
7. self.source = dispatch_source_create(DISPATCH_SOURCE_TYPE_PROC, pid,  
8.   DISPATCH_PROC_EXIT, DISPATCH_TARGET_QUEUE_DEFAULT); 
9. dispatch_source_set_event_handler(self.source, ^(){ 
10.     NSLog(@"Mail quit."); 
11. }); 
12. dispatch_resume(self.source); 

1. NSURL *directoryURL; // assume this is set to a directory 
2. int const fd = open([[directoryURL path] fileSystemRepresentation], O_EVTONLY); 
3. if (fd < 0) { 
4.     char buffer[80]; 
5.     strerror_r(errno, buffer, sizeof(buffer)); 
6.     NSLog(@"Unable to open \"%@\": %s (%d)", [directoryURL path], buffer, errno); 
7.     return; 
8. } 
9. dispatch_source_t source = dispatch_source_create(DISPATCH_SOURCE_TYPE_VNODE, fd,  
10.   DISPATCH_VNODE_WRITE | DISPATCH_VNODE_DELETE, DISPATCH_TARGET_QUEUE_DEFAULT); 
11. dispatch_source_set_event_handler(source, ^(){ 
12.     unsigned long const data = dispatch_source_get_data(source); 
13.     if (data & DISPATCH_VNODE_WRITE) { 
14.         NSLog(@"The directory changed."); 
15.     } 
16.     if (data & DISPATCH_VNODE_DELETE) { 
17.         NSLog(@"The directory has been deleted."); 
18.     } 
19. }); 
20. dispatch_source_set_cancel_handler(source, ^(){ 
21.     close(fd); 
22. }); 
23. self.source = source; 
24. dispatch_resume(self.source); 

1. dispatch_source_t source = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER,  
2.   0, 0, DISPATCH_TARGET_QUEUE_DEFAULT); 
3. dispatch_source_set_event_handler(source, ^(){ 
4.     NSLog(@"Time flies."); 
5. }); 
6. dispatch_time_t start 
7. dispatch_source_set_timer(source, DISPATCH_TIME_NOW, 5ull * NSEC_PER_SEC,  
8.   100ull * NSEC_PER_MSEC); 
9. self.source = source; 
10. dispatch_resume(self.source); 

0: HTTP/1.1 200 OK\r\nDate: Mon, 23 May 2005 22:38
1: :34 GMT\r\nServer: Apache/1.3.3.7 (Unix) (Red-H
2: at/Linux)\r\nLast-Modified: Wed, 08 Jan 2003 23
3: :11:55 GMT\r\nEtag: "3f80f-1b6-3e1cb03b"\r\nCon
4: tent-Type: text/html; charset=UTF-8\r\nContent-
5: Length: 131\r\nConnection: close\r\n\r\n<html>\r
6: \n<head>\r\n  <title>An Example Page</title>\r\n
7: </head>\r\n<body>\r\n  Hello World, this is a ve

dispatch_data_t a; // Assume this hold some valid data
dispatch_data_t b; // Assume this hold some valid data
dispatch_data_t c = dispatch_data_create_concat(a, b);

dispatch_data_apply(c, ^(dispatch_data_t region, size_t offset, const void *buffer, size_t size) {
    fprintf(stderr, "region with offset %zu, size %zu\n", offset, size);
    return true;
});

dispatch_io_t dispatch_io_create(dispatch_io_type_t type, dispatch_fd_t fd, 
  dispatch_queue_t queue, void (^cleanup_handler)(int error));

_isolation = dispatch_queue_create([[self description] UTF8String], 0);
_nativeSocket = socket(PF_INET6, SOCK_STREAM, IPPROTO_TCP);
struct sockaddr_in sin = {};
sin.sin_len = sizeof(sin);
sin.sin_family = AF_INET6;
sin.sin_port = htons(port);
sin.sin_addr.s_addr= INADDR_ANY;
int err = bind(result.nativeSocket, (struct sockaddr *) &sin, sizeof(sin));
NSCAssert(0 <= err, @"");
 
_eventSource = dispatch_source_create(DISPATCH_SOURCE_TYPE_READ, _nativeSocket, 0, _isolation);
dispatch_source_set_event_handler(result.eventSource, ^{
    acceptConnection(_nativeSocket);
});

typedef union socketAddress {
    struct sockaddr sa;
    struct sockaddr_in sin;
    struct sockaddr_in6 sin6;
} socketAddressUnion;
 
socketAddressUnion rsa; // remote socket address
socklen_t len = sizeof(rsa);
int native = accept(nativeSocket, &rsa.sa, &len);
if (native == -1) {
    // Error. Ignore.
    return nil;
}
 
_remoteAddress = rsa;
_isolation = dispatch_queue_create([[self description] UTF8String], 0);
_channel = dispatch_io_create(DISPATCH_IO_STREAM, native, _isolation, ^(int error) {
    NSLog(@"An error occured while listening on socket: %d", error);
});
 
//dispatch_io_set_high_water(_channel, 8 * 1024);
dispatch_io_set_low_water(_channel, 1);
dispatch_io_set_interval(_channel, NSEC_PER_MSEC * 10, DISPATCH_IO_STRICT_INTERVAL);
 
socketAddressUnion lsa; // remote socket address
socklen_t len = sizeof(rsa);
getsockname(native, &lsa.sa, &len);
_localAddress = lsa;

如果我们想要设置SO_KEEPALIVE（如果我们使用了HTTP的keep-alive等级），我们需要在调用dispatch_io_create前这么做。 创建好I/O通道后，我们可以设置读取处理程序：

dispatch_io_read(_channel, 0, SIZE_MAX, _isolation, ^(bool done, dispatch_data_t data, int error){
    if (data != NULL) {
        if (_data == NULL) {
            _data = data;
        } else {
            _data = dispatch_data_create_concat(_data, data);
        }
        [self processData];
    }
});

uint64_t dispatch_benchmark(size_t count, void (^block)(void));

size_t const objectCount = 1000;
uint64_t n = dispatch_benchmark(10000, ^{
    @autoreleasepool {
        id obj = @42;
        NSMutableArray *array = [NSMutableArray array];
        for (size_t i = 0; i < objectCount; ++i) {
            [array addObject:obj];
        }
    }
});
NSLog(@"-[NSMutableArray addObject:] : %llu ns", n);

-[NSMutableArray addObject:] : 31803 ns
 

curl "http://opensource.apple.com/source/libdispatch/libdispatch-84.5/man/dispatch_benchmark.3?txt" 
  | /usr/bin/groffer --tty -T utf8
 

void * sharedBuffer(void)
{
    static void * buffer;
    if (buffer == NULL) {
        void * newBuffer = calloc(1, 1024);
        if (!OSAtomicCompareAndSwapPtrBarrier(NULL, newBuffer, &buffer)) {
            free(newBuffer);
        }
    }
    return buffer;
}

@interface MyTableViewCell : UITableViewCell
 
@property (readonly, nonatomic, copy) NSDictionary *amountAttributes;
 
@end
 
 
 
@implementation MyTableViewCell
{
    NSDictionary *_amountAttributes;
}
 
- (NSDictionary *)amountAttributes;
{
    if (_amountAttributes == nil) {
        static __weak NSDictionary *cachedAttributes = nil;
        static OSSpinLock lock = OS_SPINLOCK_INIT;
        OSSpinLockLock(&lock);
        _amountAttributes = cachedAttributes;
        if (_amountAttributes == nil) {
            NSMutableDictionary *attributes = [[self subtitleAttributes] mutableCopy];
            attributes[NSFontAttributeName] = [UIFont fontWithName:@"ComicSans" size:36];
            attributes[NSParagraphStyleAttributeName] = [NSParagraphStyle defaultParagraphStyle];
            _amountAttributes = [attributes copy];
            cachedAttributes = _amountAttributes;
        }
        OSSpinLockUnlock(&lock);
    }
    return _amountAttributes;
}

底层并发APIs_源自objc.io