与Operation Queues的关系
Operation Queues 在GCD之前就有了，其中的某些设计原理因Operation Queues而流行，GCD就是基于这些原理构建的。从iOS4与Mac OSX 10.6开始，Operation Queues在底层就是用GCD来实现的。
GCD是纯C的API，而Operation Queues是Objective-C的对象。
GCD基本用法
- 替代同步锁，实现线程安全
将同步与异步派发结合起来，可以实现与普通加锁机制一样的同步功能，且不会阻塞执行异步派发的线程
使用同步队列及栅栏块，可以令同步行为更加高效
_syncQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
- (void)setName:(NSString *)name{
dispatch_barrier_async(_syncQueue, ^{
_name = name;
});
}
- (NSString *)name{
__block NSString *localString;
dispatch_async(_syncQueue, ^{
localString = _name;
});
return _name;
}
- (void)addPhoto:(Photo *)photo
{
if (photo) { // 1
dispatch_barrier_async(self.concurrentPhotoQueue, ^{ // 2
[_photosArray addObject:photo]; // 3
dispatch_async(dispatch_get_main_queue(), ^{ // 4
[self postContentAddedNotification];
});
});
}
}
dispatch_after
延后执行某项任务
[self performSelector:@selector(doSomething)
withObject:nil
afterDelay:3];
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
[self doSomething];
});
- (void)showOrHideNavPrompt
{
NSUInteger count = [[PhotoManager sharedManager] photos].count;
double delayInSeconds = 1.0;
dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC)); // 1
dispatch_after(popTime, dispatch_get_main_queue(), ^(void){ // 2
if (!count) {
[self.navigationItem setPrompt:@"Add photos with faces to Googlyify them!"];
} else {
[self.navigationItem setPrompt:nil];
}
});
}
dispatch_sync
dispatch_sync 添加任务到一个队列并等待直到任务完成。
- (void)viewDidLoad
{
[super viewDidLoad];
dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSLog(@"First Log");
});
NSLog(@"Second Log");
}
dispatch_async
dispatch_async 做类似的事情，但不同之处是它不会等待任务的完成，而是立即继续“调用线程”的其它任务。
//把任务放到主线程执行也有两种方法，但优先使用GCD
[self performSelectorOnMainThread:@selector(doSomething) withObject:nil waitUntilDone:NO];
//waitUntilDone= YES dispatch_sync
dispatch_async(dispatch_get_main_queue(), ^{
[self doSomething];
});
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{ // 1
UIImage *overlayImage = [self faceOverlayImageFromImage:_image];
dispatch_async(dispatch_get_main_queue(), ^{ // 2
[self fadeInNewImage:overlayImage]; // 3
});
});
- (void)viewDidLoad
{
[super viewDidLoad];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
NSLog(@"First Log");
});
NSLog(@"Second Log");
}
dispatch_once
+ (instancetype)sharedManager
{
static PhotoManager *sharedPhotoManager = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
sharedPhotoManager = [[PhotoManager alloc] init];
sharedPhotoManager->_photosArray = [NSMutableArray array];
});
return sharedPhotoManager;
}
Dispatch Groups
Dispatch Group 会在整个组的任务都完成时通知你。这些任务可以是同步的，也可以是异步的，即便在不同的队列也行。而且在整个组的任务都完成时，Dispatch Group 可以用同步的或者异步的方式通知你。因为要监控的任务在不同队列，那就用一个 dispatch_group_t 的实例来记下这些不同的任务。
- (void)downloadPhotosWithCompletionBlock:(BatchPhotoDownloadingCompletionBlock)completionBlock
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{ // 1
__block NSError *error;
dispatch_group_t downloadGroup = dispatch_group_create(); // 2
for (NSInteger i = 0; i < 3; i++) {
NSURL *url;
switch (i) {
case 0:
url = [NSURL URLWithString:kOverlyAttachedGirlfriendURLString];
break;
case 1:
url = [NSURL URLWithString:kSuccessKidURLString];
break;
case 2:
url = [NSURL URLWithString:kLotsOfFacesURLString];
break;
default:
break;
}
dispatch_group_enter(downloadGroup); // 3
Photo *photo = [[Photo alloc] initwithURL:url
withCompletionBlock:^(UIImage *image, NSError *_error) {
if (_error) {
error = _error;
}
dispatch_group_leave(downloadGroup); // 4
}];
[[PhotoManager sharedManager] addPhoto:photo];
}
dispatch_group_wait(downloadGroup, DISPATCH_TIME_FOREVER); // 5
dispatch_async(dispatch_get_main_queue(), ^{ // 6
if (completionBlock) { // 7
completionBlock(error);
}
});
});
}
- (void)downloadPhotosWithCompletionBlock:(BatchPhotoDownloadingCompletionBlock)completionBlock
{
// 1
__block NSError *error;
dispatch_group_t downloadGroup = dispatch_group_create();
for (NSInteger i = 0; i < 3; i++) {
NSURL *url;
switch (i) {
case 0:
url = [NSURL URLWithString:kOverlyAttachedGirlfriendURLString];
break;
case 1:
url = [NSURL URLWithString:kSuccessKidURLString];
break;
case 2:
url = [NSURL URLWithString:kLotsOfFacesURLString];
break;
default:
break;
}
dispatch_group_enter(downloadGroup); // 2
Photo *photo = [[Photo alloc] initwithURL:url
withCompletionBlock:^(UIImage *image, NSError *_error) {
if (_error) {
error = _error;
}
dispatch_group_leave(downloadGroup); // 3
}];
[[PhotoManager sharedManager] addPhoto:photo];
}
dispatch_group_notify(downloadGroup, dispatch_get_main_queue(), ^{ // 4
if (completionBlock) {
completionBlock(error);
}
});
}
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, queue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"group1");
});
dispatch_group_async(group, queue, ^{
[NSThread sleepForTimeInterval:2];
NSLog(@"group2");
});
dispatch_group_async(group, queue, ^{
[NSThread sleepForTimeInterval:3];
NSLog(@"group3");
});
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
NSLog(@"updateUi");
});
dispatch_release(group);
dispatch_apply
执行某个代码片段N次,
dispatch_apply(5, globalQ, ^(size_t index) {
// 执行5次
});
未完待续...
https://github.com/nixzhu/dev-blog/blob/master/2014-04-19-grand-central-dispatch-in-depth-part-1.md