WWDC2010 Session211 使用Grad Central Dispa

GCD 概述

1. GCD 包含于 libSystem.dylib

2. 可供所有程序使用.

- #include <dispatch/dispatch.h>

3. GCD API 提供 block-based and function-based variants

- 目前仅提供 block-based API

GCD总结

1. Blocks

- dispatch_async()

2. Queues

- Lightweight list of blocks

- Enqueue/dequeue is FIFO

3. dispatch_get_main_queue()

- Main thread/main runloop

4. dispatch_queue_create()

- Automatic helper thread

GCD的好处

1. 高效 - More CPU cycles available for your code

2. 使用方便

- Blocks are easy to use

- Queues are inherently producer/consumer

3. Systemwide perspective

- Only the OS can balance unrelated subsystems

兼容性

1. Existing threading and synchronization primitives are 100% compatible

2. GCD threads are wrapped POSIX threads

- Do not cancel, exit, kill, join, or detach GCD threads

3. GCD reuses threads

- Restore any per-thread state changed within a block

多线程

Session 206已提到此内容。

锁定资源

1. 对关键资源进行互斥访问。

2. 在线程中按序访问共享资源。

3. Ensure data integrity

没有GCD的代码

- (void)updateImageCacheWithImg:(UIImage*)img {

NSLock *l = self.imageCacheLock;

[l lock];

// Critical section

if ([self.imageCache containsObj:img]) {

[l unlock]; // Don't forget to unlock

return;

}

[self.imageCache addObj:img];

[l unlock];

}

GCD代码

- (void)updateImageCacheWithImg:(NSImage*)img {

dispatch_queue_t queue = self.imageCacheQueue;

dispatch_sync(queue, ^{ //You can change dispatch_sync to dispatch_async to defer critical section

// Critical section

if ([self.imageCache containsObj:img]) {

return;

}

[self.imageCache addObj:img];

});

}

线程间通信

通过以下方法 (Without GCD)

– performSelectorOnMainThread:withObject:waitUntilDone:

– performSelector:onThread:withObject:waitUntilDone:

– performSelector:withObject:afterDelay:

– performSelectorInBackground:withObject:

GCD 代码，等同于 performSelector:onThread:withObject:waitUntilDone:

// waitUntilDone: NO

dispatch_async(queue, ^{

[myObject doSomething:foo withData:bar];

});

// waitUntilDone: YES

dispatch_sync(queue, ^{

[myObject doSomething:foo withData:bar];

});

GCD代码， 等同于 performSelector:withObject:afterDelay:dispatch_time_t delay;delay = dispatch_time(DISPATCH_TIME_NOW, 50000 /* 50μs */);dispatch_after(delay, queue, ^{ [myObject doSomething:foo withData:bar];});GCD代码，等同于 performSelectorInBackground:withObject:dispatch_queue_t queue = dispatch_get_global_queue(0, 0);dispatch_async(queue, ^{ [myObject doSomething:foo withData:bar];});
Global Queues1. Enqueue/dequeue is FIFO

2. Concurrent execution- Non-FIFO completion order

3. dispatch_get_global_queue(priority, 0)

4. Global queues map GCD activity to real threads 5. Priority bands- DISPAT