GCD 概述

1. GCD 包含於 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

- 目前僅提供 block-based API

GCD總結

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的好處

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

多線程

Session 206已提到此內容。

鎖定資源

1. 對關鍵資源進行互斥訪問。

2. 在線程中按序訪問共享資源。

3. Ensure data integrity

沒有GCD的代碼

- (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代碼

- (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];

});

}

線程間通信

通過以下方法 (Without GCD)

– performSelectorOnMainThread:withObject:waitUntilDone:

– performSelector:onThread:withObject:waitUntilDone:

– performSelector:withObject:afterDelay:

– performSelectorInBackground:withObject:

GCD 代碼，等同於 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代碼， 等同於 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代碼，等同於 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- DISPATCH_QUEUE_PRIORITY_HIGH - DISPATCH_QUEUE_PRIORITY_DEFAULT - DISPATCH_QUEUE_PRIORITY_LOW
下面三節講述了很多內容，但我還沒有理解。如有興趣，請看原始視頻。- Dispatch Sources
- Source Cancellation- Target Queues
GCD對象

GCD方法大全

管理對象的生命周期

1. Ensure objects captured by blocks are valid when blocks are executed

- Objects must be retained and released around asynchronous operations

2. Objective-C objects captured by blocks are auto-retained and auto-released

3. Other objects captured by blocks must be retained by your code

- CFRetain()/CFRelease()

- dispatch_retain()/dispatch_release()

掛起和恢復

1. Suspend and resume only affects queues and sources you create

- Sources are created suspended

2. Suspension is asynchronous

- Takes effect between blocks

3. Your queues can predictably suspend objects that target them

程序上下文

1. Applications can attach custom data to GCD objects

- dispatch_set_context()/dispatch_get_context()

2. Optional finalizer callback

- dispatch_set_finalizer_f()

- Allows attached context to be freed with object

- Called on the target queue of the object

WWDC2010 Session211 Simplifying iPhone App Development with Grand Central Dispatch

Daniel Steffen - Core OS