iOS多线程开发（五）---GCD（Grand Central Dispatch）

GCD 原理

-将一段（Block）代码作为特定任务，放到指定的队列（queue：Serial(串行，同时只能执行一个任务)/globel（并行，同时可执行多个任务，但执行顺序是随机的）/main（全局的Serial，在主线程里面执行任务） dispatch queue）里，进行同步/异步(dispatch_sync/dispatch_async)处理。根据可用的处理资源，安排他们在任何可用的处理器核心上执行任务。dispatch_sync/dispatch_async的意思就是将任务进行同步/异步并行处理，不一定需要一个任务处理完后才能处理下一个

-一个任务可以是一个函数(function)或者是一个block。 GCD的底层依然是用线程实现，不过这样可以让程序员不用关注实现的细节。

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

-dispatch queue分为下面三种：

   -Serial 又称为private dispatch queues，同时只执行一个任务。Serial queue通常用于同步访问特定的资源或数据。当你创建多个Serial queue时，虽然它们各自是同步执行的，但Serial queue与Serial queue之间是并发执行的。

   -Concurrent 又称为global dispatch queue，可以并发地执行多个任务，但是执行完成的顺序是随机的。

   -Main dispatch queue 它是全局可用的serial queue，它是在应用程序主线程上执行任务的。

3. dispatch_get_main_queue()

- Main thread/main runloop（获取主线程的队列）

4. dispatch_queue_create()

- Automatic helper thread， private dispatch queues

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 *nLock = self.imageCacheLock;
  [nLock lock];
  // Critical section
  if ([self.imageCache containsObj:img])
  {
    [nLock unlock]; // Don't forget to unlock
    return;
  }
  [self.imageCache addObj:img];
  [nLock 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

         UIImage *cellImage = [self loadMyImageFromNetwork:image_url];

         // 将图片cache到本地

         [self cacheImage:cellImage];

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

             return;

          }

          [self.imageCache addObj:cellImage];

          });

}

dispatch_sync/dispatch_async的意思就是将任务进行同步/异步并行处理，不一定需要一个任务处理完后才能处理下一个。以上代码loadMyImageFromNetwork的意思就是从网络中读取图片，这个任务交给network_queue来处理。这样读取图片的时间过长也不会阻塞主线程界面的处理。

• dispatch_async(network_queue, ^{    
•     UIImage *cellImage = [self loadMyImageFromNetwork:image_url];    
•     // 将图片cache到本地    
•     [self cacheImage:cellImage];    
•     
•    // 回到主线程    
•    dispatch_async(dispatch_get_main_queue(), ^{    
•       // 显示图片到界面    
•       [self displayImageToTableView:cellImage];    
•    }];    
•         
• } );  

• 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 Queues

1. 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
下面三节讲述了很多内容，但我还没有理解。如有兴趣，请看原始视频。- Dispatch Sources
- Source Cancellation- Target Queues

GCD对象

GCD方法大全

管理对象的生命周期

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()

挂起和恢复

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