版本记录
| 版本号 | 时间 |
|---|---|
| V1.0 | 2018.02.23 |
前言
dispatch_barrier_async和dispatch_barrier_sync大家在做执行任务的时候经常用到的函数，这一篇就主要分析一下这两个函数的用法和区别。
dispatch_barrier_sync
先看一下这个函数的API
/*!
* @function dispatch_barrier_sync
*
* @abstract
* Submits a barrier block for synchronous execution on a dispatch queue.
为队列上的同步执行提交阻塞的代码块。
*
* @discussion
* Submits a block to a dispatch queue like dispatch_sync(), but marks that
* block as a barrier (relevant only on DISPATCH_QUEUE_CONCURRENT queues).
将块提交到dispatch_sync（）等调度队列，但将该块标记为阻塞（仅与DISPATCH_QUEUE_CONCURRENT队列相关）。
*
* See dispatch_sync() for details.
*
* @param queue
* The target dispatch queue to which the block is submitted.
* The result of passing NULL in this parameter is undefined.
block要提交的目标队列，传递NULL参数的结果未定义。
*
* @param block
* The block to be invoked on the target dispatch queue.
* The result of passing NULL in this parameter is undefined.
*/
#ifdef __BLOCKS__
API_AVAILABLE(macos(10.7), ios(4.3))
DISPATCH_EXPORT DISPATCH_NONNULL_ALL DISPATCH_NOTHROW
void
dispatch_barrier_sync(dispatch_queue_t queue,
DISPATCH_NOESCAPE dispatch_block_t block);
#endif
下面大家看一下简单的示例：
- (void)initSyncBarrier
{
//1 创建并发队列
dispatch_queue_t concurrentQueue = dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT);
//2 向队列中添加任务
dispatch_async(concurrentQueue, ^{
NSLog(@"Task 1,%@",[NSThread currentThread]);
});
dispatch_async(concurrentQueue, ^{
NSLog(@"Task 2,%@",[NSThread currentThread]);
});
dispatch_async(concurrentQueue, ^{
NSLog(@"Task 3,%@",[NSThread currentThread]);
});
dispatch_barrier_sync(concurrentQueue, ^{
[NSThread sleepForTimeInterval:1.0];
NSLog(@"barrier");
});
NSLog(@"aa, %@", [NSThread currentThread]);
dispatch_async(concurrentQueue, ^{
NSLog(@"Task 4,%@",[NSThread currentThread]);
});
NSLog(@"bb, %@", [NSThread currentThread]);
dispatch_async(concurrentQueue, ^{
NSLog(@"Task 5,%@",[NSThread currentThread]);
});
}
看一下输出结果
2018-02-23 23:16:46.987147+0800 JJWebImage[1399:39117] Task 2,<NSThread: 0x6040002653c0>{number = 4, name = (null)}
2018-02-23 23:16:46.987145+0800 JJWebImage[1399:39125] Task 3,<NSThread: 0x604000265200>{number = 5, name = (null)}
2018-02-23 23:16:46.987154+0800 JJWebImage[1399:39116] Task 1,<NSThread: 0x60000027cc40>{number = 3, name = (null)}
2018-02-23 23:16:47.987597+0800 JJWebImage[1399:39014] barrier
2018-02-23 23:16:47.987860+0800 JJWebImage[1399:39014] aa, <NSThread: 0x60000006c900>{number = 1, name = main}
2018-02-23 23:16:47.988025+0800 JJWebImage[1399:39014] bb, <NSThread: 0x60000006c900>{number = 1, name = main}
2018-02-23 23:16:47.988055+0800 JJWebImage[1399:39116] Task 4,<NSThread: 0x60000027cc40>{number = 3, name = (null)}
2018-02-23 23:16:47.988263+0800 JJWebImage[1399:39118] Task 5,<NSThread: 0x6040002652c0>{number = 6, name = (null)}
我们可以看到：
- Task1,2,3不是顺序执行的因为是异步，但是都在barrier的前面，Task4,5在barrier的后面执行。
- aa和bb都在主线程进行输出。
- 执行完barrier，才会将后面的任务4，5插入到队列执行。
dispatch_barrier_async
首先看一下这个函数的API
/*!
* @function dispatch_barrier_async
*
* @abstract
* Submits a barrier block for asynchronous execution on a dispatch queue.
为队列上的异步执行提交阻塞的代码块。
*
* @discussion
* Submits a block to a dispatch queue like dispatch_async(), but marks that
* block as a barrier (relevant only on DISPATCH_QUEUE_CONCURRENT queues).
提交block块到dispatch_async()队列，但是标记该代码块为阻塞（仅与DISPATCH_QUEUE_CONCURRENT队列相关）。
*
* See dispatch_async() for details.
*
* @param queue
* The target dispatch queue to which the block is submitted.
* The system will hold a reference on the target queue until the block
* has finished.
* The result of passing NULL in this parameter is undefined.
block要提交到的目标队列，系统会保持对目标队列的引用，直到block完成。传递NULL参数的结果未定义。
*
* @param block
* The block to submit to the target dispatch queue. This function performs
* Block_copy() and Block_release() on behalf of callers.
* The result of passing NULL in this parameter is undefined.
要提交到的目标队列的block， 这个函数代表调用者执行Block_copy（）和Block_release（）。传递NULL参数的结果未定义。
*/
#ifdef __BLOCKS__
API_AVAILABLE(macos(10.7), ios(4.3))
DISPATCH_EXPORT DISPATCH_NONNULL_ALL DISPATCH_NOTHROW
void
dispatch_barrier_async(dispatch_queue_t queue, dispatch_block_t block);
#endif
下面看一个简单的实例
- (void)initAsyncBarrier
{
//1 创建并发队列
dispatch_queue_t concurrentQueue = dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT);
//2 向队列中添加任务
dispatch_async(concurrentQueue, ^{
NSLog(@"Task 1,%@",[NSThread currentThread]);
});
dispatch_async(concurrentQueue, ^{
NSLog(@"Task 2,%@",[NSThread currentThread]);
});
dispatch_async(concurrentQueue, ^{
NSLog(@"Task 3,%@",[NSThread currentThread]);
});
dispatch_barrier_async(concurrentQueue, ^{
[NSThread sleepForTimeInterval:1.0];
NSLog(@"barrier");
});
NSLog(@"aa, %@", [NSThread currentThread]);
dispatch_async(concurrentQueue, ^{
NSLog(@"Task 4,%@",[NSThread currentThread]);
});
NSLog(@"bb, %@", [NSThread currentThread]);
dispatch_async(concurrentQueue, ^{
NSLog(@"Task 5,%@",[NSThread currentThread]);
});
}
看一下输出结果
2018-02-23 23:10:43.338610+0800 JJWebImage[1362:36007] aa, <NSThread: 0x604000062480>{number = 1, name = main}
2018-02-23 23:10:43.338665+0800 JJWebImage[1362:36154] Task 1,<NSThread: 0x604000275100>{number = 3, name = (null)}
2018-02-23 23:10:43.338664+0800 JJWebImage[1362:36153] Task 3,<NSThread: 0x60000007e640>{number = 4, name = (null)}
2018-02-23 23:10:43.339671+0800 JJWebImage[1362:36007] bb, <NSThread: 0x604000062480>{number = 1, name = main}
2018-02-23 23:10:43.338731+0800 JJWebImage[1362:36152] Task 2,<NSThread: 0x604000275040>{number = 5, name = (null)}
2018-02-23 23:10:44.341556+0800 JJWebImage[1362:36153] barrier
2018-02-23 23:10:44.341849+0800 JJWebImage[1362:36152] Task 5,<NSThread: 0x604000275040>{number = 5, name = (null)}
2018-02-23 23:10:44.341855+0800 JJWebImage[1362:36153] Task 4,<NSThread: 0x60000007e640>{number = 4, name = (null)}
大家可以看到：
- Task1,2,3不是顺序执行的因为是异步，但是都在barrier的前面，Task4,5在barrier的后面执行。
- aa和bb都在主线程进行输出。
- 不用执行完barrier，就可以将任务4，5插入到队列中，但是仍然需要执行完barrier，才会执行任务4和5。
总结
你也可以这么理解，它们二者的差别在于插入barrier后面任务的时机不同。后面任务执行顺序都要在barrier之后，这一点是相同的。
1. 相同点
等待在它前面插入队列的任务先执行完
等待他们自己的任务执行完再执行后面的任务
2. 不同点
dispatch_barrier_sync将自己的任务插入到队列的时候，需要等待自己的任务结束之后才会继续插入被写在它后面的任务，然后执行它们。dispatch_barrier_async将自己的任务插入到队列之后，不会等待自己的任务结束，它会继续把后面的任务插入到队列，然后等待自己的任务结束后才执行后面任务。
后记
本篇已结束，后面更精彩~~~
