dispatch_barrier_async、dispatch_
https://blog.csdn.net/Lu_Ca/article/details/80691965
1、dispatch_barrier_async
dispatch_barrier_async用于等待前面的任务执行完毕后自己才执行，而它后面的任务需等待它完成之后才执行。一个典型的例子就是数据的读写，通常为了防止文件读写导致冲突，我们会创建一个串行的队列，所有的文件操作都是通过这个队列来执行，比如FMDB，这样就可以避免读写冲突。不过其实这样效率是有提升的空间的，当没有更新数据时，读操作其实是可以并行进行的，而写操作需要串行的执行，如何实现呢：
dispatch_queue_t queue = dispatch_queue_create("Database_Queue", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(queue, ^{
NSLog(@"reading data1");
});
dispatch_async(queue, ^{
NSLog(@"reading data2");
});
dispatch_barrier_async(queue, ^{
NSLog(@"writing data1");
[NSThread sleepForTimeInterval:1];
});
dispatch_async(queue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"reading data3");
});
执行结果如下：
GCDTests[13360:584316] reading data2
GCDTests[13360:584317] reading data1
GCDTests[13360:584317] writing data1
GCDTests[13360:584317] reading data3
我们将写数据的操作放在dispatch_barrier_async中，这样能确保在写数据的时候会等待前面的读操作完成，而后续的读操作也会等到写操作完成后才能继续执行，提高文件读写的执行效率。
2、dispatch_apply
dispatch_apply类似一个for循环，会在指定的dispatch queue中运行block任务n次，如果队列是并发队列，则会并发执行block任务，dispatch_apply是一个同步调用，block任务执行n次后才返回。
简单的使用方法：
dispatch_queue_t queue = dispatch_queue_create("myqueue", DISPATCH_QUEUE_CONCURRENT);
//并发的运行一个block任务5次
dispatch_apply(5, queue, ^(size_t i) {
NSLog(@"do a job %zu times",i+1);
});
NSLog(@"go on");
输出结果：
GCDTests[10029:760640] do a job 2 times
GCDTests[10029:760640] do a job 1 times
GCDTests[10029:760640] do a job 3 times
GCDTests[10029:760640] do a job 5 times
GCDTests[10029:760640] do a job 4 times
GCDTests[10029:760640] go on
3、dispatch_block_notify
dispatch_block_notify当观察的某个block执行结束之后立刻通知提交另一特定的block到指定的queue中执行，该函数有三个参数，第一参数是需要观察的block，第二个参数是被通知block提交执行的queue，第三参数是当需要被通知执行的block，函数的原型:
void dispatch_block_notify(dispatch_block_t block, dispatch_queue_t queue,
dispatch_block_t notification_block);
具体使用的方法:
dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_SERIAL);
dispatch_block_t previousBlock = dispatch_block_create(0, ^{
NSLog(@"previousBlock begin");
[NSThread sleepForTimeInterval:1];
NSLog(@"previousBlock done");
});
dispatch_async(queue, previousBlock);
dispatch_block_t notifyBlock = dispatch_block_create(0, ^{
NSLog(@"notifyBlock");
});
//当previousBlock执行完毕后，提交notifyBlock到global queue中执行
dispatch_block_notify(previousBlock, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), notifyBlock);
运行结果：
GCDTests[17129:895673] previousBlock begin
GCDTests[17129:895673] previousBlock done
GCDTests[17129:895673] notifyBlock
4、dispatch_group_wait
dispatch_group_wait会同步地等待group中所有的block执行完毕后才继续执行,类似于dispatch barrier
dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT);
dispatch_group_t group = dispatch_group_create();
//将任务异步地添加到group中去执行
dispatch_group_async(group,queue,^{ NSLog(@"block1"); });
dispatch_group_async(group,queue,^{ NSLog(@"block2"); });
dispatch_group_wait(group,DISPATCH_TIME_FOREVER);
NSLog(@"go on");
执行结果如下，只有block1跟block2执行完毕后才会执行dispatch_group_wait后面的内容。
GCDTests[954:41031] block2
GCDTests[954:41032] block1
GCDTests[954:40847] go on
5、dispatch_group_notify
功能与dispatch_group_wait类似，不过该过程是异步的，不会阻塞该线程，dispatch_group_notify有三个参数
void dispatch_group_notify(dispatch_group_t group, //要观察的group
dispatch_queue_t queue, //block执行的队列
dispatch_block_t block); //当group中所有任务执行完毕之后要执行的block
简单的示意用法:
dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group,queue,^{ NSLog(@"block1"); });
dispatch_group_async(group,queue,^{ NSLog(@"block2"); });
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
NSLog(@"done");
});
NSLog(@"go on");
可以看到如下的执行结果
GCDTests[1046:45104] go on
GCDTests[1046:45153] block1
GCDTests[1046:45152] block2
GCDTests[1046:45104] done
