IOS-GCD多线程

同步 & 异步：

区别在于会不会阻塞线程：同步会等待当前任务结束再去执行下一个任务；异步是同时进行执行任务。

串行 & 并行：

 dispatch_get_main_queue   -->主线程的队列 dispatch_get_global_queue -->全局线程的队列(全局并发的)

简单的例子：

dispatch_async(dispatch_get_global_queue(0,0), ^{    NSLog(@"Task Thread!");    [NSThread sleepTimeInterval:3];    dispatch_async(dispatch_get_main_queue(), ^{        NSLog(@" UI Thread！")    });});

自定义线程：

串行：
创建一个自定义的线程，该线程是一个同步线程。如下执行的了两个dispatch_async，实际上是同一个线程，task1和task2会按照先后顺序执行。
默认NULL 就是 DISPATCH_QUEUE_SEARIAL

dispatch_queue_t queue = dispatch_queue_create(@"gcd_queue", NULL);dispatch_async(queue, ^{    NSLog(@"task 1");    [NSthread sleepForTimeInterval:2];    NSLog(@"task 1 over");});dispatch_async(queue, ^{    NSLog(@"task 2");    [NSthread sleepForTimeInterval:2];    NSLog(@"task 2 over");});

并行：
DISPATCH_QUEUE_CONCURRENT 参数创建一个并行的队列。

dispatch_queue_t queue = dispatch_queue_create(@"gcd_queue", DISPATCH_QUEUE_CONCURRENT);dispatch_async(queue, ^{    NSLog(@"task 1");    [NSthread sleepForTimeInterval:2];    NSLog(@"task 1 over");});dispatch_async(queue, ^{    NSLog(@"task 2");    [NSthread sleepForTimeInterval:2];    NSLog(@"task 2 over");});

GCD_group的使用：

dispatch_queue_t queue = dispatch_queue_create(@"gcd_queue", DISPATCH_QUEUE_CONCURRENT);//创建一个groupdispatch_group_t group = dispatch_group_create();dispatch_group_async(group, queue, ^{    NSLog(@"task 1");    [NSthread sleepForTimeInterval:2];    NSLog(@"task 1 over");});dispatch_group_async(group, queue, ^{    NSLog(@"task 2");    [NSthread sleepForTimeInterval:2];    NSLog(@"task 2 over");});dispatch_group_notify(group, queue, ^{    NSLog(@"All task over!");    dispatch_async(dispatch_get_main_queue(), ^{        NSLog(@" UI Thread！")    });});

异步线程控制方法：

dispatch_queue_t queue = dispatch_queue_create(@"gcd_queue", DISPATCH_QUEUE_CONCURRENT);//创建一个groupdispatch_group_t group = dispatch_group_create();//载入groupdispatch_group_enter(group);    .    .     //执行内容    .//用完释放groupdispatch_group_leave(group);