iOS中多线程编程工具主要有:html
这三种方法都简单易用,各有千秋.但无疑GCD是最有诱惑力的,由于其自己是apple为多核的并行运算提出的解决方案.虽然当前移动平台用双核的很少,但不影响GCD做为多线程编程的利器(ipad2已是双核了,这无疑是一个趋势).ios
http://www.cnblogs.com/scorpiozj/archive/2011/07/25/2116459.html程序员
GCD是和block紧密相连的,因此最好先了解下block(能够查看这里).GCD是C level的函数,这意味着它也提供了C的函数指针做为参数,方便了C程序员.编程
1、下面首先来看GCD的使用:多线程
dispatch_async(dispatch_queue_t queue, dispatch_block_t block);
async代表异步运行,block表明的是你要作的事情,queue则是你把任务交给谁来处理了.(除了async,还有sync,delay,本文以async为例).并发
之因此程序中会用到多线程是由于程序每每会须要读取数据,而后更新UI.为了良好的用户体验,读取数据的操做会倾向于在后台运行,这样以免阻塞主线程.GCD里就有三种queue来处理.app
先来介绍一下 Main queue:异步
顾名思义,运行在主线程,由dispatch_get_main_queue得到.和ui相关的就要使用Main Queue.async
//GCD下载图片刷新主界面的例子 /* - (IBAction)touchUpInsideByThreadOne:(id)sender { dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ NSURL * url = [NSURL URLWithString:@"http://avatar.csdn.net/2/C/D/1_totogo2010.jpg"]; NSData * data = [[NSData alloc]initWithContentsOfURL:url]; UIImage *image = [[UIImage alloc]initWithData:data]; if (data != nil) { dispatch_async(dispatch_get_main_queue(), ^{ self.imageView.image = image; }); } }); }*/
经过与线程池的配合,dispatch queue分为下面两种:而系统默认就有一个串行队列main_queue和并行队列global_queue:ide
而系统默认就有一个串行队列main_queue和并行队列global_queue:
dispatch_queue_t globalQ = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); dispatch_queue_t mainQ = dispatch_get_main_queue();一般,咱们能够在global_queue中作一些long-running的任务,完成后在main_queue中更新UI,避免UI阻塞,没法响应用户操做:
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ // long-running task dispatch_async(dispatch_get_main_queue(), ^{ // update UI }); });
1.Serial quque(private dispatch queue)
每次运行一个任务,能够添加多个,执行次序FIFO. 一般是指程序员生成的,好比:
NSDate *da = [NSDate date]; NSString *daStr = [da description]; const char *queueName = [daStr UTF8String]; dispatch_queue_t myQueue = dispatch_queue_create(queueName, DISPATCH_QUEUE_PRIORITY_DEFAULT);
下面仍是下载图片例子:
- (IBAction)touchUpInsideByThreadOne:(id)sender { NSDate *da = [NSDate date]; NSString *daStr = [da description]; const char *queueName = [daStr UTF8String]; dispatch_queue_t myQueue = dispatch_queue_create(queueName, NULL); dispatch_async(myQueue, ^{ NSURL * url = [NSURL URLWithString:@"http://avatar.csdn.net/2/C/D/1_totogo2010.jpg"]; NSData * data = [[NSData alloc]initWithContentsOfURL:url]; UIImage *image = [[UIImage alloc]initWithData:data]; if (data != nil) { dispatch_async(dispatch_get_main_queue(), ^{ self.imageView.image = image; }); } }); dispatch_release(myQueue); }为了验证Serial queue的FIFO特性,写了以下的验证代码:发现的确是顺序执行的。
- (IBAction)touchUpInsideByThreadOne:(id)sender { NSDate *da = [NSDate date]; NSString *daStr = [da description]; const char *queueName = [daStr UTF8String]; dispatch_queue_t myQueue = dispatch_queue_create(queueName, DISPATCH_QUEUE_SERIAL); dispatch_async(myQueue, ^{ [NSThread sleepForTimeInterval:6]; NSLog(@"[NSThread sleepForTimeInterval:6];"); }); dispatch_async(myQueue, ^{ [NSThread sleepForTimeInterval:3]; NSLog(@"[NSThread sleepForTimeInterval:3];"); }); dispatch_async(myQueue, ^{ [NSThread sleepForTimeInterval:1]; NSLog(@"[NSThread sleepForTimeInterval:1];"); }); dispatch_release(myQueue); }运行结果为:
2013-07-24 16:37:14.397 NSThreadAndBlockDemo[1924:12303] [NSThread sleepForTimeInterval:6]; 2013-07-24 16:37:17.399 NSThreadAndBlockDemo[1924:12303] [NSThread sleepForTimeInterval:3]; 2013-07-24 16:37:18.401 NSThreadAndBlockDemo[1924:12303] [NSThread sleepForTimeInterval:1];
3. Concurrent queue(global dispatch queue):
能够同时运行多个任务,每一个任务的启动时间是按照加入queue的顺序,结束的顺序依赖各自的任务.使用dispatch_get_global_queue得到.
- (IBAction)touchUpInsideByThreadOne:(id)sender { dispatch_queue_t myQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); dispatch_async(myQueue, ^{ [NSThread sleepForTimeInterval:6]; NSLog(@"[NSThread sleepForTimeInterval:6];"); }); dispatch_async(myQueue, ^{ [NSThread sleepForTimeInterval:3]; NSLog(@"[NSThread sleepForTimeInterval:3];"); }); dispatch_async(myQueue, ^{ [NSThread sleepForTimeInterval:1]; NSLog(@"[NSThread sleepForTimeInterval:1];"); }); dispatch_release(myQueue); }运行的结果为:
2013-07-24 16:38:41.660 NSThreadAndBlockDemo[1944:12e03] [NSThread sleepForTimeInterval:1]; 2013-07-24 16:38:43.660 NSThreadAndBlockDemo[1944:12b03] [NSThread sleepForTimeInterval:3]; 2013-07-24 16:38:46.660 NSThreadAndBlockDemo[1944:12303] [NSThread sleepForTimeInterval:6];
2、dispatch_group_async的使用
dispatch_group_async能够实现监听一组任务是否完成,完成后获得通知执行其余的操做。这个方法颇有用,好比你执行三个下载任务,当三个任务都下载完成后你才通知界面说完成的了。下面是一段例子代码:
- (IBAction)touchUpInsideByThreadOne:(id)sender { dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); dispatch_group_t group = dispatch_group_create(); dispatch_group_async(group, queue, ^{ [NSThread sleepForTimeInterval:6]; NSLog(@"group1 [NSThread sleepForTimeInterval:6];"); }); dispatch_group_async(group, queue, ^{ [NSThread sleepForTimeInterval:3]; NSLog(@"group2 [NSThread sleepForTimeInterval:3];"); }); dispatch_group_async(group, queue, ^{ [NSThread sleepForTimeInterval:1]; NSLog(@"group3 [NSThread sleepForTimeInterval:1];"); }); dispatch_group_notify(group, dispatch_get_main_queue(), ^{ NSLog(@"main thread."); }); dispatch_release(group); }执行结果为:
2013-07-24 16:48:23.063 NSThreadAndBlockDemo[2004:12e03] group3 [NSThread sleepForTimeInterval:1]; 2013-07-24 16:48:25.063 NSThreadAndBlockDemo[2004:12b03] group2 [NSThread sleepForTimeInterval:3]; 2013-07-24 16:48:28.063 NSThreadAndBlockDemo[2004:12303] group1 [NSThread sleepForTimeInterval:6]; 2013-07-24 16:48:28.065 NSThreadAndBlockDemo[2004:11303] main thread.
三、dispatch_barrier_async的使用
dispatch_barrier_async是在前面的任务执行结束后它才执行,并且它后面的任务等它执行完成以后才会执行
例子代码以下:
- (IBAction)touchUpInsideByThreadOne:(id)sender { dispatch_queue_t queue = dispatch_queue_create("gcdtest.rongfzh.yc", DISPATCH_QUEUE_CONCURRENT); dispatch_async(queue, ^{ [NSThread sleepForTimeInterval:3]; NSLog(@"dispatch_async1"); }); dispatch_async(queue, ^{ [NSThread sleepForTimeInterval:1]; NSLog(@"dispatch_async2"); }); dispatch_barrier_async(queue, ^{ NSLog(@"dispatch_barrier_async"); [NSThread sleepForTimeInterval:0.5]; }); dispatch_async(queue, ^{ [NSThread sleepForTimeInterval:1]; NSLog(@"dispatch_async3"); }); }执行结果为:
2013-07-24 17:01:54.580 NSThreadAndBlockDemo[2153:12b03] dispatch_async2 2013-07-24 17:01:56.580 NSThreadAndBlockDemo[2153:12303] dispatch_async1 2013-07-24 17:01:56.580 NSThreadAndBlockDemo[2153:12303] dispatch_barrier_async 2013-07-24 17:01:58.083 NSThreadAndBlockDemo[2153:12303] dispatch_async3若是使用dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);会发现运行结果为:
2013-07-24 17:07:17.577 NSThreadAndBlockDemo[2247:12e03] dispatch_barrier_async 2013-07-24 17:07:18.579 NSThreadAndBlockDemo[2247:15207] dispatch_async3 2013-07-24 17:07:19.578 NSThreadAndBlockDemo[2247:12b03] dispatch_async2 2013-07-24 17:07:20.577 NSThreadAndBlockDemo[2247:12303] dispatch_async1
四、dispatch_apply
执行某个代码片断N次。
dispatch_apply(5, globalQ, ^(size_t index) {
// 执行5次
});
五、dispatch_once
dispatch_once这个函数,它能够保证整个应用程序生命周期中某段代码只被执行一次!
static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ // code to be executed once });六、dispatch_after
double delayInSeconds = 2.0; dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC); dispatch_after(popTime, dispatch_get_main_queue(), ^(void){ // code to be executed on the main queue after delay });
dispatch_set_target_queue(serialQ, globalQ);