ios 使用GCD 多线程 教程
2015-07-29 11:22
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什么是GCD
Grand Central Dispatch (GCD)是Apple开发的一个多核编程的解决方法。该方法在Mac OS X 10.6雪豹中首次推出,并随后被引入到了iOS4.0中。GCD是一个替代诸如NSThread, NSOperationQueue, NSInvocationOperation等技术的很高效和强大的技术,它看起来象就其它语言的闭包(Closure)一样,但苹果把它叫做blocks。
应用举例
让我们来看一个编程场景。我们要在iphone上做一个下载网页的功能,该功能非常简单,就是在iphone上放置一个按钮,点击该按钮时,显示一个转动的圆圈,表示正在进行下载,下载完成之后,将内容加载到界面上的一个文本控件中。
不用GCD前
虽然功能简单,但是我们必须把下载过程放到后台线程中,否则会阻塞UI线程显示。所以,如果不用GCD, 我们需要写如下3个方法:someClick 方法是点击按钮后的代码,可以看到我们用NSInvocationOperation建了一个后台线程,并且放到NSOperationQueue中。后台线程执行download方法。
download 方法处理下载网页的逻辑。下载完成后用performSelectorOnMainThread执行download_completed 方法。
download_completed 进行clear up的工作,并把下载的内容显示到文本控件中。
这3个方法的代码如下。可以看到,虽然 开始下载 -> 下载中 -> 下载完成 这3个步骤是整个功能的三步。但是它们却被切分成了3块。他们之间因为是3个方法,所以还需要传递数据参数。如果是复杂的应用,数据参数很可能就不象本例子中的NSString那么简单了,另外,下载可能放到Model的类中来做,而界面的控制放到View Controller层来做,这使得本来就分开的代码变得更加散落。代码的可读性大大降低。
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">9</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">10</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">11</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">12</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">13</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">14</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">15</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">16</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">17</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">18</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">19</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">20</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">21</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">22</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">23</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">24</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">25</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">26</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">27</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">28</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">29</span> | static NSOperationQueue * queue; - (IBAction)someClick:(id)sender { self.indicator.hidden = NO; [self.indicator startAnimating]; queue = [[NSOperationQueue alloc] init]; NSInvocationOperation * op = [[[NSInvocationOperation alloc] initWithTarget:self selector:@selector(download) object:nil] autorelease]; [queue addOperation:op]; } - (void)download { NSURL * url = [NSURL URLWithString:@"http://www.youdao.com"]; NSError * error; NSString * data = [NSString stringWithContentsOfURL:url encoding:NSUTF8StringEncoding error:&error]; if (data != nil) { [self performSelectorOnMainThread:@selector(download_completed:) withObject:data waitUntilDone:NO]; } else { NSLog(@"error when download:%@", error); [queue release]; } } - (void) download_completed:(NSString *) data { NSLog(@"call back"); [self.indicator stopAnimating]; self.indicator.hidden = YES; self.content.text = data; [queue release]; } |
使用GCD后
如果使用GCD,以上3个方法都可以放到一起,如下所示:<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">9</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">10</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">11</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">12</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">13</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">14</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">15</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">16</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">17</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">18</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">19</span> | // 原代码块一 self.indicator.hidden = NO; [self.indicator startAnimating]; dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ // 原代码块二 NSURL * url = [NSURL URLWithString:@"http://www.youdao.com"]; NSError * error; NSString * data = [NSString stringWithContentsOfURL:url encoding:NSUTF8StringEncoding error:&error]; if (data != nil) { // 原代码块三 dispatch_async(dispatch_get_main_queue(), ^{ [self.indicator stopAnimating]; self.indicator.hidden = YES; self.content.text = data; }); } else { NSLog(@"error when download:%@", error); } }); |
另外,代码变清楚了,虽然是异步的代码,但是它们被GCD合理的整合在一起,逻辑非常清晰。如果应用上MVC模式,我们也可以将View Controller层的回调函数用GCD的方式传递给Modal层,这相比以前用@selector的方式,代码的逻辑关系会更加清楚。
GCD的定义
简单GCD的定义有点象函数指针,差别是用 ^ 替代了函数指针的 * 号,如下所示:<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">9</span> | // 申明变量 (void) (^loggerBlock)(void); // 定义 loggerBlock = ^{ NSLog(@"Hello world"); }; // 调用 loggerBlock(); |
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span> | dispatch_async(dispatch_get_global_queue(0, 0), ^{ // something }); |
程序块可以在代码中以内联的方式来定义。
程序块可以访问在创建它的范围内的可用的变量。
系统提供的dispatch方法
为了方便地使用GCD,苹果提供了一些方法方便我们将block放在主线程 或 后台线程执行,或者延后执行。使用的例子如下:<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">9</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">10</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">11</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">12</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">13</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">14</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">15</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">16</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">17</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">18</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">19</span> | // 后台执行: dispatch_async(dispatch_get_global_queue(0, 0), ^{ // something }); // 主线程执行: dispatch_async(dispatch_get_main_queue(), ^{ // something }); // 一次性执行: static dispatch_once_t onceToken; dispatch_once(&onceToken, ^{ // code to be executed once }); // 延迟2秒执行: 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 }); |
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span> | dispatch_queue_t urls_queue = dispatch_queue_create("blog.devtang.com", NULL); dispatch_async(urls_queue, ^{ // your code }); dispatch_release(urls_queue); |
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">9</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">10</span> | dispatch_group_t group = dispatch_group_create(); dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{ // 并行执行的线程一 }); dispatch_group_async(group, dispatch_get_global_queue(0,0), ^{ // 并行执行的线程二 }); dispatch_group_notify(group, dispatch_get_global_queue(0,0), ^{ // 汇总结果 }); |
修改block之外的变量
默认情况下,在程序块中访问的外部变量是复制过去的,即写操作不对原变量生效。但是你可以加上 __block来让其写操作生效,示例代码如下:<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span> | __block int a = 0; void (^foo)(void) = ^{ a = 1; } foo(); // 这里,a的值被修改为1 |
后台运行
GCD的另一个用处是可以让程序在后台较长久的运行。在没有使用GCD时,当app被按home键退出后,app仅有最多5秒钟的时候做一些保存或清理资源的工作。但是在使用GCD后,app最多有10分钟的时间在后台长久运行。这个时间可以用来做清理本地缓存,发送统计数据等工作。让程序在后台长久运行的示例代码如下:
<span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">1</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">2</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">3</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">4</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">5</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">6</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">7</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">8</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">9</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">10</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">11</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">12</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">13</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">14</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">15</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">16</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">17</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">18</span> <span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">19</span><span class="line-number" style="margin: 0px; padding: 0px; border: 0px; font-family: inherit; font-style: inherit; font-variant: inherit; line-height: inherit; vertical-align: baseline; color: rgb(88, 110, 117) !important;">20</span> | // AppDelegate.h文件 @property (assign, nonatomic) UIBackgroundTaskIdentifier backgroundUpdateTask; // AppDelegate.m文件 - (void)applicationDidEnterBackground:(UIApplication *)application { [self beingBackgroundUpdateTask]; // 在这里加上你需要长久运行的代码 [self endBackgroundUpdateTask]; } - (void)beingBackgroundUpdateTask { self.backgroundUpdateTask = [[UIApplication sharedApplication] beginBackgroundTaskWithExpirationHandler:^{ [self endBackgroundUpdateTask]; }]; } - (void)endBackgroundUpdateTask { [[UIApplication sharedApplication] endBackgroundTask: self.backgroundUpdateTask]; self.backgroundUpdateTask = UIBackgroundTaskInvalid; } |
总结
总体来说,GCD能够极大地方便开发者进行多线程编程。如果你的app不需要支持iOS4.0以下的系统,那么就应该尽量使用GCD来处理后台线程和UI线程的交互。
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