系统提供的dispatch方法

h1, h2, h3, h4, h5, h6, p, blockquote { margin: 0; padding: 0 }
body { font-family: "Helvetica Neue", Helvetica, "Hiragino Sans GB", Arial, sans-serif; font-size: 13px; line-height: 18px; color: rgba(115, 115, 115, 1); background-color: rgba(255, 255, 255, 1); margin: 10px 13px }
table { margin: 10px 0 15px; border-collapse: collapse }
td, th { border: 1px solid rgba(221, 221, 221, 1); padding: 3px 10px }
th { padding: 5px 10px }
a { color: rgba(0, 105, 214, 1) }
a:hover { color: rgba(0, 80, 163, 1); text-decoration: none }
a img { border: none }
p { margin-bottom: 9px }
h1, h2, h3, h4, h5, h6 { color: rgba(64, 64, 64, 1); line-height: 36px }
h1 { margin-bottom: 18px; font-size: 30px }
h2 { font-size: 24px }
h3 { font-size: 18px }
h4 { font-size: 16px }
h5 { font-size: 14px }
h6 { font-size: 13px }
hr { margin: 0 0 19px; border-top: 0; border-right: 0; border-bottom: 1px solid rgba(204, 204, 204, 1); border-left: 0 }
blockquote { padding: 13px 13px 21px 15px; margin-bottom: 18px; font-family: georgia, serif; font-style: italic }
blockquote:before { content: "“"; font-size: 40px; margin-left: -10px; font-family: georgia, serif; color: rgba(238, 238, 238, 1) }
blockquote p { font-size: 14px; font-weight: 300; line-height: 18px; margin-bottom: 0; font-style: italic }
code, pre { font-family: Monaco, Andale Mono, Courier New, monospace }
code { background-color: rgba(254, 233, 204, 1); color: rgba(0, 0, 0, 0.75); padding: 1px 3px; font-size: 12px; -webkit-border-radius: 3px; -moz-border-radius: 3px; border-radius: 3px }
pre { display: block; padding: 14px; margin: 0 0 18px; line-height: 16px; font-size: 11px; border: 1px solid rgba(217, 217, 217, 1); white-space: pre-wrap; word-wrap: break-word }
pre code { background-color: rgba(255, 255, 255, 1); color: rgba(115, 115, 115, 1); font-size: 11px; padding: 0 }
sup { font-size: 0.83em; vertical-align: super; line-height: 0 }
* { -webkit-print-color-adjust: exact }
@media screen and (min-width: 914px) { body { width: 854px; margin: 10px auto } }
@media print { body, code, pre code, h1, h2, h3, h4, h5, h6 { color: rgba(0, 0, 0, 1) } table, pre { page-break-inside: avoid } }

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。

//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
});

自定义dispatch_queue_t可以用dispatch_queue_create,如：

dispatch_queue_t urls_queue = dispatch_queue_create("blog.devtang.com", NULL);
dispatch_async(urls_queue, ^{
 // your code
});
dispatch_release(urls_queue);

GCD的高级用法，让后台2个线程并行执行，然后等2个线程都结束后，再汇总执行结果。这个可以使用dispatch_group,dispatch_group_async,dispatch_group_notify来实现：

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
});