swift 异步线程用法

//1.全局-异步回调主线程（加优先级）
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), {
     dispatch_sync(dispatch_get_main_queue(),{
          // do something
     })
})
//2.全局异步回调（优先级）
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)){
          // do something
}
//3.异步回调主线程
dispatch_async(dispatch_get_main_queue(),{
          // do something
}