swift：异步任务+完成

Question

我对swift中的异步任务感到非常困惑。 我想做的是这样的......

 func buttonPressed(button: UIButton) {
   // display an "animation" tell the user that it is calculating (do not want to freeze the screen
   // do some calculations (take very long time) at the background
   // the calculations result are needed to update the UI
 }

我试着这样做：

func buttonPressed(button: UIButton) {
    let queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
    dispatch_async(queue) { () -> Void in
       // display the animation of "updating"
       // do the math here
        dispatch_async(dispatch_get_main_queue(), {
          // update the UI
       }
    }
}

但是，我发现UI已更新，无需等待我的计算完成。 我对使用异步队列感到很困惑。 有人帮吗？ 谢谢。

Answer 1

您需要一个具有异步完成处理程序的函数。

在计算结束时调用completion()

func doLongCalculation(completion: () -> ())
{
  // do something which takes a long time
  completion()
}

在buttonPressed函数中，在后台线程上调度计算函数，并在完成后返回主线程以更新UI

func buttonPressed(button: UIButton) {
  // display the animation of "updating"
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {  
    self.doLongCalculation {
      dispatch_async(dispatch_get_main_queue()) {
        // update the UI
        print("completed")
      }
    }
  }
}

Answer 2

dispatch_queue_t dispatchqueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_async(dispatchqueue, ^(void){
    while ([self calculate]) {
        NSLog(@"calculation finished");
        dispatch_async(dispatch_get_main_queue(), ^{
            // update the UI
        });
    }
});

- (BOOL)calculate
{
    //do calculation
    //return true or false based on calculation success or failure
    return true;
}