swift：異步任務+完成

Question

我對swift中的異步任務感到非常困惑。 我想做的是這樣的......

 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
 }

我試着這樣做：

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，我發現UI已更新，無需等待我的計算完成。 我對使用異步隊列感到很困惑。 有人幫嗎？ 謝謝。

Answer 1

您需要一個具有異步完成處理程序的函數。

在計算結束時調用completion()

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

在buttonPressed函數中，在后台線程上調度計算函數，並在完成后返回主線程以更新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;
}