由于项目需要, 做了一个滚动视图, 不过需要它停留在一些固定的离散位置, 所以要在用户变动滚动视图偏移后, 进行一次位置的校正. 所以要在合适的实际进行校正函数的调用.

Objective C 下的 hacks

在栈溢出上很快找到之前的通解. ```objc - (void)scrollViewDidScroll:(UIScrollView *)sender { [NSObject cancelPreviousPerformRequestsWithTarget:self]; //ensure that the end of scroll is fired. [self performSelector:@selector(scrollViewDidEndScrollingAnimation:) withObject:nil afterDelay:0.3]; ... }

(void)scrollViewDidEndScrollingAnimation:(UIScrollView *)scrollView { [NSObject cancelPreviousPerformRequestsWithTarget:self]; // ... } ```

Swift 的悲剧

翻文档的时候发现悲了个剧

Sending Messages
performSelector:withObject:afterDelay:  Not available in Swift
performSelector:withObject:afterDelay:inModes: Not available in Swift
performSelectorOnMainThread:withObject:waitUntilDone: Not available in Swift
performSelectorOnMainThread:withObject:waitUntilDone:modes: Not available in Swift
performSelector:onThread:withObject:waitUntilDone: Not available in Swift
performSelector:onThread:withObject:waitUntilDone:modes: Not available in Swift
performSelectorInBackground:withObject: Not available in Swift
cancelPreviousPerformRequestsWithTarget(_:)
Cancels perform requests previously registered with the performSelector:withObject:afterDelay: instance method.

Swift 下没法使用相关接口..

新的思路

静下了仔细想一下, 用户所带来的停止, 其实是两种情况

drag 完后, 有一个decelerated motion 的过程, did end decelerating 后, 就是停止这个其实很好处理, 在scrollViewDidEndDecelerating(scrollView: UIScrollView)的回调中就是捕获.
drag 完后, 没有产生惯性, 直接停下来了, 所以我们要在did end dragging 后就做一次判断, 所以我们可以在scrollViewDidEndDragging(scrollView: UIScrollView, willDecelerate decelerate: Bool)中, 对参数decelerate进行判断.

// MARK: UIScrollViewDelegate
extension YourViewControllerOrViewClass: UIScrollViewDelegate {
  // case 1:
  func scrollViewDidEndDecelerating(scrollView: UIScrollView) {
    adjustScrollViewPostion(scrollView)
  }
  // case 2:
  func scrollViewDidEndDragging(scrollView: UIScrollView, willDecelerate decelerate: Bool) {
    if !decelerate {
      adjustScrollViewPostion(scrollView)
    }
  }

  // MARK: Handle method
  func adjustScrollViewPostion(scrollView: UIScrollView) {
    // ...
  }
}