iphone - 检查 UIScrollView 中的滚动方向

Question

我正在尝试实现方法scrollViewWillBeginDragging。 调用此方法时，我检查用户是否选择了 ScrollView 内的按钮之一处于状态:已选择。 如果不是，那么我会显示一个 UIAlert 来通知用户。

我的问题是，如果用户从从右到左滚动(从右侧拉出下一个 View )，我只想调用已选择的按钮 (NextQuestion) 方法. 但如果它们从从左到右滚动，那么我希望它正常滚动。

目前，无论用户向哪个方向滚动，检查器方法都会被调用。我如何才能只在他们从从右到左滚动时调用方法？

这是我目前的实现方式:

- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
{
    [self NextQuestion:scrollView];
}

-(IBAction)NextQuestion:(id)sender
{
    CGFloat pageWidth = scrollView.frame.size.width;
    int page = floor((scrollView.contentOffset.x - pageWidth) / pageWidth) + 1;
    NSInteger npage = 0;
    CGRect frame = scrollView.frame;

    // Check to see if the question has been answered. Call method from another class.
    if([QuestionControl CheckQuestionWasAnswered:page])
    {
        pageNumber++;

        NSLog(@"Proceed");
       if(([loadedQuestionnaire count] - 1) != page)
       {
           [self loadScrollViewWithPage:page - 1];
           [self loadScrollViewWithPage:page];
           [self loadScrollViewWithPage:page + 1];
       }

       // update the scroll view to the appropriate page
       frame = scrollView.frame;
       frame.origin.x = (frame.size.width * page) + frame.size.width;
       frame.origin.y = 0;
       [self.scrollView scrollRectToVisible:frame animated:YES];

   }

   // If question has not been answered show a UIAlert with instructions.
   else
   {
       UIAlertView *alertNotAnswered = [[UIAlertView alloc] initWithTitle:@"Question Not Answered" 
                                                                 message:@"You must answer this question to continue the questionnaire." 
                                                                delegate:nil 
                                                       cancelButtonTitle:@"OK" 
                                                       otherButtonTitles:nil, nil];
       [alertNotAnswered show];
   }
} 

解决方案 1 的代码:

- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
{
    NSLog(@"%f",scrollView.contentOffset.x);
    if (scrollView.contentOffset.x < lastOffset) // has scrolled left..
    {
        lastOffset = scrollView.contentOffset.x;
        [self NextQuestion:scrollView];
    }
}

Answer 1

在 scrollViewWillBeginDragging 中， ScrollView 尚未移动(或注册移动)，因此 contentOffset 将为 0。从 IOS 5 开始，您可以改为查看 ScrollView 的panGestureRecognizer 确定用户滚动手势的方向和幅度。

- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
{ 
    CGPoint translation = [scrollView.panGestureRecognizer translationInView:scrollView.superview];

    if(translation.x > 0)
    {
        // react to dragging right
    } else
    {
        // react to dragging left
    }
}