检查UIScrollView中的滚动方向 [英] Check direction of scroll in UIScrollView

问题描述

我正在尝试实现方法 scrollViewWillBeginDragging。
调用此方法时，我会检查用户是否已选择其中一个滚动视图中的按钮处于状态：已选中。
如果没有，那么我会显示一个UIAlert来通知用户。

I am trying to implement the method scrollViewWillBeginDragging. When this method is called I check that the user has selected one of the buttons that are within the scrollview is in state:selected. If not then I display a UIAlert to inform the user.

我的问题是我只想调用所选按钮（ NextQuestion ）方法，如果用户从向右滚动（从右侧拉下一个视图）。
但是如果他们从从左向右滚动那么我希望它能够正常滚动。

My problem here is I only want to call the button selected (NextQuestion) method if the user scrolls from right to left (pulling the next view from the right). But if they are scrolling from Left to Right then I want it to scroll as normal.

目前检查方法是无论用户滚动到哪个方向，都会调用。如何从从右到左滚动时调用方法？

At present the checker method is called no matter what direction the user scrolls in. How can I only call a method when they scroll from Right To Left?

以下是我目前已实现它：

Here is how i've currently implemented it:

- (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];
    }
}

推荐答案

In scrollViewWillBeginDragging 滚动视图尚未移动（或注册移动），因此 contentOffset 将为0.自IOS起5你可以改为查看scrollview的 panGestureRecognizer 来确定用户滚动手势的方向和大小。

In scrollViewWillBeginDragging the scroll view has not yet moved (or registered the move) and so contentOffset will by 0. As of IOS 5 you can instead look in the scrollview's panGestureRecognizer to determine the direction and magnitude of the user's scrolling gesture.

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

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

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