Setting contentOffset programmatically triggers scrollViewDidScroll

问题

I've got a a few UIScrollView on a page. You can scroll them independently or lock them together and scroll them as one. The problem occurs when they are locked.

I use UIScrollViewDelegate and scrollViewDidScroll: to track movement. I query the contentOffset of the UIScrollView which changed and then reflect change to other scroll views by setting their contentOffset property to match.

Great.... except I noticed a lot of extra calls. Programmatically changing the contentOffset of my scroll views triggers the delegate method scrollViewDidScroll: to be called. I've tried using setContentOffset:animated: instead, but I'm still getting the trigger on the delegate.

How can I modify my contentOffsets programmatically to not trigger scrollViewDidScroll:?

Implementation notes.... Each UIScrollView is part of a custom UIView which uses delegate pattern to call back to the presenting UIViewController subclass that handles coordinating the various contentOffset values.

回答1:

It is possible to change the content offset of a UIScrollView without triggering the delegate callback scrollViewDidScroll:, by setting the bounds of the UIScrollView with the origin set to the desired content offset.

CGRect scrollBounds = scrollView.bounds;
scrollBounds.origin = desiredContentOffset;
scrollView.bounds = scrollBounds;

回答2:

Try

id scrollDelegate = scrollView.delegate;
scrollView.delegate = nil;
scrollView.contentOffset = point;
scrollView.delegate = scrollDelegate;

Worked for me.

回答3:

What about using existing properties of UIScrollView?

if(scrollView.isTracking || scrollView.isDragging || scrollView.isDecelerating) {
    //your code
}

回答4:

Simplifying @Tark's answer, you can position the scrollview without firing scrollViewDidScroll in one line like this:

scrollView.bounds.origin = CGPoint(x:0, y:100); // whatever values you'd like

回答5:

Another approach is to add some logic in your scrollViewDidScroll delegate to determine whether or not the change in content offset was triggered programatically or by the user's touch.

Add an 'isManualScroll' boolean variable to your class.
Set its initial value to false.
In scrollViewWillBeginDragging set it to true.
In your scrollViewDidScroll check to see that is it true and only respond if it is.
In scrollViewDidEndDecelerating set it to false.
In scrollViewWillEndDragging add logic to set it to false if the velocity is 0 (as scrollViewDidEndDecelerating won't be called in this case).

回答6:

This is not a direct answer to the question, but if you are getting what appear to be spurious such messages, it can ALSO be because you are changing the bounds. I am using some Apple sample code with a "tilePages" method that removes and adds subview to a scrollview. This infrequently results in additional scrollViewDidScroll: messages called immediately, so you get into a recursion which you for sure didn't expect. In my case I got a nasty impossible to find crash.

What I ended up doing was queuing the call on the main queue:

- (void)scrollViewDidScroll:(UIScrollView *)scrollView
{
    if(scrollView == yourScrollView) {
        // dispatch fixes some recursive call to scrollViewDidScroll in tilePages (related to removeFromSuperView)
        // The reason can be found here: http://stackoverflow.com/questions/9418311
        dispatch_async(dispatch_get_main_queue(), ^{ [self tilePages]; });
    }
}

来源：https://stackoverflow.com/questions/9418311/setting-contentoffset-programmatically-triggers-scrollviewdidscroll

标签

ios

uiscrollview

uiscrollviewdelegate