How would you implement the recycling of views during scrolling in an UIScrollView?

Question

Well, there is the delegate that will get called when:

• the scroll view beginns scrolling
• the finger liftet of from the scroll view after initializing a scroll-movement
• the scroll view stopped decelerating

but now the problem is, if you want to recycle those View's upon scrolling, you must do that very quickly during scrolling. Something must trigger a method that will either remove one or three views from top to bottom, or from bottom to top, as the user scrolls.

Problems:

• The user might scroll very slowly, or very fast. I don't know.

• As soon as scrolling begins, I must check i.e. 50 times per second what the offset is, and determine if it's time to recycle views or not. Next problem is, how to invoke a method 50 times per second, and how to stop that as scrolling stopps.

Which patterns do you know to solve that problem? I am sure that there are some good ones out there.

Answer 1

Table views are excellent for this kind of thing, and very flexible.

Answer 2

As a matter of fact, scrollViewDidScroll method is called continuously during scrolling, not when the finger is lifted. (Actually it is called every time contentOffset is changed, be it because of user scrolling or a programmatic change.) So you can safely use it to recycle the views.

Be sure to preload the views within one screen size distance from the current view, because the user can potentially scroll one screen width or height distance in one gesture.

And of course, if you were going to arrange your views vertically, UITableView is your friend and does the recycling for you. (It also supports variable-height rows, if you are interested.)

Answer 3

If paging is enabled then your delegate receives a

-(void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView

when scrolling is at a page boundary which is a good time to recycle views. Otherwise you will have to monitor content offset in the delegate's

- (void)scrollViewDidScroll:(UIScrollView *)scrollView;

and work out when you can recycle a view.