How to know when UIScrollView is about to stop scrolling in iOS ?

UIScrollViewDelegate - that's all we have.
There is scrollViewDidEndDecelerating.
I'm not sure, looks like scrollViewWillEndDragging may help.

I want to be notified when scroll view is about to stop scrolling (few instant(1 or 1/2 second) before stopping not exactly at the time of stopping).

Will scrollViewWillEndDragging will help here ?

scrollViewWillEndDragging means end of the dragging.

scrollViewDidEndScrollingAnimation - when the automation ends.

There is contentOffset and visible rectangle properties.
I do not know why you need to know when the scroller is about to stop. Sometimes you may solve your question if you'll ask it a bit different. Looks like now you trying to solve a problem derived from another problem.

Issue is more on scrolling than dragging  Requirement is like..

I want to initiate a call  to fetch the data from database in secondary thread  to render  UIView .  This data should be available before stopping the UIScrollView. So I can render that data in UI when UIScrollVIew stop.

You can work with GUI only in the main thread. You cannot make with a UIView something from any other thread. You can fetch data in a secondary thread, but in order to update a UIView you will have to switch to the main thread. If the last one is busy, you will have to wait.
You know there is a special refresh control. But you can arrange everything as you wish. You can launch the fetch when the scroll stopped dragging and begin scrolling. Let it finish the scroll. The fetch, probably, may happen in a secondary thread and in the end of it switch back to the main thread an update the related UIView object.
For example so:

[[NSOperationQueue mainQueue] addOperationWithBlock:^{
    [self updateView];
}];

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By going this way UI is waiting for data and flickering after data loaded. So I want to load data only when UI scrollview is about to stop.

 Is there any other way to do it ?