How is layoutIfNeeded used?

layoutIfNeeded forces the receiver to layout its subviews immediately if required.

Suppose you have overridden layoutSubviews, and UIKit feels that your view requires layout for whatever reason (e.g. you called setNeedsLayout when handling some user action). Then, your custom layoutSubviews method will be called immediately instead of when it would normally be called in the regular UIKit run loop event sequence (after event handling, but before drawRect:).

An example of why you might need to call layoutIfNeeded within a single run loop:

1. You resize a custom view containing a table view with a custom layout. setNeedsLayout is set so that layoutSubviews will be called later.
2. A controller object asks the table view to scroll to some particular cell when handling a user event.
3. Your custom view performs some custom sizing of the table view in layoutSubviews that changes the table view size.

The problem is when the controller asked the table view to scroll (step 2), the table view had bounds that were stale. The updated bounds would only be set on the table view later (step 3). What the controller wanted the table view to scroll to may not actually be visible after layoutSubviews is done. A solution then would be for the controller to call layoutIfNeeded in situations where it knows this might occur.

The difference between these two methods can be now be described by referencing the update cycle.

The method setNeedsLayout for a UIView tells the system that you want it to layout and redraw that view and all of its subviews, when it is time for the update cycle. This is an asynchronous activity, because the method completes and returns immediately, but it isn’t until some later time that the layout and redraw actually happens, and you don’t know when that update cycle will be.

In contrast, the method layoutIfNeeded is a synchronous call that tells the system you want a layout and redraw of a view and its subviews, and you want it done immediately without waiting for the update cycle. When the call to this method is complete, the layout has already been adjusted and drawn based on all changes that had been noted prior to the method call.

So, stated succinctly, layoutIfNeeded says update immediately please, whereas setNeedsLayout says please update but you can wait until the next update cycle.