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iOS swift — setNeedsLayout vs layoutIfNeeded vs layoutSubviews()
I always wondered about these methods while coding in iOS. I tried to read more and this is what I understood.
When an iOS app launches, UIApplication in iOS starts the main run loop for an app, which runs on the main thread. The main run loop processes events (such as user touches) and handles updates to view-based interfaces. As events occur, such as touch, location updates, motion, and multimedia control, the run loop finds the appropriate handler for the events, calling appropriate methods, which call other methods, and so on. At some moment in time, all events will have been handled and control will return to the run loop. Let’s label this point where control is returned to the run loop as the update cycle.
While the events are being processed, and some changes are requested to the view, these changes are not updated immediately. Instead the system wait for the existing process to finish and when the next redraw cycle is going to happen. There is a periodic interval between the event processing and UI layout update handling. This is why we need to understand the above three methods properly.
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.
Apple doc says: Call this method on your application’s main thread when you want to adjust the layout of a view’s subviews. This method makes a note of the request and returns immediately. Because this method does not force an immediate update, but instead waits for the next update cycle, you can use it to invalidate the layout of multiple views before any of those views are updated. This behavior allows you to consolidate all of your layout updates to one update cycle, which is usually better for performance.
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.
The default implementation uses any constraints you have set to determine the size and position of any subviews.
Subclasses can override this method as needed to perform more precise layout of their subviews. You should override this method only if the autoresizing and constraint-based behaviors of the subviews do not offer the behavior you want. You can use your implementation to set the frame rectangles of your subviews directly.
You should not call this method directly. If you want to force a layout update, call the
setNeedsLayout() method instead to do so prior to the next drawing update. If you want to update the layout of your views immediately, call the
For a better understanding , download and run the code in Github. I found this from the link mentioned below.