In iOS, is there a way to find out if any view is rendered on screen?

Question

I want to monitor how long it takes between a UIViewController starts running init method and a user really sees something shown on screen. I know viewDidAppear is a good place to end my timer, but in some cases, my app needs to fetch data from web and then UITableViewCells or other views are rendered after download is complete. I'd like to know if there is any general way to handle all cases.

Answer 1

One good place would be viewDidLayoutSubviews. As per apple docs, viewDidLayoutSubviews it's called to notify the view controller that it's view has just laid out it's subviews so this method is called every time the view is updated (rotated, changed, etc.). The downside of this method is that it gets called A LOT, for every layout change so probably there you should do some additional computations (did you received your response from the server, is your data source updated). This answers your specific question - "In iOS, is there a way to find out if any view is rendered on screen?" - but I would not recommend this.

Probably you want to implement some analytics logging for the API that you are using so the best way, in my opinion would be to have one logging class, that implements two methods (loading started - loading finished). And call those methods in your ViewControllers. (Maybe you can create a base ViewController class and all your viewcontrollers should inherit that ViewController - in that base ViewController you can call the loading started method only once, in viewDidLoad - because you override that viewDidLoad - this will save you a few lines and the risk to forget calling the method in your VC).

The OS usually is fast enough when laying out your UI (unless you do some really strange stuff with your UI) so the only thing that is slowing down the Loading are the WebAPI Calls so you should not mind the layout methods. Please let me know if this makes sense.