Where should this render to MTKView while loop go in UIKit?

Question

Regarding this tutorial...

It describes how to load frames from a video file and draw them one after another to a Metal view.

There is a while loop that iterates over each frame and calls draw on the view...

while sampleBuffer != nil {
    
    ...

    displayView.draw()

    ...
}

Where should this loop go? I have tried it in viewDidLoad and after a delay and lots of print outputs it only seems to show the final frame. It's as though you can't see updates to a view before viewDidLoad has finished.

Answer 1

I suppose I would put it in viewWillAppear(_:) or viewDidAppear(_:). However, this is the first time I've seen a draw command in a super tight loop like that, and it seems less than ideal.

If you would like to control the draw manually, I would recommend the use of a CADisplayLink instead. The purpose of CADisplayLink is it is timed to be in sync with the system update of the screen. You can also set a preferred frame rate, which is nice.

weak var displayLink: CADisplayLink?

func createDisplayLink() {
    displayLink?.invalidate() // in case existing timer
    let displayLink = CADisplayLink(target: self, selector: #selector(update(_:)))
    displayLink.add(to: .main, forMode: .common) 
    displayLink.preferredFramesPerSecond = 30 // Optional set desired frame rate 
    self.displayLink = displayLink
}

@objc func update(_ displayLink: CADisplayLink) {
    displayView.draw()
}

func viewWillAppear(_ animated: Bool) {
    // You can create it earlier, but I prefer here 
    createDisplayLink() 
}

func viewWillDisappear(_ animated: Bool) {
    // Stop rending when view disappears
    displayLink?.invalidate()
}