Hi everyone,
I'm not an experienced developer. I'm interested in the low-latency related APIs in UIUpdateLink, but I failed to write even a minimal demo that works.
UIUpdateInfo.isImmediatePresentationExpected is always false here. My understanding must be wrong. I've totally no idea so I'm asking for help here. I appreciate anyone who gives suggestions of any kind.
Here's my (failed) demo about tracking touch inputs (of the 1st finger) and draw some shape at that place:
import UIKit
class ContentUIView: UIView {
// MARK: - About UIUpdateLink and drawing
required init?(coder: NSCoder) {
super.init(coder: coder)
initializeUpdateLink()
}
override init(frame: CGRect) {
super.init(frame: frame)
initializeUpdateLink()
}
private func initializeUpdateLink() {
self.updateLink = UIUpdateLink(view: self)
self.updateLink.addAction(to: .beforeCADisplayLinkDispatch,
target: self,
selector: #selector(update))
self.updateLink.wantsImmediatePresentation = true
self.updateLink.isEnabled = true
}
@objc func update(updateLink: UIUpdateLink,
updateInfo: UIUpdateInfo) {
print(updateInfo.isImmediatePresentationExpected) // FIXME: Why always false?
CATransaction.begin()
defer { CATransaction.commit() }
layer.setNeedsDisplay()
layer.displayIfNeeded()
}
override func draw(_ rect: CGRect) {
// FIXME: Any way to support opacity?
guard let context = UIGraphicsGetCurrentContext() else { return }
context.clear(rect)
guard let lastTouch = self.lastTouch else { return }
let location = lastTouch.location(in: self)
let circleBounds = CGRect(x: location.x - 16, y: location.y - 16, width: 32, height: 32)
context.setFillColor(.init(red: 1/2, green: 1/2, blue: 1/2, alpha: 1))
context.addLines(between: [])
context.fillEllipse(in: circleBounds)
}
// MARK: - Touch input
override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
super.touchesBegan(touches, with: event)
guard lastTouch == nil else { return }
lastTouch = touches.first
}
override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {
super.touchesEnded(touches, with: event)
guard let lastTouch, touches.contains(lastTouch) else { return }
self.lastTouch = nil
}
override func touchesCancelled(_ touches: Set<UITouch>, with event: UIEvent?) {
self.touchesEnded(touches, with: event)
}
private var lastTouch: UITouch?
private var updateLink: UIUpdateLink!
}
#Preview { ContentUIView() }
Anyway, I'm not meant to find alternative APIs and I'd be willing to know what it can't do.
