Why do UIView & UIImageView animate differently?

问题

I have 4 buttons, all use the same animate func

• imageViewWithoutDelay
• imageViewWithDelay
• ViewWithoutDelay
• ViewWithDelay

my code:

import UIKit

class ViewController: UIViewController {

    @IBAction func imageViewithoutDelay(_ sender: AnyObject) {
        let circleImage = UIImageView()
        // kindly add your own image.
        circleImage.image = UIImage(named: "empty")
        animate(inputView: circleImage, delay: false)
    }

    @IBAction func imageViewWithDelay(_ sender: UIButton) {

        let circleImage = UIImageView()
        circleImage.image = UIImage(named: "empty")
        animate(inputView: circleImage, delay: true)
    }

    func animate(inputView : UIView, delay: Bool){

        inputView.layer.cornerRadius = inputView.frame.size.width / 2
        inputView.clipsToBounds = true
        inputView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(inputView)

        let screenSize = UIScreen.main.bounds
        let screenHeight = screenSize.height * 0.10
        inputView.center.y = screenHeight
        view.setNeedsLayout()
        view.setNeedsDisplay()

        if delay == true{
        DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(1), execute: {
            UIView.animate(withDuration: 2.5, animations: {
                inputView.alpha = 0.0
                let screenSize = UIScreen.main.bounds
                let screenHeight = screenSize.height * 0.10
                inputView.center.y -= screenHeight
            })
        })} else{
            UIView.animate(withDuration: 2.5, animations: {
                inputView.alpha = 0.0
                let screenSize = UIScreen.main.bounds
                let screenHeight = screenSize.height * 0.10
                inputView.center.y -= screenHeight
            })

        }

    }

    override func viewDidLoad() {
        super.viewDidLoad()
    }

}

What amuses me is the different behavior of the UIview & UIImageView.

The UIImageView instance only animates as intended (going up from origin) if I use delay. However, without using a delay block, UIImageView instance first drops down, then animates going up to its origin).

The UIView doesn’t show different behaviors. It animates from its origin and goes up.

Is this a bug?

This question is a tangent to this original question.

回答1:

What you're doing is wrong no matter how you slice it. You cannot add a view and animate it all in the same breath. You can only animate a view that is already in the interface.

That is why it works for the image view if you add the delay: by the time you animate, the image view has gotten into the interface and the interface has settled down.

The fact that it seems to work for the non-imageview is just dumb luck.

来源：https://stackoverflow.com/questions/42168704/why-do-uiview-uiimageview-animate-differently