Q:

Frames changing suddenly instead of animating [iOS]

I have a view with a scroll view in my app (a percentage calculator).  I am trying to animate the view to fly out and then fly in again from the left albeit with different contents, on the press of a button. 
Here's my code:
func next()
{
    UIView.animateWithDuration(0.5, animations: {
        self.mainView.frame = CGRectMake(-500, self.view.bounds.height/2-self.mainHeight/2, self.mainWidth, self.mainHeight)
        self.mainView.center.x -= 600
    }) { (success) in
        self.mainView.center.x += 1200    
    }

    UIView.animateWithDuration(0.5, animations: {
        self.mainView.frame = CGRectMake(self.view.bounds.width/2-self.mainWidth/2, self.view.bounds.height/2-self.mainHeight/2, self.mainWidth, self.mainHeight)
    }) { (success) in
        print("Animation Successful!") 
        self.drawView()
    }
}

I don't understand why, but the animation doesn't occur, the view disappears completely and I see "Animation Successful" in the logs. 
To be clear, I have created the view programmatically. 
Here's the code in viewDidLoad(); the view displays correctly when it's called:
    mainHeight = self.view.bounds.height * 0.85
    mainWidth = self.view.bounds.width * 0.85
    let viewHeight = self.view.bounds.height
    let viewWidth = self.view.bounds.width

    mainView.frame = CGRectMake(viewWidth/2-mainWidth/2, viewHeight/2-mainHeight/2, mainWidth, mainHeight)
    mainView.layer.cornerRadius = 8
    mainView.layer.masksToBounds = true
    mainView.backgroundColor = getColor(0, green: 179, blue: 164)
    self.view.addSubview(mainView)

    scrollView.frame = CGRectMake(0, 0, mainWidth, mainHeight)
    scrollView.contentSize = CGSizeMake(mainWidth, 800)
    self.mainView.addSubview(scrollView)

    contentView.frame = CGRectMake(0, 0, mainWidth, 800);
    contentView.backgroundColor = getColor(0, green: 179, blue: 164)
    contentView.layer.masksToBounds = true
    contentView.clipsToBounds = true
    self.scrollView.addSubview(contentView)

A:

Place self.layoutViewIfNeeded() before frame/center changes. 
Additionally you may need to put your next animation within a completion handler of the next animation. 
func next() {
    UIView.animateWithDuration(0.5, animations: {
        self.view.layoutIfNeeded() // add this
        self.mainView.frame = CGRectMake(-500, self.view.bounds.height/2-self.mainHeight/2, self.mainWidth, self.mainHeight)
        self.mainView.center.x -= 600

    }) { (success) in
        self.view.layoutIfNeeded() // add this
        self.mainView.center.x += 1200

        // your next animation within a completion handler of previous animation
        UIView.animateWithDuration(0.5, animations: {

            self.mainView.frame = CGRectMake(self.view.bounds.width/2-self.mainWidth/2, self.view.bounds.height/2-self.mainHeight/2, self.mainWidth, self.mainHeight)

        }) { (success) in

            print("Animation Successful!") 
            self.drawView()
        }
    }
}   

I haven't tested it yet, perhaps some changes or lines ordering is needed. 

A:

In Swift 3, I had to set the frame and then call layoutIfNeeded(). 
For example, an animation with spring:
UIView.animate(withDuration: 0.3, delay: 0.1, usingSpringWithDamping: 0.7, initialSpringVelocity: 0.5, options: .curveEaseInOut, animations: {
        self.subView.frame = CGRect(x: 0, y: 0, width: 500, height: 200)
        self.view.layoutIfNeeded()
    }) { (_) in
        // Follow up animations...
    }

