viewがスクリーンに登場したことを検知したいです。

以下のコードを空のプロジェクトにそのまま貼り付けて実行してみてください。
UIScrollViewに乗せたUIViewが画面に現れた時に背景色をアニメーションで変えています。

import UIKit

class ViewController: UIViewController, UIScrollViewDelegate {

    var scrollView = UIScrollView()
    var targetView = UIView()
    var isAnimation = false

    override func viewDidLoad() {
        super.viewDidLoad()

        let screenSize = UIScreen.mainScreen().bounds
        scrollView.frame = screenSize
        scrollView.backgroundColor = UIColor.greenColor()
        self.scrollView.contentSize = CGSize(width: screenSize.width, height: screenSize.height * 5)
        scrollView.backgroundColor = UIColor.greenColor()
        scrollView.delegate = self
        view.addSubview(scrollView)

        targetView.backgroundColor = UIColor.redColor()
        targetView.frame = CGRect(x: 100, y: 1500, width: 200, height: 200)
        scrollView.addSubview(targetView)
    }


    func scrollViewDidScroll(scrollView: UIScrollView) {
        let visibleRect = CGRect(origin: scrollView.contentOffset, size: scrollView.bounds.size)

        if CGRectIntersectsRect(visibleRect, targetView.frame) {

            if !isAnimation {
                isAnimation = true
                startAnimation()
            }
        } else {
            isAnimation = false
            self.targetView.backgroundColor = UIColor.redColor()
        }
    }

    func startAnimation()  {

        UIView.animateWithDuration(1.5, animations: {
            self.targetView.backgroundColor = UIColor.yellowColor()
        })
    }
}

回答追記 複数のViewの場合

import UIKit

class ViewController: UIViewController, UIScrollViewDelegate {

    var scrollView = UIScrollView()
    var targetViewArray: [ViewAnimation] = []
    var isAnimation = false

    override func viewDidLoad() {
        super.viewDidLoad()

        let screenSize = UIScreen.mainScreen().bounds
        scrollView.frame = screenSize
        scrollView.backgroundColor = UIColor.greenColor()
        self.scrollView.contentSize = CGSize(width: screenSize.width, height: screenSize.height * 5)
        scrollView.backgroundColor = UIColor.greenColor()
        scrollView.delegate = self
        view.addSubview(scrollView)

        let targetView = UIView(frame: CGRect(x: 20, y: 1500, width: 50, height: 100))
        targetView.backgroundColor = UIColor.redColor()
        scrollView.addSubview(targetView)
        targetViewArray.append(ViewAnimation(view: targetView))

        let targetView1 = UIView(frame: CGRect(x: 120, y: 1500, width: 50, height: 200))
        targetView1.backgroundColor = UIColor.redColor()
        scrollView.addSubview(targetView1)
        targetViewArray.append(ViewAnimation(view: targetView1))

        let targetView2 = UIView(frame: CGRect(x: 220, y: 1500, width: 50, height: 150))
        targetView2.backgroundColor = UIColor.redColor()
        scrollView.addSubview(targetView2)
        targetViewArray.append(ViewAnimation(view: targetView2))
    }


    func scrollViewDidScroll(scrollView: UIScrollView) {
        let visibleRect = CGRect(origin: scrollView.contentOffset, size: scrollView.bounds.size)

        for target in targetViewArray {

            if CGRectIntersectsRect(visibleRect, target.view.frame) {

                if !target.isAnimation {
                    target.isAnimation = true
                    startAnimation(target.view)
                }
            } else {
                target.isAnimation = false
                target.view.backgroundColor = UIColor.redColor()
            }
        }
    }

    func startAnimation(targetView: UIView)  {

        UIView.animateWithDuration(1.5, animations: {
            targetView.backgroundColor = UIColor.yellowColor()
        })
    }
}

class ViewAnimation {
    var view: UIView!
    var isAnimation = false

    init(view: UIView) {
        self.view = view
    }
}