我可以拦截UIScrollview.contentOffset的'willSet'和'didSet'吗？

Question

I have a situation where I need to gracefully move a frame when the contentOffset.y is a negative value.

The problem is I don't want to get the contentOffset call AFTER the content has already been dropped below 0.

Is there a willSet call where I can intercept it before the UIScrollView offsets its content?

Answer 1

It seems, this works:

class MyScrollView: UIScrollView {
    override var contentOffset:CGPoint {
        willSet {
            println("newOffset: \(newValue)")
        }
    }
}

In your subclass of UIScrollView , just override the willSet of contentOffset . It seems that is called before layoutSubviews() .

Answer 2

You can add a willSet pretty easily in swift. Just subclass UIScrollView and only override that, like so. Obviously you'll wanna do something besides println, but I verified this definitely fires before the didScroll delegate does. I don't know that it will resolve your animation issue, but this is what you asked for.

import UIKit

class MyScrollView: UIScrollView {

    override var contentOffset: CGPoint { willSet { println("old value \(newValue)") } }

}

Answer 3

You can use the delegate methods scrollViewDidScroll or scrollViewWillBeginDecelerating and intercept the contentOffset.y . I hope this help you.