ios - Using ScrollView Programmatically in Swift 3

Question

I have searched other questions and seem to still have some trouble creating my scrollView programmatically with autolayout in swift 3. I am able to get my scrollview to show up as shown in the picture below, but when I scroll to the bottom my other label does not show up and the 'scroll top' label does not disappear.

Hoping someone can help review my code below!

import UIKit

class ViewController: UIViewController {

let labelOne: UILabel = {
    let label = UILabel()
    label.text = "Scroll Top"
    label.backgroundColor = .red
    label.translatesAutoresizingMaskIntoConstraints = false
    return label
}()

let labelTwo: UILabel = {
    let label = UILabel()
    label.text = "Scroll Bottom"
    label.backgroundColor = .green
    label.translatesAutoresizingMaskIntoConstraints = false
    return label
}()


override func viewDidLoad() {
    super.viewDidLoad()

    let screensize: CGRect = UIScreen.main.bounds
    let screenWidth = screensize.width
    let screenHeight = screensize.height
    var scrollView: UIScrollView!
    scrollView = UIScrollView(frame: CGRect(x: 0, y: 120, width: screenWidth, height: screenHeight))
    scrollView.contentSize = CGSize(width: screenWidth, height: 2000)
    scrollView.addSubview(labelOne)
    scrollView.addSubview(labelTwo)


    view.addSubview(labelOne)
    view.addSubview(labelTwo)
    view.addSubview(scrollView)

    // Visual Format Constraints
    view.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "H:|[v0]|", options: NSLayoutFormatOptions(), metrics: nil, views: ["v0": labelOne]))
    view.addConstraints(NSLayoutConstraint.constraints(withVisualFormat: "V:|-100-[v0]", options: NSLayoutFormatOptions(), metrics: nil, views: ["v0": labelOne]))

    // Using iOS 9 Constraints in order to place the label past the iPhone 7 view
    view.addConstraint(NSLayoutConstraint(item: labelTwo, attribute: .top, relatedBy: .equal, toItem: labelOne, attribute: .bottom, multiplier: 1, constant: screenHeight + 200))
    view.addConstraint(NSLayoutConstraint(item: labelTwo, attribute: .right, relatedBy: .equal, toItem: labelOne, attribute: .right, multiplier: 1, constant: 0))
    view.addConstraint(NSLayoutConstraint(item: labelTwo, attribute: .left, relatedBy: .equal, toItem: labelOne, attribute: .left, multiplier: 1, constant: 0)     

    }

}

Answer 1

It is easy to use constraints to define the scroll content size - so you don't have to do any manual calculations.

Just remember:

1. The content elements of your scroll view must have left / top / width / height values. In the case of objects such as labels, they have intrinsic sizes, so you only have to define the left & top.
2. The content elements of your scroll view also define the bounds of the scrollable area - the contentSize - but they do so with the bottom & right constraints.
3. Combining those two concepts, you see that you need a "continuous chain" with at least one element defining the top / left / bottom / right extents.

Here is a simple example, that will run directly in a Playground page:

import UIKit
import PlaygroundSupport

class TestViewController : UIViewController {

    let labelOne: UILabel = {
        let label = UILabel()
        label.text = "Scroll Top"
        label.backgroundColor = .red
        label.translatesAutoresizingMaskIntoConstraints = false
        return label
    }()

    let labelTwo: UILabel = {
        let label = UILabel()
        label.text = "Scroll Bottom"
        label.backgroundColor = .green
        label.translatesAutoresizingMaskIntoConstraints = false
        return label
    }()

    let scrollView: UIScrollView = {
        let v = UIScrollView()
        v.translatesAutoresizingMaskIntoConstraints = false
        v.backgroundColor = .cyan
        return v
    }()


    override func viewDidLoad() {
        super.viewDidLoad()

        // add the scroll view to self.view
        self.view.addSubview(scrollView)

        // constrain the scroll view to 8-pts on each side
        scrollView.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 8.0).isActive = true
        scrollView.topAnchor.constraint(equalTo: view.topAnchor, constant: 8.0).isActive = true
        scrollView.rightAnchor.constraint(equalTo: view.rightAnchor, constant: -8.0).isActive = true
        scrollView.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: -8.0).isActive = true

        // add labelOne to the scroll view
        scrollView.addSubview(labelOne)

        // constrain labelOne to left & top with 16-pts padding
        // this also defines the left & top of the scroll content
        labelOne.leadingAnchor.constraint(equalTo: scrollView.leadingAnchor, constant: 16.0).isActive = true
        labelOne.topAnchor.constraint(equalTo: scrollView.topAnchor, constant: 16.0).isActive = true

        // add labelTwo to the scroll view
        scrollView.addSubview(labelTwo)

        // constrain labelTwo at 400-pts from the left
        labelTwo.leadingAnchor.constraint(equalTo: scrollView.leadingAnchor, constant: 400.0).isActive = true

        // constrain labelTwo at 1000-pts from the top
        labelTwo.topAnchor.constraint(equalTo: scrollView.topAnchor, constant: 1000).isActive = true

        // constrain labelTwo to right & bottom with 16-pts padding
        labelTwo.rightAnchor.constraint(equalTo: scrollView.rightAnchor, constant: -16.0).isActive = true
        labelTwo.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor, constant: -16.0).isActive = true

    }

}


let vc = TestViewController()
vc.view.backgroundColor = .yellow
PlaygroundPage.current.liveView = vc