How to implement a scrollView inside inputAccessoryView [Swift]

In my swift app I'm working with inputActivityView, really hard, and my idea is to add a scroll view to this view with 2 subviews and paging enable. Here's what I've done, I think the problem are constraints but I don't know how to solve it.

  lazy var scrollView: UIScrollView = {
        let sv = UIScrollView(frame: self.bounds)
        sv.backgroundColor = .blue
        sv.isPagingEnabled = true
        sv.contentSize = .init(width: 2 * self.frame.width, height: 54)
        return sv
    }()

    override init(frame: CGRect) { // the init of the customInputAccessoryView
        super.init(frame: frame)
        setup()
    }

    override var intrinsicContentSize: CGSize {
        return .zero
    }

    func setup() {
        backgroundColor = .red
        autoresizingMask = .flexibleHeight

        addSubview(scrollView)
        scrollView.fillSuperview()
        scrollView.heightAnchor.constraint(equalToConstant: 54).isActive = true


        firstView = UIView(frame: .init(origin: .zero, size: .init(width: frame.width, height: 54)))
        firstView.frame.origin = .zero
        firstView.backgroundColor = .gray
        firstView.translatesAutoresizingMaskIntoConstraints = false
        scrollView.addSubview(firstView)

        secondView = UIView(frame: firstView.bounds)
        secondView.frame.origin.x = frame.width
        secondView.backgroundColor = .lightGray
        secondView.translatesAutoresizingMaskIntoConstraints = false
        scrollView.addSubview(secondView)

        addConstraints()
    }

    private func addConstraints() {
        NSLayoutConstraint.activate([
            firstView.widthAnchor.constraint(equalToConstant: frame.width),
            firstView.heightAnchor.constraint(equalToConstant: 54)
       ])
    }

How can I set the constraints for the subviews, because in this way appear only the first view, and I can't scroll to the second one.

Yes, you're missing some constraints.

First, no need to instantiate views with UIView(frame: ...) if you are then setting .translatesAutoresizingMaskIntoConstraints = false because the frame you just gave it will be ignored.

Second, if you have your constraints setup correctly, no need to set a scroll view's .contentSize

// don't do this
//sv.contentSize = .init(width: 2 * self.frame.width, height: 54)

Third, when configuring subviews of a scroll view, make sure your constraints define Top / Leading / Bottom / Trailing AND Width and Height.

Here's an edited version of your code to try:

class MyInputAccessoryView: UIView {

    lazy var scrollView: UIScrollView = {
        let sv = UIScrollView()
        sv.backgroundColor = .blue
        sv.isPagingEnabled = true
        // no need for this
        //sv.contentSize = .init(width: 2 * self.frame.width, height: 54)
        return sv
    }()

    var firstView: UIView!
    var secondView: UIView!

    override init(frame: CGRect) { // the init of the customInputAccessoryView
        super.init(frame: frame)
        setup()
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override var intrinsicContentSize: CGSize {
        return .zero
    }

    func setup() {
        backgroundColor = .red
        autoresizingMask = .flexibleHeight

        addSubview(scrollView)
        //scrollView.fillSuperview()
        scrollView.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            scrollView.topAnchor.constraint(equalTo: topAnchor),
            scrollView.bottomAnchor.constraint(equalTo: bottomAnchor),
            scrollView.leadingAnchor.constraint(equalTo: leadingAnchor),
            scrollView.trailingAnchor.constraint(equalTo: trailingAnchor),
            scrollView.heightAnchor.constraint(equalToConstant: 54),
        ])


        //firstView = UIView(frame: .init(origin: .zero, size: .init(width: frame.width, height: 54)))
        //firstView.frame.origin = .zero
        firstView = UIView()
        firstView.backgroundColor = .gray
        firstView.translatesAutoresizingMaskIntoConstraints = false
        scrollView.addSubview(firstView)

        //secondView = UIView(frame: firstView.bounds)
        //secondView.frame.origin.x = frame.width
        secondView = UIView()
        secondView.backgroundColor = .lightGray
        secondView.translatesAutoresizingMaskIntoConstraints = false
        scrollView.addSubview(secondView)

        addConstraints()
    }

    private func addConstraints() {
        NSLayoutConstraint.activate([

            // make both subviews equal width and height to scrollView
            firstView.widthAnchor.constraint(equalTo: scrollView.widthAnchor),
            firstView.heightAnchor.constraint(equalTo: scrollView.heightAnchor),
            secondView.widthAnchor.constraint(equalTo: scrollView.widthAnchor),
            secondView.heightAnchor.constraint(equalTo: scrollView.heightAnchor),

            // constrain firstView Leading and Top to scrollView contentLayoutGuide Leading and Top
            firstView.leadingAnchor.constraint(equalTo: scrollView.contentLayoutGuide.leadingAnchor),
            firstView.topAnchor.constraint(equalTo: scrollView.contentLayoutGuide.topAnchor),

            // constrain secondView Leading to firstView Trailing
            secondView.leadingAnchor.constraint(equalTo: firstView.trailingAnchor),

            // constrain secondView Top / Bottom / Trailing Top to scrollView contentLayoutGuide Top / Bottom / Trailing
            secondView.topAnchor.constraint(equalTo: scrollView.contentLayoutGuide.topAnchor),
            secondView.bottomAnchor.constraint(equalTo: scrollView.contentLayoutGuide.bottomAnchor),
            secondView.trailingAnchor.constraint(equalTo: scrollView.contentLayoutGuide.trailingAnchor),

        ])
    }

}