Use a SwiftUI View as inputAccessoryView, dynamically adjusting to height changes

InputAccessoryHostingController.swift

// (c) Hannes Oud @hannesoid

/// Hosts a SwiftUI view for use as an`inputAccessoryView`
///
/// - Implements a subclass of `UIInputViewController`, this allows setting it as `UITextView.inputAccessoryViewController`
/// - Has as a `.view` a `UIView` subclass that provides a `.intrinsicContentSize` which returns the height of a SwiftUI view
/// - Has a `UIHostingViewController` subclass as a child view controller. Invalidates the `.view`'s `intrinsicContentSize` when the SwiftUI view layouts, in order to inform the system to update the size of the `inputAccessoryView`
///
/// **Usage**
///
/// ```
///    /// Subclass `UITextView` in order to be able to set `inputAccessoryViewController`
///    final class TextView: UITextView {
///      private var _inputAccessoryViewController: UIInputViewController?
///
///      // Overrides the readonly `UITextView.inputAccessoryViewController` property in order to be able to set it (as described in docs)
///      override var inputAccessoryViewController: UIInputViewController? {
///          get { _inputAccessoryViewController }
///          set { _inputAccessoryViewController = newValue }
///      }
///    }
///
///    // Configure your textview with a InputAccessoryHostingController
///    let myTextView = TextView()
///    myTextView.inputAccessoryViewController = InputAccessoryHostingController {
///        Text("Yolo") // SwiftUI ViewBuilder
///    }
///```
class InputAccessoryHostingController<InputAccessoryViewType: View>: UIInputViewController {

    typealias ViewMaker = () -> InputAccessoryViewType

    /// Serves as self.view, has a custom `intrinsicContentSize` implementation which fetches the actual fitting size of the swift UI view
    private lazy var dynamicIntrinsicHeightView: DynamicIntrinsicHeightView = .init(frame: .init(origin: .zero, size: self.initialDimensions))
    private lazy var hostingController = InputAccessoryInnerHostingController(
        rootView: self.makeAccessoryView(),
        didUpdateSize: {
            self.dynamicIntrinsicHeightView.invalidateIntrinsicContentSize()
    })
    private let initialDimensions: CGSize = .init(width: 300, height: 100)
    private var makeAccessoryView: ViewMaker

    init(@ViewBuilder makeAccessoryView: @escaping ViewMaker) {
        self.makeAccessoryView = makeAccessoryView
        super.init(nibName: nil, bundle: nil)
    }

    // MARK: - UIView

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

    override func loadView() {
        self.view = self.dynamicIntrinsicHeightView
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        // Add hostingController as child and its SwiftUI view as subview
        self.addChild(self.hostingController)
        self.view.addSubview(self.hostingController.view)
        self.hostingController.didMove(toParent: self)
        self.hostingController.view.frame = self.view.frame
        // self.hostingController.view.backgroundColor = UIColor.yellow // enable to see areas
        self.hostingController.view.autoresizingMask = [.flexibleHeight, .flexibleWidth]

        self.dynamicIntrinsicHeightView.fetchFittedHeight = { [unowned self] in
            self.hostingController.hostedViewRecommendedSize(forWidth: self.view.frame.width).height
        }

        self.view.translatesAutoresizingMaskIntoConstraints = false
        // self.view.backgroundColor = .blue

        UIView.performWithoutAnimation {
            self.view.layoutSubviews()
        }
    }

    // MARK: - DynamicIntrinsicHeightView

    /// Has a dynamic`intrinsicContentSize` height which is determined by an injected closure
    private final class DynamicIntrinsicHeightView: UIView {

        var fetchFittedHeight: () -> CGFloat = { 0 }

        override init(frame: CGRect) {
            super.init(frame: frame)
            self.fetchFittedHeight = { [unowned self] in self.frame.height }
        }

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

        override var intrinsicContentSize: CGSize {
            // note: a difference between height here and initial frame height causes a jerky appearance animation
            return .init(width: UIView.noIntrinsicMetric, height: self.fetchFittedHeight())
        }
    }

    // MARK: - InputAccessoryInnerHostingController

    final class InputAccessoryInnerHostingController: UIHostingController<InputAccessoryViewType> {

        var didUpdateSize: () -> Void

        init(rootView: InputAccessoryViewType, didUpdateSize: @escaping () -> Void) {
            self.didUpdateSize = didUpdateSize
            super.init(rootView: rootView)
        }

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

        override func viewDidLayoutSubviews() {
            super.viewDidLayoutSubviews()
            self.didUpdateSize()
        }

        func hostedViewRecommendedSize(forWidth width: CGFloat) -> CGSize {
            let fittingSize = self.view.systemLayoutSizeFitting(.init(width: self.view.frame.width, height: .greatestFiniteMagnitude), withHorizontalFittingPriority: .defaultHigh, verticalFittingPriority: .defaultHigh)
            return .init(width: width, height: fittingSize.height)
        }
    }
}