A bottom anchored modal (bottom sheet) that allows adding a child view controller. It will self-size itself based on the content in the child view controller. It respects the safe area, including having a max height. To ensure content can be fully viewed if it hits the max height, the child view controller needs to have a tableview or scrollview.

BottomAnchoredModalTransitioner.swift

//
//  BottomAnchoredModalTransitioner.swift
//  BB Links
//
//  Created by Justin Stanley on 2017-09-07.
//  Copyright © 2017 Justin Stanley. All rights reserved.
//
import Foundation
import UIKit

internal final class BottomAnchoredModalTransitioner: NSObject, UIViewControllerTransitioningDelegate {
    internal let transitionAnimator = CustomModalTransition()
    internal var modalPresentAnimations: TransitionAnimations?
    internal var modalDismissAnimations: TransitionAnimations?
    internal var interactionIsInProgress = false
    internal var shouldCompleteTransition = false

    public func animationController(forPresented _: UIViewController,
                                    presenting _: UIViewController,
                                    source _: UIViewController) -> UIViewControllerAnimatedTransitioning?
    {
        transitionAnimator.context = .presenting
        return transitionAnimator
    }

    public func animationController(forDismissed _: UIViewController) -> UIViewControllerAnimatedTransitioning? {
        transitionAnimator.context = .dismissing
        return transitionAnimator
    }

    public func interactionControllerForPresentation(using _: UIViewControllerAnimatedTransitioning) -> UIViewControllerInteractiveTransitioning? {
        nil
    }

    public func interactionControllerForDismissal(using _: UIViewControllerAnimatedTransitioning) -> UIViewControllerInteractiveTransitioning? {
        transitionAnimator.context = .dismissing
        return transitionAnimator
    }

    public func presentationController(forPresented presented: UIViewController,
                                       presenting: UIViewController?,
                                       source _: UIViewController) -> UIPresentationController?
    {
        CustomModalPresentationController(presentedViewController: presented, presenting: presenting)
    }
}

BottomAnchoredModalViewController.swift

//
//  BottomAnchoredModalViewController.swift
//  BB Links
//
//  Created by Justin Stanley on 2017-09-09.
//  Copyright © 2017 Justin Stanley. All rights reserved.
//
import Foundation
import SnapKit
import UIKit

// MARK: - Direction
internal enum Direction {
    case upwards
    case downwards
    case left
    case right
    case undetermined
}

// MARK: - UIPanGestureRecognizer
internal extension UIPanGestureRecognizer {
    var direction: Direction {
        let vel = velocity(in: view)
        let isVertical = abs(vel.y) > abs(vel.x)

        switch (isVertical, vel.x, vel.y) {
        case (true, _, let yVelocity) where yVelocity < 0: return .upwards
        case (true, _, let yVelocity) where yVelocity > 0: return .downwards
        case (false, let xVelocity, _) where xVelocity > 0: return .right
        case (false, let xVelocity, _) where xVelocity < 0: return .left
        default: return .undetermined
        }
    }

    var isDownwardPan: Bool {
        switch direction {
        case .downwards: return true
        case .upwards, .left, .right, .undetermined: return false
        }
    }
}

// MARK: - BottomAnchoredModalViewControllerProtocol
internal protocol BottomAnchoredModalViewControllerProtocol: AnyObject {
    var view: UIView! { get }
    var title: String? { get set }
    var onCloseButtonTap: (() -> Void)? { get set }
    var contentView: UIScrollView { get }
}

internal typealias TransitionAnimations = (() -> Void)
internal typealias TransitionCompletion = (() -> Void)

// MARK: - BottomAnchoredModalViewController
internal final class BottomAnchoredModalViewController: UIViewController {
    // MARK: Outlets
    @IBOutlet private var modalContainer: UIView!
    @IBOutlet private var childVCContainer: UIView!
    @IBOutlet private var maskView: UIView!
    @IBOutlet private var headerContainer: UIView!
    @IBOutlet private var titleLabel: UILabel!
    @IBOutlet private var closeButton: UIButton!
    @IBOutlet private var closeHandle: UIImageView!

    // MARK: Properties
    private let dimmedView = UIView()
    private let childVC: BottomAnchoredModalViewControllerProtocol
    private let transitioner = BottomAnchoredModalTransitioner()
    private var modalShowingConstraint: Constraint?
    private var modalHiddenConstraint: Constraint?
    private static let presentedAlpha: CGFloat = 1
    private let minTopGapSize: CGFloat = 20
    private var fullModalContentHeight: CGFloat {
        childVC.contentView.contentSize.height + childVC.contentView.adjustedContentInset.top + childVC.contentView.adjustedContentInset.bottom
    }

    private var maxModalContentHeight: CGFloat {
        view.bounds.height - headerContainer.bounds.height - topGapMinHeight
    }

    private var topGapMinHeight: CGFloat {
        view.safeAreaInsets.top + minTopGapSize
    }

    internal init(childVC: BottomAnchoredModalViewControllerProtocol) {
        self.childVC = childVC

        super.init(nibName: String(describing: type(of: self)), bundle: nil)

        transitioningDelegate = transitioner
        modalPresentationStyle = .overFullScreen
        transitioner.transitionAnimator.presentAnimator = presentAnimatorClosure()
        transitioner.transitionAnimator.dismissAnimator = dismissAnimatorClosure()
        transitioner.transitionAnimator.onPanCancelled = onPanCancelled()
        transitioner.transitionAnimator.onDismissEnd = onDismissEnd()
    }

    @available(*, unavailable)
    internal required init?(coder _: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override internal func viewDidLoad() {
        super.viewDidLoad()

        NotificationCenter.default.addObserver(self, selector: #selector(updateDisplay),
                                               name: NSNotification.Name(rawValue: ThemeChangedNotification), object: nil)

        setupViews()
    }

    override func viewSafeAreaInsetsDidChange() {
        super.viewSafeAreaInsetsDidChange()

        childVC.view.layoutIfNeeded()
    }

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()

        childVC.view.layoutIfNeeded()

        childVC.view.snp.remakeConstraints {
            $0.edges.equalToSuperview()
            $0.height.equalTo(min(self.fullModalContentHeight, self.maxModalContentHeight))
        }
    }

    // MARK: Setup
    private func setupViews() {
        view.translatesAutoresizingMaskIntoConstraints = false

        setupMainViewColours()

        titleLabel.font = .systemFont(ofSize: 22, weight: .black)
        titleLabel.text = childVC.title?.uppercased()

        dimmedView.alpha = 0
        dimmedView.isUserInteractionEnabled = true

        let tapRecognizer = UITapGestureRecognizer(target: self, action: #selector(handleDimmedViewTap))
        dimmedView.addGestureRecognizer(tapRecognizer)

        view.addSubview(dimmedView)

        dimmedView.snp.makeConstraints { make in
            make.edges.equalToSuperview()
        }

        setupModalVC()
    }

    private func setupMainViewColours() {
        let theme = ThemeManager.shared.theme

        view.backgroundColor = .clear
        headerContainer.backgroundColor = theme.primaryBackgroundColor

        let titleTextColor: UIColor
        if ThemeManager.shared.useSecondaryTintForPrimaryBackgroundColorOption {
            titleTextColor = theme.textColorOnSecondaryTintColor
        } else {
            titleTextColor = theme.primaryTextColor
        }

        titleLabel.textColor = titleTextColor

        dimmedView.backgroundColor = theme.dimmedTransparentBackgroundColor
    }

    private func setupModalVC() {
        setupModalVCColors()

        childVC.onCloseButtonTap = { [unowned self] in
            self.dismiss(wantsInteractiveStart: false)
        }

        modalContainer.applyShadow(.Modal)

        setupPanGestureRecognizer(on: modalContainer)

        view.addSubview(modalContainer)
        modalContainer.snp.makeConstraints {
            $0.centerX.equalToSuperview()
            $0.leading.trailing.lessThanOrEqualToSuperview().priority(.medium)
            $0.top.lessThanOrEqualTo(self.view.safeAreaLayoutGuide).offset(minTopGapSize).priority(.high)
            $0.width.lessThanOrEqualTo(500).priority(.required)
            self.modalShowingConstraint = $0.bottom.equalTo(self.view.snp.bottom).constraint
            self.modalHiddenConstraint = $0.top.equalTo(self.view.snp.bottom).offset(20).constraint
        }

        updateModalConstraints(isShowing: false)

        maskView.addCorners()

        if let childVC = childVC as? UIViewController {
            addChild(childVC)
            childVCContainer.addSubview(childVC.view)
            childVC.didMove(toParent: self)

            childVC.view.snp.makeConstraints {
                $0.edges.equalToSuperview()
                // temporary to let it calculate its content height
                $0.height.equalTo(200)
            }

            childVC.view.layoutIfNeeded()
        }
    }

    private func setupModalVCColors() {
        let theme = ThemeManager.shared.theme

        let alpha: CGFloat = 0.35
        let tintColor: UIColor
        if ThemeManager.shared.useSecondaryTintForPrimaryBackgroundColorOption {
            tintColor = theme.textColorOnSecondaryTintColor.withAlphaComponent(alpha)
        } else {
            tintColor = theme.primaryTextColor.withAlphaComponent(alpha)
        }

        closeButton.tintColor = tintColor
        closeHandle.tintColor = tintColor

        modalContainer.backgroundColor = .clear

        maskView.backgroundColor = theme.primaryBackgroundColor
    }

    private func setupPanGestureRecognizer(on view: UIView) {
        let gestureRecognizer = UIPanGestureRecognizer(target: self, action: #selector(handlePanGesture(_:)))
        view.addGestureRecognizer(gestureRecognizer)
    }

    private func updateModalConstraints(isShowing: Bool) {
        if isShowing {
            modalHiddenConstraint?.deactivate()
            modalShowingConstraint?.activate()
        } else {
            modalShowingConstraint?.deactivate()
            modalHiddenConstraint?.activate()
        }
    }

    // MARK: Gestures
    @objc
    internal func handleDimmedViewTap(_ recognizer: UITapGestureRecognizer) {
        if recognizer.state == .ended {
            dismiss(wantsInteractiveStart: false)
        }
    }

    @IBAction private func closeButtonTapped() {
        dismiss(wantsInteractiveStart: false)
    }

    @objc
    private func handlePanGesture(_ gestureRecognizer: UIPanGestureRecognizer) {
        guard let gestureRecognizerView = gestureRecognizer.view else { return }

        let translation = gestureRecognizer.translation(in: gestureRecognizerView)
        var progress = translation.y / gestureRecognizerView.bounds.height
        progress = min(max(progress, 0), 1)

        switch gestureRecognizer.state {
        case .began:
            transitioner.interactionIsInProgress = true
            dismiss(wantsInteractiveStart: true)
        case .changed:
            transitioner.shouldCompleteTransition = (gestureRecognizer.isDownwardPan && translation.y > 40) || progress > 0.3
            transitioner.transitionAnimator.update(progress)
        case .ended:
            transitioner.interactionIsInProgress = false
            if transitioner.shouldCompleteTransition {
                transitioner.transitionAnimator.finish()
            } else {
                transitioner.transitionAnimator.cancel()
            }
        case .cancelled:
            transitioner.interactionIsInProgress = false
            transitioner.transitionAnimator.cancel()
        case .failed, .possible:
            break
        @unknown default:
            fatalError("Unknown UIGestureRecognizer.State: \(gestureRecognizer.state)")
        }
    }

    internal func onPanCancelled() -> (() -> Void) {
        return { [weak self] in
            self?.updateModalConstraints(isShowing: true)
        }
    }

    internal func onDismissEnd() -> (() -> Void) {
        return { [weak self] in
            self?.childVC.view.removeFromSuperview()
            self?.view.removeFromSuperview()
        }
    }

    private func dismiss(wantsInteractiveStart: Bool) {
        transitioner.transitionAnimator.wantsInteractiveStart = wantsInteractiveStart
        dismiss(animated: true)
    }

    @objc
    private func updateDisplay() {
        setupMainViewColours()
        setupModalVCColors()
    }

    // MARK: Animators
    private func presentAnimatorClosure() -> (() -> UIViewPropertyAnimator) {
        return { [unowned self] in
            let animator = UIViewPropertyAnimator(duration: Constants.AnimationDuration.bottomModalPresent, dampingRatio: 0.78)
            let dimmedViewAnimations = { [unowned self] in
                self.dimmedView.alpha = BottomAnchoredModalViewController.presentedAlpha
            }
            let modalAnimations = { [unowned self] in
                self.updateModalConstraints(isShowing: true)
                self.view.layoutIfNeeded()
            }

            animator.addAnimations(dimmedViewAnimations)
            animator.addAnimations(modalAnimations)

            return animator
        }
    }

    private func dismissAnimatorClosure() -> (() -> UIViewPropertyAnimator) {
        return { [unowned self] in
            let animator = UIViewPropertyAnimator(duration: Constants.AnimationDuration.bottomModalDismiss, curve: .easeInOut)
            let dimmedViewAnimations = { [unowned self] in
                self.dimmedView.alpha = 0
            }
            let modalAnimations = { [unowned self] in
                self.updateModalConstraints(isShowing: false)

                self.view.layoutIfNeeded()
            }

            animator.addAnimations(dimmedViewAnimations)
            animator.addAnimations(modalAnimations)

            return animator
        }
    }
}

BottomAnchoredModalViewController.xib

<?xml version="1.0" encoding="UTF-8"?>
<document type="com.apple.InterfaceBuilder3.CocoaTouch.XIB" version="3.0" toolsVersion="16097" targetRuntime="iOS.CocoaTouch" propertyAccessControl="none" useAutolayout="YES" useTraitCollections="YES" useSafeAreas="YES" colorMatched="YES">
    <device id="retina4_7" orientation="portrait" appearance="light"/>
    <dependencies>
        <deployment identifier="iOS"/>
        <plugIn identifier="com.apple.InterfaceBuilder.IBCocoaTouchPlugin" version="16087"/>
        <capability name="Safe area layout guides" minToolsVersion="9.0"/>
        <capability name="documents saved in the Xcode 8 format" minToolsVersion="8.0"/>
    </dependencies>
    <objects>
        <placeholder placeholderIdentifier="IBFilesOwner" id="-1" userLabel="File's Owner" customClass="BottomAnchoredModalViewController" customModule="BB_Links" customModuleProvider="target">
            <connections>
                <outlet property="closeButton" destination="Wze-Dd-bEZ" id="J6X-6X-Oca"/>
                <outlet property="closeHandle" destination="sej-t9-lQE" id="Xsu-Oj-y91"/>
                <outlet property="headerContainer" destination="a5B-dp-gse" id="Q0y-u1-s2Q"/>
                <outlet property="maskView" destination="DvA-6U-B1q" id="1vm-cr-4CY"/>
                <outlet property="childVCContainer" destination="mQ6-Qt-rN3" id="Qci-wn-afI"/>
                <outlet property="modalContainer" destination="btX-6M-PDH" id="VPn-Cf-rs1"/>
                <outlet property="titleLabel" destination="WPO-Gd-5Hb" id="xtY-qi-hB7"/>
                <outlet property="view" destination="iN0-l3-epB" id="nST-Cg-JJa"/>
            </connections>
        </placeholder>
        <placeholder placeholderIdentifier="IBFirstResponder" id="-2" customClass="UIResponder"/>
        <view contentMode="scaleToFill" translatesAutoresizingMaskIntoConstraints="NO" id="iN0-l3-epB">
            <rect key="frame" x="0.0" y="0.0" width="330" height="550"/>
            <subviews>
                <view contentMode="scaleToFill" translatesAutoresizingMaskIntoConstraints="NO" id="btX-6M-PDH" userLabel="Modal Container">
                    <rect key="frame" x="0.0" y="250" width="330" height="300"/>
                    <subviews>
                        <view contentMode="scaleToFill" translatesAutoresizingMaskIntoConstraints="NO" id="DvA-6U-B1q" userLabel="Modal Mask View">
                            <rect key="frame" x="0.0" y="0.0" width="330" height="350"/>
                            <subviews>
                                <view contentMode="scaleToFill" translatesAutoresizingMaskIntoConstraints="NO" id="mQ6-Qt-rN3" userLabel="Visible Content">
                                    <rect key="frame" x="0.0" y="52" width="330" height="248"/>
                                    <color key="backgroundColor" white="1" alpha="1" colorSpace="calibratedWhite"/>
                                </view>
                                <view contentMode="scaleToFill" translatesAutoresizingMaskIntoConstraints="NO" id="a5B-dp-gse" userLabel="Modal Header">
                                    <rect key="frame" x="0.0" y="0.0" width="330" height="52"/>
                                    <subviews>
                                        <label opaque="NO" userInteractionEnabled="NO" contentMode="left" horizontalHuggingPriority="251" verticalHuggingPriority="251" verticalCompressionResistancePriority="1000" text="Label" textAlignment="center" lineBreakMode="tailTruncation" baselineAdjustment="alignBaselines" minimumScaleFactor="0.5" translatesAutoresizingMaskIntoConstraints="NO" id="WPO-Gd-5Hb">
                                            <rect key="frame" x="30" y="17" width="270" height="27"/>
                                            <constraints>
                                                <constraint firstAttribute="height" constant="27" id="N56-4b-nuj"/>
                                            </constraints>
                                            <fontDescription key="fontDescription" type="system" weight="black" pointSize="22"/>
                                            <nil key="textColor"/>
                                            <nil key="highlightedColor"/>
                                        </label>
                                        <button opaque="NO" contentMode="scaleToFill" contentHorizontalAlignment="center" contentVerticalAlignment="center" buttonType="roundedRect" lineBreakMode="middleTruncation" translatesAutoresizingMaskIntoConstraints="NO" id="Wze-Dd-bEZ">
                                            <rect key="frame" x="302" y="3" width="25" height="25"/>
                                            <constraints>
                                                <constraint firstAttribute="height" constant="25" id="Gmn-Uc-bCc"/>
                                                <constraint firstAttribute="width" constant="25" id="Gq2-Wp-E2V"/>
                                            </constraints>
                                            <state key="normal" image="close-glyph"/>
                                            <connections>
                                                <action selector="closeButtonTapped" destination="-1" eventType="touchUpInside" id="ygu-RX-I9j"/>
                                            </connections>
                                        </button>
                                        <imageView userInteractionEnabled="NO" contentMode="center" horizontalHuggingPriority="251" verticalHuggingPriority="251" image="handle-glyph" translatesAutoresizingMaskIntoConstraints="NO" id="sej-t9-lQE">
                                            <rect key="frame" x="146.5" y="8" width="37" height="5"/>
                                            <constraints>
                                                <constraint firstAttribute="height" constant="5" id="2jf-fp-9Ih"/>
                                                <constraint firstAttribute="width" constant="37" id="gvS-2g-sX4"/>
                                            </constraints>
                                        </imageView>
                                    </subviews>
                                    <color key="backgroundColor" white="1" alpha="1" colorSpace="calibratedWhite"/>
                                    <constraints>
                                        <constraint firstAttribute="bottom" secondItem="WPO-Gd-5Hb" secondAttribute="bottom" constant="8" id="6DX-C3-OiV"/>
                                        <constraint firstAttribute="trailing" secondItem="WPO-Gd-5Hb" secondAttribute="trailing" constant="30" id="LIG-Vz-ZKX"/>
                                        <constraint firstItem="WPO-Gd-5Hb" firstAttribute="centerX" secondItem="a5B-dp-gse" secondAttribute="centerX" id="MkO-Bv-fZF"/>
                                        <constraint firstItem="WPO-Gd-5Hb" firstAttribute="top" secondItem="a5B-dp-gse" secondAttribute="top" constant="17" id="Mo8-NZ-ArP"/>
                                        <constraint firstItem="Wze-Dd-bEZ" firstAttribute="top" secondItem="a5B-dp-gse" secondAttribute="top" constant="3" id="RSx-xK-eto"/>
                                        <constraint firstItem="sej-t9-lQE" firstAttribute="centerX" secondItem="a5B-dp-gse" secondAttribute="centerX" id="Ryd-Wc-1Sa"/>
                                        <constraint firstAttribute="trailing" secondItem="Wze-Dd-bEZ" secondAttribute="trailing" constant="3" id="VmX-2i-D0f"/>
                                        <constraint firstItem="WPO-Gd-5Hb" firstAttribute="leading" secondItem="a5B-dp-gse" secondAttribute="leading" constant="30" id="aGE-jq-oZx"/>
                                        <constraint firstItem="sej-t9-lQE" firstAttribute="top" secondItem="a5B-dp-gse" secondAttribute="top" constant="8" id="frd-87-dQJ"/>
                                    </constraints>
                                </view>
                            </subviews>
                            <color key="backgroundColor" white="1" alpha="1" colorSpace="calibratedWhite"/>
                            <constraints>
                                <constraint firstItem="a5B-dp-gse" firstAttribute="top" secondItem="DvA-6U-B1q" secondAttribute="top" id="08G-zb-8EK"/>
                                <constraint firstAttribute="trailing" secondItem="a5B-dp-gse" secondAttribute="trailing" id="8A9-V8-3Yd"/>
                                <constraint firstAttribute="bottom" secondItem="mQ6-Qt-rN3" secondAttribute="bottom" constant="50" id="EXW-yv-XTa"/>
                                <constraint firstItem="a5B-dp-gse" firstAttribute="bottom" secondItem="mQ6-Qt-rN3" secondAttribute="top" id="IEQ-Lx-l0G"/>
                                <constraint firstItem="a5B-dp-gse" firstAttribute="leading" secondItem="DvA-6U-B1q" secondAttribute="leading" id="aT9-yg-0AA"/>
                                <constraint firstAttribute="trailing" secondItem="mQ6-Qt-rN3" secondAttribute="trailing" id="eki-mY-hgI"/>
                                <constraint firstItem="mQ6-Qt-rN3" firstAttribute="leading" secondItem="DvA-6U-B1q" secondAttribute="leading" id="mTI-bY-SlW"/>
                            </constraints>
                        </view>
                    </subviews>
                    <color key="backgroundColor" white="1" alpha="1" colorSpace="calibratedWhite"/>
                    <constraints>
                        <constraint firstItem="DvA-6U-B1q" firstAttribute="leading" secondItem="btX-6M-PDH" secondAttribute="leading" id="Gaq-Uf-i4e"/>
                        <constraint firstAttribute="trailing" secondItem="DvA-6U-B1q" secondAttribute="trailing" id="VKR-Z1-AMb"/>
                        <constraint firstAttribute="height" constant="300" placeholder="YES" id="aaY-8G-289"/>
                        <constraint firstAttribute="bottom" secondItem="DvA-6U-B1q" secondAttribute="bottom" constant="-50" id="h7c-yA-wFZ"/>
                        <constraint firstItem="DvA-6U-B1q" firstAttribute="top" secondItem="btX-6M-PDH" secondAttribute="top" id="rWV-Be-XaQ"/>
                    </constraints>
                </view>
            </subviews>
            <color key="backgroundColor" cocoaTouchSystemColor="groupTableViewBackgroundColor"/>
            <constraints>
                <constraint firstAttribute="trailing" secondItem="btX-6M-PDH" secondAttribute="trailing" placeholder="YES" id="CUV-9L-55H"/>
                <constraint firstItem="btX-6M-PDH" firstAttribute="leading" secondItem="iN0-l3-epB" secondAttribute="leading" placeholder="YES" id="Emd-Ct-ZZG"/>
                <constraint firstItem="btX-6M-PDH" firstAttribute="top" relation="greaterThanOrEqual" secondItem="iN0-l3-epB" secondAttribute="top" placeholder="YES" id="dKN-ff-Zk0"/>
                <constraint firstAttribute="bottom" secondItem="btX-6M-PDH" secondAttribute="bottom" placeholder="YES" id="oBZ-vd-sep"/>
            </constraints>
            <freeformSimulatedSizeMetrics key="simulatedDestinationMetrics"/>
            <viewLayoutGuide key="safeArea" id="Xke-q2-Jgy"/>
            <point key="canvasLocation" x="-8" y="-283.35832083958024"/>
        </view>
    </objects>
    <resources>
        <image name="close-glyph" width="15" height="15"/>
        <image name="handle-glyph" width="37" height="5"/>
    </resources>
</document>

CustomModalPresentationController.swift

//
//  CustomModalPresentationController.swift
//  BB Links
//
//  Created by Justin Stanley on 2017-09-07.
//  Copyright © 2017 Justin Stanley. All rights reserved.
//

import Foundation
import UIKit

internal final class CustomModalPresentationController: UIPresentationController {
    override internal var shouldPresentInFullscreen: Bool {
        false
    }

    override internal var frameOfPresentedViewInContainerView: CGRect {
        guard let containerView = containerView else { return CGRect() }
        // We don't want the presented view to fill the whole container view, so inset it's frame
        return (containerView.bounds).insetBy(dx: 50, dy: 50)
    }

    lazy var dimmingView: UIView = {
        let view = UIView(frame: self.containerView!.bounds)
        view.backgroundColor = UIColor.blue.withAlphaComponent(0.3)
        view.alpha = 0.0
        return view
    }()

    override internal func presentationTransitionWillBegin() {
        guard let containerView = containerView, let presentedView = presentedView else { return }

        // Add the dimming view and the presented view to the heirarchy
        dimmingView.frame = frameOfPresentedViewInContainerView
        containerView.addSubview(dimmingView)
        containerView.addSubview(presentedView)

        // Fade in the dimming view alongside the transition
        if let transitionCoordinator = presentingViewController.transitionCoordinator {
            transitionCoordinator.animate(alongsideTransition: { _ in
                self.dimmingView.alpha = 1
            }, completion: nil)
        }
    }

    override func presentationTransitionDidEnd(_ completed: Bool) {
        // If the presentation didn't complete, remove the dimming view
        if !completed {
            dimmingView.removeFromSuperview()
        }
    }

    override func dismissalTransitionWillBegin() {
        // Fade out the dimming view alongside the transition
        if let transitionCoordinator = presentingViewController.transitionCoordinator {
            transitionCoordinator.animate(alongsideTransition: { context in
                self.dimmingView.alpha = 1 - (0.5 * context.percentComplete)
            }, completion: nil)
        }
    }

    override func dismissalTransitionDidEnd(_ completed: Bool) {
        // If the dismissal completed, remove the dimming view
        if completed {
            dimmingView.removeFromSuperview()
        }
    }

    // ---- UIContentContainer protocol methods
    override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
        super.viewWillTransition(to: size, with: coordinator)

        guard let containerView = containerView else { return }

        coordinator.animate(alongsideTransition: { _ in
            self.dimmingView.frame = containerView.bounds
        }, completion: nil)
    }
}

CustomModalTransition.swift

//
//  CustomModalTransition.swift
//  BB Links
//
//  Created by Justin Stanley on 2017-09-09.
//  Copyright © 2017 Justin Stanley. All rights reserved.
//

import Foundation
import SnapKit
import UIKit

internal enum CustomModalTransitionContext {
    case presenting, dismissing
}

internal final class CustomModalTransition: UIPercentDrivenInteractiveTransition, UIViewControllerAnimatedTransitioning {
    internal var presentAnimator: (() -> UIViewPropertyAnimator)!
    internal var dismissAnimator: (() -> UIViewPropertyAnimator)!
    internal var onPanCancelled: (() -> Void)?
    internal var onDismissEnd: (() -> Void)?
    internal var context: CustomModalTransitionContext = .presenting

    /// Used when tapped button/view
    internal func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
        transitionAnimator(using: transitionContext).startAnimation()
    }

    /// Used when panning
    internal func interruptibleAnimator(using transitionContext: UIViewControllerContextTransitioning) -> UIViewImplicitlyAnimating {
        transitionAnimator(using: transitionContext)
    }

    internal func transitionDuration(using _: UIViewControllerContextTransitioning?) -> TimeInterval {
        switch context {
        case .presenting: return Constants.AnimationDuration.bottomModalPresent
        case .dismissing: return Constants.AnimationDuration.bottomModalDismiss
        }
    }

    private func transitionAnimator(using transitionContext: UIViewControllerContextTransitioning) -> UIViewImplicitlyAnimating {
        switch context {
        case .presenting:
            guard let toView = transitionContext.view(forKey: .to) else {
                fatalError("Failed to get View for the VC transitioning to.")
            }

            // Add the view of the VC being presented to the presentation container view and layout
            transitionContext.containerView.addSubview(toView)
            toView.snp.remakeConstraints { make in
                make.edges.equalToSuperview()
            }

            transitionContext.containerView.layoutIfNeeded()

            let animator = presentAnimator()
            animator.addCompletion { _ in
                transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
            }
            return animator
        case .dismissing:
            let animator = dismissAnimator()
            animator.addCompletion { [unowned self] position in
                switch position {
                case .end where transitionContext.transitionWasCancelled:
                    transitionContext.completeTransition(false)
                case .end:
                    self.onDismissEnd?()
                    transitionContext.completeTransition(true)
                case .start where transitionContext.transitionWasCancelled:
                    // Need to remake constraints since the values were already changed
                    self.onPanCancelled?()
                    transitionContext.completeTransition(false)
                case .current, .start:
                    transitionContext.completeTransition(false)
                @unknown
                default:
                    fatalError("Unknown UIViewAnimatingPosition: \(position)")
                }
            }
            return animator
        }
    }
}

UIView+Extensions.swift

//
//  UIView.swift
//  BBLinks
//
//  Created by Justin Stanley on 2016-07-02.
//  Copyright © 2016 Justin Stanley. All rights reserved.
//
import Foundation
import UIKit

// MARK: Styling
enum ShadowDirection {
    case above
    case below

    fileprivate func height(elevation: ShadowElevation) -> CGFloat {
        switch self {
        case .above: return -elevation.absoluteHeight
        case .below: return elevation.absoluteHeight
        }
    }
}

enum ShadowElevation {
    case modal
    case floating
    case chip
    case bar
    case low
    case none

    fileprivate var absoluteHeight: CGFloat {
        switch self {
        case .modal: return 10
        case .floating: return 2
        case .chip: return 1
        case .low: return 1
        case .bar: return 3
        case .none: return 0
        }
    }

    fileprivate var radius: CGFloat {
        switch self {
        case .modal: return 16
        case .floating: return 5
        case .chip: return 4
        case .low: return 2
        case .bar: return 2
        case .none: return 0
        }
    }

    private var lightThemeOpacity: Float {
        switch self {
        case .modal: return 0.5
        case .floating: return 0.2
        case .chip: return 0.2
        case .low: return 0.2
        case .bar: return 0.2
        case .none: return 0
        }
    }

    fileprivate var opacity: Float {
        let isLightTheme = ThemeManager.shared.theme.isLightTheme
        return isLightTheme ? lightThemeOpacity : 2 * lightThemeOpacity
    }
}

struct ShadowStyle {
    let direction: ShadowDirection
    let elevation: ShadowElevation

    init(direction: ShadowDirection = .below, elevation: ShadowElevation) {
        self.direction = direction
        self.elevation = elevation
    }

    fileprivate var offset: CGSize {
        CGSize(width: 0, height: direction.height(elevation: elevation))
    }

    fileprivate var radius: CGFloat {
        elevation.radius
    }

    fileprivate var opacity: Float {
        elevation.opacity
    }

    fileprivate var color: CGColor {
        UIColor.black.cgColor
    }

    static var Floating: ShadowStyle {
        ShadowStyle(elevation: .floating)
    }

    static var Chip: ShadowStyle {
        ShadowStyle(elevation: .chip)
    }

    static var Standard: ShadowStyle {
        ShadowStyle(elevation: .bar)
    }

    static var TabBar: ShadowStyle {
        ShadowStyle(direction: .above, elevation: .bar)
    }

    static var Modal: ShadowStyle {
        ShadowStyle(elevation: .modal)
    }

    static var Low: ShadowStyle {
        ShadowStyle(elevation: .low)
    }

    static var None: ShadowStyle {
        ShadowStyle(elevation: .none)
    }
}

public enum CornerRadius: Equatable {
    case small
    case smallMedium
    case medium
    case large
    case extraLarge
    case buttonSmall
    case buttonLarge
    case custom(value: CGFloat)

    public var value: CGFloat {
        switch self {
        case .small: return 5
        case .smallMedium: return 8
        case .medium: return 12
        case .large, .buttonSmall: return 15
        case .extraLarge: return 20
        case .buttonLarge: return 17
        case let .custom(value): return value
        }
    }
}

internal extension UIView {
    /// If the view has rounded corners, apply the shadow to a parent view with a nil background colour instead.
    func applyShadow(_ style: ShadowStyle) {
        layer.masksToBounds = false
        layer.shadowColor = style.color
        layer.shadowOpacity = style.opacity
        layer.shadowRadius = style.radius
        layer.shadowOffset = style.offset
        layer.shouldRasterize = true
        layer.rasterizationScale = UIScreen.main.scale
    }

    /// Add corners to a view without a shadow. If the view requires a shadow, apply the shadow to a parent view with a nil background colour.
    func addCorners(radius: CornerRadius = .large, useContinuousCurve: Bool = true) {
        layer.masksToBounds = true
        layer.cornerRadius = radius.value
        layer.cornerCurve = useContinuousCurve ? .continuous : .circular
    }

    var safeAreaBottomInset: CGFloat {
        safeAreaInsets.bottom
    }
}

// MARK: - Layout Extensions
public extension UIView {
    /**
     Sets the view's constraints equal to its superview. Padding is set to zero unless provided.
     You must add the view as a subview of its parent before calling
     */
    func addEdgeConstraints(_ view: UIView, padding: UIEdgeInsets? = nil) {
        view.snp.makeConstraints { make in
            if let padding = padding {
                make.edges.equalToSuperview().inset(padding)
            } else {
                make.edges.equalToSuperview()
            }
        }
    }

    /**
     Adds the view as a subview and sets the view's constraints equal to its superview. Optionally provide padding to inset by.
     */
    func addSubviewAndEdgeConstraints(_ view: UIView, padding: UIEdgeInsets? = nil) {
        addSubview(view)
        addEdgeConstraints(view, padding: padding)
    }

    /**
     Sets constraints that enable the child to be centered horizontally with the ability to grow vertically. Optionally provide padding to inset by.
     You must add the view as a subview of its parent before calling.
     */
    func addCenteringConstraints(_ view: UIView, padding: UIEdgeInsets? = nil) {
        view.snp.makeConstraints { make in
            if let padding = padding {
                make.top.bottom.equalToSuperview().inset(padding)
                make.leading.greaterThanOrEqualToSuperview().inset(padding)
                make.trailing.lessThanOrEqualToSuperview().inset(padding)
            } else {
                make.top.bottom.equalToSuperview()
                make.leading.greaterThanOrEqualToSuperview()
                make.trailing.lessThanOrEqualToSuperview()
            }

            make.centerX.equalToSuperview()
        }
    }

    /**
     Adds the view as a subview and sets constraints that enable the child to be centered horizontally with the ability to grow vertically. Optionally provide padding to inset by.
     */
    func addSubviewAndCenteringConstraints(_ view: UIView, padding: UIEdgeInsets? = nil) {
        addSubview(view)
        addCenteringConstraints(view, padding: padding)
    }
}

extension UIView {
    static func emptyFullWidthView(height: CGFloat = 15) -> UIView {
        UIView(frame: CGRect(x: 0,
                             y: 0,
                             width: min(UIScreen.main.bounds.width, UIScreen.main.bounds.height),
                             height: height))
    }
}

// MARK: Rounding Corners
public extension UIView {
    /// Note: Since UIRectCorner is a bitmask, you can provide a single value or array of values.
    func roundCorners(_ corners: UIRectCorner, cornerRadius: CGFloat) {
        layer.cornerRadius = cornerRadius
        layer.maskedCorners = corners.maskedCorners
    }
}

private extension UIRectCorner {
    var maskedCorners: CACornerMask {
        var cornerMask = CACornerMask()

        if contains(.allCorners) {
            cornerMask.insert([.layerMaxXMaxYCorner, .layerMaxXMinYCorner, .layerMinXMaxYCorner, .layerMinXMinYCorner])
            return cornerMask
        } else {
            if contains(.topLeft) {
                cornerMask.insert(.layerMinXMinYCorner)
            }
            if contains(.topRight) {
                cornerMask.insert(.layerMaxXMinYCorner)
            }
            if contains(.bottomLeft) {
                cornerMask.insert(.layerMinXMaxYCorner)
            }
            if contains(.bottomRight) {
                cornerMask.insert(.layerMaxXMaxYCorner)
            }
            return cornerMask
        }
    }
}

UsageExample.swift

import UIKit

class SomeVC: UIViewController {
    override func viewDidAppear() {
        super.viewDidAppear()
        
        // instantiate and setup the child view controller that conforms to the protocol
        // the tableView/scrollview probably should have these properties set:
        // tableView.insetsContentViewsToSafeArea = true
        // tableView.contentInsetAdjustmentBehavior = .scrollableAxes
        
        let childVC = SomeViewController()
        // the title will show in the top bar
        childVC.title = "Justin was here"
        
        // instantiate the bottom modal vc
        let bottomModalVC = BottomAnchoredModalViewController(childVC: childVC)
        
        // present the bottom modal vc
        present(bottomModalVC, animated: true)
    }
}

XibAssets.md

These are the assets used in the .xib file

• Close Button asset: https://www.dropbox.com/s/uu9cjo61ehpfx0e/close-glyph.pdf?dl=0
• Handle asset: https://www.dropbox.com/s/8gf7sty07m3zg17/handle-glyph.pdf?dl=0