FloatingPanel/Framework/Sources/FloatingPanelController.swift

//
//  Created by Shin Yamamoto on 2018/09/18.
//  Copyright © 2018 Shin Yamamoto. All rights reserved.
//

import UIKit

public protocol FloatingPanelControllerDelegate: class {
    // if it returns nil, FloatingPanelController uses the default layout
    func floatingPanel(_ vc: FloatingPanelController, layoutFor newCollection: UITraitCollection) -> FloatingPanelLayout?

    // if it returns nil, FloatingPanelController uses the default behavior
    func floatingPanel(_ vc: FloatingPanelController, behaviorFor newCollection: UITraitCollection) -> FloatingPanelBehavior?

    func floatingPanelDidMove(_ vc: FloatingPanelController) // any offset changes

    // called on start of dragging (may require some time and or distance to move)
    func floatingPanelWillBeginDragging(_ vc: FloatingPanelController)
    // called on finger up if the user dragged. velocity is in points/second.
    func floatingPanelDidEndDragging(_ vc: FloatingPanelController, withVelocity velocity: CGPoint, targetPosition: FloatingPanelPosition)
    func floatingPanelWillBeginDecelerating(_ vc: FloatingPanelController) // called on finger up as we are moving
    func floatingPanelDidEndDecelerating(_ vc: FloatingPanelController) // called when scroll view grinds to a halt
}

public extension FloatingPanelControllerDelegate {
    func floatingPanel(_ vc: FloatingPanelController, layoutFor newCollection: UITraitCollection) -> FloatingPanelLayout? {
        return nil
    }
    func floatingPanel(_ vc: FloatingPanelController, behaviorFor newCollection: UITraitCollection) -> FloatingPanelBehavior? {
        return nil
    }
    func floatingPanelDidMove(_ vc: FloatingPanelController) {}
    func floatingPanelWillBeginDragging(_ vc: FloatingPanelController) {}
    func floatingPanelDidEndDragging(_ vc: FloatingPanelController, withVelocity velocity: CGPoint, targetPosition: FloatingPanelPosition) {}
    func floatingPanelWillBeginDecelerating(_ vc: FloatingPanelController) {}
    func floatingPanelDidEndDecelerating(_ vc: FloatingPanelController) {}
}

public enum FloatingPanelPosition: Int {
    case full
    case half
    case tip
}

///
/// A container view controller to display a floating panel to present contents in parallel as a user wants.
///
public class FloatingPanelController: UIViewController, UIScrollViewDelegate, UIGestureRecognizerDelegate {

    /// Constants indicating how safe area insets are added to the adjusted content inset.
    public enum ContentInsetAdjustmentBehavior: Int {
        case always
        case never
    }

    /// The delegate of the floating panel controller object.
    public weak var delegate: FloatingPanelControllerDelegate?

    /// Returns the surface view managed by the controller object. It's the same as `self.view`.
    public var surfaceView: FloatingPanelSurfaceView! {
        return view as? FloatingPanelSurfaceView
    }

    /// Returns the backdrop view managed by the controller object.
    public var backdropView: FloatingPanelBackdropView! {
        return floatingPanel.backdropView
    }

    /// Returns the scroll view that the conroller tracks.
    public weak var scrollView: UIScrollView? {
        return floatingPanel.scrollView
    }

    /// The current position of the floating panel controller's contents.
    public var position: FloatingPanelPosition {
        return floatingPanel.state
    }

    /// The content insets of the tracking scroll view derived the safe area of the parent view
    public var adjustedContentInsets: UIEdgeInsets {
        return floatingPanel.layoutAdapter.adjustedContentInsets
    }

    /// The behavior for determining the adjusted content offsets.
    ///
    /// This property specifies how the content area of the tracking scroll view are modified using `adjustedContentInsets`. The default value of this property is FloatingPanelController.ContentInsetAdjustmentBehavior.always.
    public var contentInsetAdjustmentBehavior: ContentInsetAdjustmentBehavior = .always

    private var floatingPanel: FloatingPanel!

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }

    /// Initialize a newly created a floating panel controller.
    public init() {
        super.init(nibName: nil, bundle: nil)
    }

    /// Creates the view that the controller manages.
    override public func loadView() {
        assert(self.storyboard == nil, "Storyboard isn't supported")

        let view = FloatingPanelSurfaceView()
        view.backgroundColor = .white

        self.view = view as UIView

        let layout = fetchLayout(for: self.traitCollection)
        let behavior = fetchBehavior(for: self.traitCollection)
        floatingPanel = FloatingPanel(self,
                                      layout: layout,
                                      behavior: behavior)
    }

    public override func willTransition(to newCollection: UITraitCollection, with coordinator: UIViewControllerTransitionCoordinator) {
        super.willTransition(to: newCollection, with: coordinator)

        // Change layout for a new trait collection
        floatingPanel.layoutAdapter.layout = fetchLayout(for: newCollection)
        floatingPanel.behavior = fetchBehavior(for: newCollection)

        guard let parent = parent else { fatalError() }

        floatingPanel.layoutAdapter.prepareLayout(toParent: parent)
        floatingPanel.layoutAdapter.activateLayout(of: floatingPanel.state)
    }

    public override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
        super.traitCollectionDidChange(previousTraitCollection)
        guard previousTraitCollection != traitCollection else { return }

        if let parent = parent {
            self.update(safeAreaInsets: parent.layoutInsets)
        }
        floatingPanel.backdropView.isHidden = (traitCollection.verticalSizeClass == .compact)
    }

    public override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)

        // I needs to update safeAreaInsets here to ensure that the `adjustedContentInsets` has a correct value.
        // Because the parent VC does not call viewSafeAreaInsetsDidChange() expectedly and
        // `view.safeAreaInsets` has a correct value of the bottom inset here.
        if let parent = parent {
            self.update(safeAreaInsets: parent.layoutInsets)
        }
    }

    private func fetchLayout(for traitCollection: UITraitCollection) -> FloatingPanelLayout {
        switch traitCollection.verticalSizeClass {
        case .compact:
            return self.delegate?.floatingPanel(self, layoutFor: traitCollection) ?? FloatingPanelDefaultLandscapeLayout()
        default:
            return self.delegate?.floatingPanel(self, layoutFor: traitCollection) ?? FloatingPanelDefaultLayout()
        }
    }

    private func fetchBehavior(for traitCollection: UITraitCollection) -> FloatingPanelBehavior {
        return self.delegate?.floatingPanel(self, behaviorFor: traitCollection) ?? FloatingPanelDefaultBehavior()
    }

    private func update(safeAreaInsets: UIEdgeInsets) {
        floatingPanel.safeAreaInsets = safeAreaInsets
        switch contentInsetAdjustmentBehavior {
        case .always:
            scrollView?.contentInset = adjustedContentInsets
            scrollView?.scrollIndicatorInsets = adjustedContentInsets
        default:
            break
        }
    }

    // MARK: - Container view controller interface

    /// Adds the view managed the controller as a child of the specified view controller.
    /// - Parameters:
    ///     - parent: A parent view controller object that displays FloatingPanelController's view. A conatiner view controller object isn't applicable.
    ///     - belowView: Insert the surface view managed by the controller below the specified view. As default, the surface view will be added to the end of the parent list of subviews.
    ///     - animated: Pass true to animate the presentation; otherwise, pass false.
    public func addPanel(toParent parent: UIViewController, belowView: UIView? = nil, animated: Bool = false) {
        guard self.parent == nil else {
            log.warning("Already added to a parent(\(parent))")
            return
        }
        precondition((parent is UINavigationController) == false, "UINavigationController displays only one child view controller at a time.")
        precondition((parent is UITableViewController) == false, "UITableViewController should not be the parent because the view hierarchy will be break in reusing cells.")
        precondition((parent is UICollectionViewController) == false, "UICollectionViewController should not be the parent because the view hierarchy will be break in reusing cells.")

        view.frame = parent.view.bounds
        if let belowView = belowView {
            parent.view.insertSubview(self.view, belowSubview: belowView)
        } else {
            parent.view.addSubview(self.view)
        }

        parent.addChild(self)

        // Must set a layout again here because `self.traitCollection` is applied correctly on it's added to a parent VC
        floatingPanel.layoutAdapter.layout = fetchLayout(for: traitCollection)
        floatingPanel.layoutViews(in: parent)
        floatingPanel.present(animated: animated) { [weak self] in
            guard let self = self else { return }
            self.didMove(toParent: parent)
        }
    }

    /// Removes the controller and the managed view from its parent view controller
    /// - Parameters:
    ///     - animated: Pass true to animate the presentation; otherwise, pass false.
    ///     - completion: The block to execute after the view controller is dismissed. This block has no return value and takes no parameters. You may specify nil for this parameter.
    public func removePanelFromParent(animated: Bool, completion: (() -> Void)? = nil) {
        guard self.parent != nil else {
            completion?()
            return
        }

        floatingPanel.dismiss(animated: animated) { [weak self] in
            guard let self = self else { return }

            self.willMove(toParent: nil)
            self.view.removeFromSuperview()
            self.removeFromParent()
            completion?()
        }
    }

    /// Moves the position to the specified position.
    /// - Parameters:
    ///     - to: Pass a FloatingPanelPosition value to move the surface view to the position.
    ///     - animated: Pass true to animate the presentation; otherwise, pass false.
    ///     - completion: The block to execute after the view controller is dismissed. This block has no return value and takes no parameters. You may specify nil for this parameter.
    public func move(to: FloatingPanelPosition, animated: Bool, completion: (() -> Void)? = nil) {
        floatingPanel.move(to: to, animated: animated, completion: completion)
    }

    /// Presents the specified view controller as the content view controller in the surface view interface.
    public override func show(_ vc: UIViewController, sender: Any?) {
        let surfaceView = self.view as! FloatingPanelSurfaceView
        surfaceView.contentView.addSubview(vc.view)
        vc.view.frame = surfaceView.contentView.bounds
        vc.view.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            vc.view.topAnchor.constraint(equalTo: surfaceView.contentView.topAnchor, constant: 0.0),
            vc.view.leftAnchor.constraint(equalTo: surfaceView.contentView.leftAnchor, constant: 0.0),
            vc.view.rightAnchor.constraint(equalTo: surfaceView.contentView.rightAnchor, constant: 0.0),
            vc.view.bottomAnchor.constraint(equalTo: surfaceView.contentView.bottomAnchor, constant: 0.0),
            ])
        addChild(vc)
        vc.didMove(toParent: self)
    }

    // MARK: - Scroll view tracking

    /// Tracks the specified scroll view to correspond with the scroll.
    ///
    /// - Attention:
    ///     The specified scroll view must be already assigned the delegate property because the controller intemediates the several delegate methods.
    ///
    public func track(scrollView: UIScrollView) {
        floatingPanel.scrollView = scrollView
        floatingPanel.userScrollViewDelegate = scrollView.delegate
        scrollView.delegate = floatingPanel
        switch contentInsetAdjustmentBehavior {
        case .always:
            if #available(iOS 11.0, *) {
                scrollView.contentInsetAdjustmentBehavior = .never
            } else {
                children.forEach { (vc) in
                    vc.automaticallyAdjustsScrollViewInsets = false
                }
            }
        default:
            break
        }
    }

    // MARK: - Helpers

    /// Returns the y-coordinate of the point at the origin of the surface view
    public func originYOfSurface(for pos: FloatingPanelPosition) -> CGFloat {
        switch pos {
        case .full:
            return floatingPanel.layoutAdapter.topY
        case .half:
            return floatingPanel.layoutAdapter.middleY
        case .tip:
            return floatingPanel.layoutAdapter.bottomY
        }
    }
}