KeyboardFrameObserver.swift

import UIKit

/// Object observing keyboard changes and passing a `KeyboardChangeInfo` to notify about changes.
final class KeyboardObserver: NSObject {

    typealias KeyboardChangeClosure = (KeyboardChangeInfo) -> Void
    let changeClosure: KeyboardChangeClosure

    // MARK: - Private

    private var showToken: NSObjectProtocol?
    private var hideToken: NSObjectProtocol?
    private let notificationCenter = NotificationCenter.default

    // MARK: - Lifecycle

    init(closure: @escaping KeyboardChangeClosure) {
        changeClosure = closure
        super.init()
        setupObservers()
    }

    deinit {
        if let showToken = showToken {
            notificationCenter.removeObserver(showToken)
        }
        if let hideToken = hideToken {
            notificationCenter.removeObserver(hideToken)
        }
    }

    // MARK: - Observation

    private func setupObservers() {
        // swiftlint:disable trailing_closure
        showToken = notificationCenter.addObserver(
            forName: UIApplication.keyboardWillShowNotification,
            object: nil,
            queue: .main,
            using: { [weak self] in self?.processNotification($0, .show) }
        )

        hideToken = notificationCenter.addObserver(
            forName: UIApplication.keyboardWillHideNotification,
            object: nil,
            queue: .main,
            using: { [weak self] in self?.processNotification($0, .hide) }
        )
        // swiftlint:enable trailing_closure
    }

    private func processNotification(_ note: Notification, _ type: KeyboardChangeInfo.ChangeType) {
        guard let info = KeyboardChangeInfo(note: note, type: type) else { return }
        changeClosure(info)
    }

}


/// Value holding keyboard change relevant animation.
struct KeyboardChangeInfo {

    enum ChangeType {
        case show, hide
    }

    let frameEnd: CGRect
    let frameBegin: CGRect
    let duration: TimeInterval
    let animationOptions: UIView.AnimationOptions
    let changeType: ChangeType

    // MARK: - Lifecycle

    init?(note: Notification, type: ChangeType) {
        let userInfo = note.userInfo
        guard let frameEnd = userInfo?[UIResponder.keyboardFrameEndUserInfoKey] as? CGRect,
            let frameBegin = userInfo?[UIResponder.keyboardFrameBeginUserInfoKey] as? CGRect,
            let duration = userInfo?[UIResponder.keyboardAnimationDurationUserInfoKey] as? Double,
            let curveValue = userInfo?[UIResponder.keyboardAnimationCurveUserInfoKey] as? UInt else { return nil }

        changeType = type
        self.frameEnd = frameEnd
        self.frameBegin = frameBegin
        self.duration = duration
        self.animationOptions = UIView.AnimationOptions(rawValue: curveValue)
    }

}

// MARK: - Helper

extension KeyboardChangeInfo {

    func animateAlongside(animations: @escaping () -> Void, completion: ((Bool) -> Void)? = nil) {
        UIView.animate(using: self, animations: animations, completion: completion)
    }

}

extension UIView {

    static func animate(
        using changeInfo: KeyboardChangeInfo,
        animations: @escaping () -> Void,
        completion: ((Bool) -> Void)? = nil
        ) {
        animate(
            withDuration: changeInfo.duration,
            delay: 0,
            options: changeInfo.animationOptions,
            animations: animations,
            completion: completion
        )
    }

}