threema-ios/Threema/KeyboardResizeCenterY.swift

//  _____ _
// |_   _| |_  _ _ ___ ___ _ __  __ _
//   | | | ' \| '_/ -_) -_) '  \/ _` |_
//   |_| |_||_|_| \___\___|_|_|_\__,_(_)
//
// Threema iOS Client
// Copyright (c) 2018-2020 Threema GmbH
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License, version 3,
// as published by the Free Software Foundation.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program. If not, see <https://www.gnu.org/licenses/>.

import Foundation

class KeyboardResizeCenterY {
    let parentView: UIView
    let resizeView: UIView
    var defaultCenterY: NSLayoutConstraint
    var offsetCenterY: NSLayoutConstraint?
    
    init(parent: UIView, resize: UIView) {
        self.parentView = parent
        self.resizeView = resize
        
        self.defaultCenterY = NSLayoutConstraint(item: self.resizeView, attribute: NSLayoutConstraint.Attribute.centerY, relatedBy: NSLayoutConstraint.Relation.equal, toItem: self.parentView, attribute: NSLayoutConstraint.Attribute.centerY, multiplier: 1.0, constant: 0)
        
        self.parentView.addConstraint(self.defaultCenterY)
        
        NotificationCenter.default.addObserver(self, selector: #selector(keyboardDidShow), name: UIResponder.keyboardDidShowNotification, object: nil)
        NotificationCenter.default.addObserver(self, selector: #selector(keyboardWillHide), name: UIResponder.keyboardWillHideNotification, object: nil)
    }
    
    deinit {
        NotificationCenter.default.removeObserver(self)
    }
    
    @objc func keyboardDidShow(notification: NSNotification) {
        if let keyboardSize = (notification.userInfo?[UIResponder.keyboardFrameEndUserInfoKey] as? NSValue)?.cgRectValue {
            let offset: CGFloat = UIScreen.main.bounds.height - self.resizeView.frame.maxY - keyboardSize.height - 10
            if offset < 0 {
                self.offsetCenterY = NSLayoutConstraint(item: self.resizeView, attribute: NSLayoutConstraint.Attribute.centerY, relatedBy: NSLayoutConstraint.Relation.equal, toItem: self.parentView, attribute: NSLayoutConstraint.Attribute.centerY, multiplier: 1.0, constant: offset)
                
                self.parentView.removeConstraint(self.defaultCenterY)
                self.parentView.addConstraint(self.offsetCenterY!)
            }
        }
    }
    
    @objc func keyboardWillHide(notification: NSNotification) {
        if let offsetCenterY = self.offsetCenterY {
            self.parentView.removeConstraint(offsetCenterY)
        }
        self.parentView.addConstraint(self.defaultCenterY)
    }
    
}