Attractor/App/Common UI/SegmentedReliefButton.swift

//
//  SegmentedReliefButton.swift
//  App
//
//  Created by James Magahern on 8/14/20.
//

import UIKit

class SegmentedReliefButton: ReliefButton
{
    var children: [ReliefButton] = [] {
        willSet { children.forEach { $0.removeFromSuperview() } }
        didSet  { children.forEach { addSubview($0) }; setNeedsLayout() }
    }
    
    override var isPointerInteractionEnabled: Bool {
        set { children.forEach { $0.isPointerInteractionEnabled = newValue } }
        get { children.first?.isPointerInteractionEnabled ?? false }
    }
    
    private let backgroundsContainerView = UIView(frame: .zero)
    private var childrenHighlighObservations: [NSKeyValueObservation] = []
    
    init(children: [ReliefButton]) {
        super.init()
        self.children = children
        
        backgroundsContainerView.clipsToBounds = true
        backgroundsContainerView.layer.cornerRadius = Self.cornerRadius - Self.borderWidth
        
        constrainedToSquare = false
        
        addSubview(backgroundsContainerView)
    }
    
    override var isHighlighted: Bool {
        // Does not highlight. Only children do. 
        set {} get { false }
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func sizeThatFits(_ size: CGSize) -> CGSize {
        let ourSize = super.sizeThatFits(size)
        let width: CGFloat = children.reduce(0.0) { (result, button) -> CGFloat in
            return result + button.sizeThatFits(size).width
        }
        
        return CGSize(width: width, height: ourSize.height)
    }
    
    override func setBackgroundInverted(_ inverted: Bool) {
        // no-op
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
        
        backgroundView.colors = [ .clear ]
        
        backgroundsContainerView.frame = backgroundView.frame
        
        childrenHighlighObservations.removeAll()
        backgroundsContainerView.subviews.forEach { $0.removeFromSuperview() }
        
        let darkMode = self.traitCollection.userInterfaceStyle == .dark
        var buttonRect = CGRect(origin: .zero, size: CGSize(width: 0, height: bounds.height))
        for (i, child) in children.enumerated() {
            child.shadowView.isHidden = true
            child.backgroundView.isHidden = true
            bringSubviewToFront(child)
            
            let childSize = child.sizeThatFits(bounds.size)
            buttonRect.size = CGSize(width: childSize.width, height: bounds.height)
            child.frame = buttonRect
            
            buttonRect.origin.x += buttonRect.width
            
            // Background
            let backgroundView = GradientView(direction: .vertical, colors: Self.gradientColors(inverted: false, darkMode: darkMode))
            backgroundView.frame = child.frame
            backgroundsContainerView.insertSubview(backgroundView, at: 0)
            childrenHighlighObservations.append(child.observe(\.isHighlighted) { [backgroundView] (button, changeEvent) in
                backgroundView.colors = Self.gradientColors(inverted: button.isHighlighted, darkMode: darkMode)
            })
            
            // Separator
            if i < children.count - 1 {
                let separatorView = UIView(frame: CGRect(
                    x: child.frame.maxX,
                    y: 0,
                    width: 1.0,
                    height: bounds.height
                ))
                separatorView.backgroundColor = .systemFill
                backgroundsContainerView.addSubview(separatorView)
            }
        }
    }
}