mvm_core_ui/MVMCoreUI/BaseControllers/ThreeLayerViewController.swift

//
//  MVMCoreUIThreeLayerViewController.swift
//  MVMCoreUI
//
//  Created by Scott Pfeil on 2/13/19.
//  Copyright © 2019 Verizon Wireless. All rights reserved.
//
//  This class has three views, a top, middle, and bottom. The top view is just the first view in the scrollview. The middle can be aligned center, top, or bottom depending on if spaceBetweenTopAndMiddle() or spaceBetweenMiddleAndBottom() return values. The bottom view, if inside the scrollview, appears pinned to the bottom unless content pushes it off the screen. If outside the scroll, it's pinned to the bottom under the scrollview.

import UIKit
import MVMAnimationFramework

open class ThreeLayerViewController: ProgrammaticScrollViewController {
    
    // The three main views
    var topView: UIView?
    var middleView: UIView?
    var bottomView: UIView?
    var useMargins: Bool = false
    
    // The bottom view can be put outside of the scrolling area.
    var bottomViewOutsideOfScroll = false
    
    private var safeAreaView: UIView?
    var heightConstraint: NSLayoutConstraint?
 
    open override func updateViews() {
        super.updateViews()
        let width = view.bounds.width
        MFStyler.setDefaultMarginsFor(contentView, size: width)
        if let topView = topView as? MVMCoreViewProtocol {
            topView.updateView(width)
        }
        if let middleView = middleView as? MVMCoreViewProtocol {
            middleView.updateView(width)
        }
        if let bottomView = bottomView as? MVMCoreViewProtocol {
            bottomView.updateView(width)
        }
    }
    
    open override func updateViewConstraints() {
        super.updateViewConstraints()
        guard let scrollView = scrollView else {
            return
        }
        
        if scrollView.contentInsetAdjustmentBehavior == UIScrollView.ContentInsetAdjustmentBehavior.automatic {
            heightConstraint?.constant = -scrollView.adjustedContentInset.top - scrollView.adjustedContentInset.bottom
        } else {
            heightConstraint?.constant = -scrollView.contentInset.top - scrollView.contentInset.bottom
        }
    }
    
    open override func loadView() {
        super.loadView()
        // The height is used to keep the bottom view at the bottom.
        if let contentView = contentView, let scrollView = scrollView {
            heightConstraint = contentView.heightAnchor.constraint(equalTo: scrollView.heightAnchor, multiplier: 1.0)
            heightConstraint?.priority = UILayoutPriority.defaultLow
        }
    }
    
    open override func handleNewData() {
        // Removes the views
        topView?.removeFromSuperview()
        middleView?.removeFromSuperview()
        bottomView?.removeFromSuperview()
        safeAreaView?.removeFromSuperview()
        if let subViews = contentView?.subviews {
            for view in subViews {
                view.removeFromSuperview()
            }
        }
        
        // Reset constraints
        bottomConstraint?.isActive = true
        heightConstraint?.isActive = false
        
        setupLayers()
        
        super.handleNewData()
    }
    
    //MARK:-Functions to subclass
    // Subclass for a top view.
    open func viewForTop() -> UIView? {
        return nil
    }
    
    // Subclass for a middle view.
    open func viewForMiddle() -> UIView? {
        return nil
    }
    
    // Subclass for a bottom view.
    open func viewForBottom() -> UIView? {
        return nil
    }
    
    // If a value is set, the middle view is pinned this value below the top view, if not, space is left to fill.
    open func spaceBetweenTopAndMiddle() -> CGFloat? {
        return nil
    }
    
    // If a value is set, the middle view is pinned this value above the bottom view, if not, space is left to fill.
    open func spaceBetweenMiddleAndBottom() -> CGFloat? {
        return nil
    }
}

//MARK:-Setup
extension ThreeLayerViewController {
    func setupViewAsTop() -> UIView? {
        if let topView = viewForTop() {
            self.topView = topView
        } else {
            topView = MVMCoreUICommonViewsUtility.commonView()
            topView?.heightAnchor.constraint(equalToConstant: 0).isActive = true
        }
        guard let topView = topView, let contentView = contentView else {
            return nil
        }
        contentView.addSubview(topView)
        NSLayoutConstraint.pinViewTop(toSuperview: topView, useMargins: useMargins, constant: 0).isActive = true
        NSLayoutConstraint.pinViewLeft(toSuperview: topView, useMargins: useMargins, constant: 0).isActive = true
        NSLayoutConstraint.pinViewRight(toSuperview: topView, useMargins: useMargins, constant: 0).isActive = true
        return topView
    }
    
    func setupViewAsMiddle() -> UIView? {
        if let middleView = viewForMiddle() {
            self.middleView = middleView
        } else {
            middleView = MVMCoreUICommonViewsUtility.commonView()
            middleView?.heightAnchor.constraint(equalToConstant: 0).isActive = true
        }
        guard let middleView = middleView, let contentView = contentView else {
            return nil
        }
        contentView.addSubview(middleView)
        NSLayoutConstraint.pinViewLeft(toSuperview: middleView, useMargins: useMargins, constant: 0).isActive = true
        NSLayoutConstraint.pinViewRight(toSuperview: middleView, useMargins: useMargins, constant: 0).isActive = true
        middleView.setContentHuggingPriority(UILayoutPriority.required, for: NSLayoutConstraint.Axis.vertical)
        return middleView
    }
    
    func setupViewAsBottom() -> UIView? {
        if let bottomView = viewForBottom() {
            self.bottomView = bottomView
        } else {
            bottomView = MVMCoreUICommonViewsUtility.commonView()
            bottomView?.heightAnchor.constraint(equalToConstant: 0).isActive = true
        }
        guard let bottomView = bottomView else {
            return nil
        }
        
        // Adds the bottom view outside the scroll if directed.
        if bottomViewOutsideOfScroll {
            bottomConstraint?.isActive = false;
            addViewInsideOfScrollViewBottom(ViewConstrainingView.empty())
            addViewOutsideOfScrollViewBottom(bottomView)
        } else {
            bottomConstraint?.isActive = true;
            addViewInsideOfScrollViewBottom(bottomView)
        }
        return bottomView
    }
    
    func setupLayers() {
        guard let contentView = contentView, let topView = setupViewAsTop(), let middleView = setupViewAsMiddle(), let bottomView = setupViewAsBottom() else {
            return
        }
        let spaceAbove = spaceBetweenTopAndMiddle()
        let spaceBelow = spaceBetweenMiddleAndBottom()
        if let spaceAbove = spaceAbove, let spaceBelow = spaceBelow {
            // Both top and bottom space set, buttons not pinned to bottom.
            middleView.topAnchor.constraint(equalTo: topView.bottomAnchor, constant: spaceAbove).isActive = true
            bottomView.topAnchor.constraint(equalTo: middleView.bottomAnchor, constant: spaceBelow).isActive = true
        } else {
            heightConstraint?.isActive = true
            
            if let spaceAbove = spaceAbove {
                // Space above is set, space below is free.
                let bottomSpacer = MVMCoreUICommonViewsUtility.commonView()
                contentView.addSubview(bottomSpacer)
                bottomSpacer.leftAnchor.constraint(equalTo: contentView.leftAnchor).isActive = true
                contentView.rightAnchor.constraint(equalTo: bottomSpacer.rightAnchor).isActive = true
                bottomSpacer.heightAnchor.constraint(greaterThanOrEqualToConstant: 0).isActive = true
                middleView.topAnchor.constraint(equalTo: topView.bottomAnchor, constant: spaceAbove).isActive = true
                bottomSpacer.topAnchor.constraint(equalTo: middleView.bottomAnchor).isActive = true
                bottomView.topAnchor.constraint(equalTo: bottomSpacer.bottomAnchor).isActive = true
            } else if let spaceBelow = spaceBelow {
                // Space below is set, space above is free.
                let topSpacer = MVMCoreUICommonViewsUtility.commonView()
                contentView.addSubview(topSpacer)
                topSpacer.leftAnchor.constraint(equalTo: contentView.leftAnchor).isActive = true
                contentView.rightAnchor.constraint(equalTo: topSpacer.rightAnchor).isActive = true
                topSpacer.heightAnchor.constraint(greaterThanOrEqualToConstant: PaddingTwo).isActive = true
                topSpacer.topAnchor.constraint(equalTo: topView.bottomAnchor).isActive = true
                middleView.topAnchor.constraint(equalTo: topSpacer.bottomAnchor).isActive = true
                bottomView.topAnchor.constraint(equalTo: middleView.bottomAnchor, constant: spaceBelow).isActive = true
            } else {
                // No set space above or below, make the spacers the same height with a default minimum.
                let topSpacer = MVMCoreUICommonViewsUtility.commonView()
                let bottomSpacer = MVMCoreUICommonViewsUtility.commonView()
                contentView.addSubview(topSpacer)
                contentView.addSubview(bottomSpacer)
                topSpacer.leftAnchor.constraint(equalTo: contentView.leftAnchor).isActive = true
                contentView.rightAnchor.constraint(equalTo: topSpacer.rightAnchor).isActive = true
                bottomSpacer.leftAnchor.constraint(equalTo: contentView.leftAnchor).isActive = true
                contentView.rightAnchor.constraint(equalTo: bottomSpacer.rightAnchor).isActive = true
                topSpacer.heightAnchor.constraint(equalTo: bottomSpacer.heightAnchor).isActive = true
                topSpacer.heightAnchor.constraint(greaterThanOrEqualToConstant: PaddingTwo).isActive = true
                topSpacer.topAnchor.constraint(equalTo: topView.bottomAnchor).isActive = true
                middleView.topAnchor.constraint(equalTo: topSpacer.bottomAnchor).isActive = true
                bottomSpacer.topAnchor.constraint(equalTo: middleView.bottomAnchor).isActive = true
                bottomView.topAnchor.constraint(equalTo: bottomSpacer.bottomAnchor).isActive = true
            }
        }
    }
    
    func addViewInsideOfScrollViewBottom(_ view: UIView) {
        guard let contentView = contentView else {
            return
        }
        contentView.addSubview(view);
        NSLayoutConstraint.pinViewLeft(toSuperview: view, useMargins: useMargins, constant: 0).isActive = true
        NSLayoutConstraint.pinViewRight(toSuperview: view, useMargins: useMargins, constant: 0).isActive = true
        NSLayoutConstraint.pinViewBottom(toSuperview: view, useMargins: useMargins, constant: 0).isActive = true
    }
    
    func addViewOutsideOfScrollViewBottom(_ view: UIView) {
        self.view?.addSubview(view)
        if let scrollView = scrollView, let parentView = self.view {
            view.topAnchor.constraint(equalTo: scrollView.bottomAnchor).isActive = true
            NSLayoutConstraint.pinViewLeft(toSuperview: view, useMargins: useMargins, constant: 0).isActive = true
            NSLayoutConstraint.pinViewRight(toSuperview: view, useMargins: useMargins, constant: 0).isActive = true
            parentView.safeAreaLayoutGuide.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
            if let safeAreaView = MVMCoreUICommonViewsUtility.getAndSetupSafeAreaView(on: parentView) {
                safeAreaView.backgroundColor = bottomView?.backgroundColor
                self.safeAreaView = safeAreaView
            }
        }
    }
}