A simple workspace builder using simple UIStackView's and a clean DSL for describing the layout.

DemoViewController.swift

//
//  ViewController.swift
//  Workspace
//
//  Created by Shaps Benkau on 12/27/2016.
//  Copyright (c) 2016 Snippex Ltd. All rights reserved.
//

import UIKit

class DemoViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let content = PaneViewController(title: "Content")
        content.view.backgroundColor = UIColor(white: 0.106, alpha: 1)
        
        let preview = PaneViewController(title: "Preview")
        preview.view.backgroundColor = UIColor(white: 0.086, alpha: 1)
        
        let timeline = PaneViewController(title: "Timeline")
        timeline.view.backgroundColor = UIColor(white: 0.106, alpha: 1)
        
        let toolbar = PaneViewController(title: "Toolbar") // 0.122
        toolbar.view.backgroundColor = UIColor(white: 0.2, alpha: 1)
        
        let workspace = WorkspaceController(nibName: nil, bundle: nil)
        
        addChildViewController(workspace)
        view.addSubview(workspace.view)
        workspace.didMove(toParentViewController:self)
        workspace.view.autoresizingMask = [.flexibleHeight, .flexibleWidth]
        
        /* This is where we define the workspace
         
           ----- -------------
           |   | |           |
           | C | |  Preview  |
           | o | |------------
           | n | |  Toolbar  |
           | t | |------------
           | e | |           |
           | n | |  Timeline |
           | t | |           |
           |   | |           |
           ----- -------------
         
         */
        
        let rightPane = WorkspacePane(axis: .vertical)
            .add(child: .controller(preview), size: .fixed(length: 320))
            .add(child: .controller(toolbar), size: .fixed(length: 44))
            .add(child: .controller(timeline), size: .fixed(length: 100), priority: UILayoutPriorityDefaultLow)
        
        let mainPane = WorkspacePane(axis: .horizontal)
            .add(child: .controller(content), size: .fixed(length: 320))
            .add(child: .pane(rightPane), size: .flexible)
        
        workspace.invalidate(with: mainPane)
        
        
        // To demonstrate animation -- we can hide the left hand view initially
        content.view.isHidden = true
        
        DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
            // Now we can un-hide the view and watch everything animate nicely ;)
            UIView.animate(withDuration: 1, delay: 0, usingSpringWithDamping: 0.6, initialSpringVelocity: 1, options: .beginFromCurrentState, animations: {
                content.view.isHidden = false
            }, completion: nil)
        }
    }
}


final class PaneViewController: UIViewController {
    init(title: String) {
        super.init(nibName: nil, bundle: nil)
        self.title = title
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let label = UILabel()
        label.text = title
        label.textColor = .white
        label.textAlignment = .center
        label.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(label)
        
        NSLayoutConstraint.activate([
            label.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            label.centerYAnchor.constraint(equalTo: view.centerYAnchor),
        ])
    }
}

WorkspaceController.swift

//
//  WorkspaceController.swift
//  Workspace
//
//  Created by Shaps Benkau on 27/12/2016.
//  Copyright © 2016 Snippex Ltd. All rights reserved.
//

import UIKit

public class WorkspaceController: UIViewController {
    
    var viewControllers: [UIViewController] = []
    
    public func invalidate(with pane: WorkspacePane) {
        for controller in viewControllers {
            controller.willMove(toParentViewController: nil)
            controller.view.removeFromSuperview()
            controller.removeFromParentViewController()
        }
        
        viewControllers.removeAll()
        view.subviews.forEach {
            $0.removeFromSuperview()
        }
        
        pane.view.frame = view.bounds
        pane.view.autoresizingMask = [.flexibleWidth, .flexibleHeight]
        view.backgroundColor = .black
        view.addSubview(pane.view)
        
        pane.invalidate(in: self)
    }
    
}

WorkspacePane.swift

//
//  WorkspacePane.swift
//  Workspace
//
//  Created by Shaps Benkau on 01/01/2017.
//  Copyright © 2017 Snippex Ltd. All rights reserved.
//

import UIKit

public final class WorkspacePane {
    
    public enum Axis {
        case horizontal
        case vertical
    }
    
    public enum SizeMode {
        case flexible
        case fixed(length: CGFloat)
    }
    
    public enum Child {
        case controller(UIViewController)
        case pane(WorkspacePane)
    }
    
    public var view: UIView {
        return stackView
    }
    
    // MARK: Public
    
    public init(axis: Axis) {
        let stackView = UIStackView()
        
        stackView.axis = axis == .horizontal ? .horizontal : .vertical
        stackView.alignment = .fill
        stackView.distribution = .fill
        stackView.spacing = 2
        
        self.stackView = stackView
        self.axis = axis
    }
    
    @discardableResult public func add(child: Child, size mode: SizeMode, priority: UILayoutPriority = UILayoutPriorityDefaultHigh) -> WorkspacePane {
        children.append((child, mode, priority))
        return self
    }
    
    // MARK: Internal
    
    func invalidate(in workspace: WorkspaceController) {
        let maxLength = max(UIScreen.main.bounds.width, UIScreen.main.bounds.height)
        
        for (child, mode, priority) in children {
            switch child {
            case .controller(let controller):
                addChild(controller: controller, to: workspace)
                applyConstraint(to: controller.view, maxLength: maxLength, size: mode, priority: priority)
            case .pane(let pane):
                addChild(pane: pane, to: workspace)
                applyConstraint(to: pane.view, maxLength: maxLength, size: mode, priority: priority)
            }
        }
    }
    
    // MARK: Private 
    
    private let axis: Axis
    private let stackView: UIStackView
    private var children: [(Child, SizeMode, UILayoutPriority)] = []
    
    private func sizeForMode(_ mode: SizeMode, maxLength: CGFloat) -> CGSize {
        switch mode {
        case .fixed(let length):
            return CGSize(width: axis == .horizontal ? length : maxLength,
                          height: axis == .vertical ? length : maxLength)
        default:
            return CGSize(width: maxLength, height: maxLength)
        }
    }
    
    private func addChild(pane: WorkspacePane, to workspace: WorkspaceController) {
        stackView.addArrangedSubview(pane.view)
        pane.invalidate(in: workspace)
    }
    
    private func addChild(controller: UIViewController, to workspace: WorkspaceController) {
        guard !workspace.viewControllers.contains(controller) else { fatalError("This controller already exists in the Workspace") }
        
        workspace.addChildViewController(controller)
        stackView.addArrangedSubview(controller.view)
        controller.didMove(toParentViewController:workspace)
        workspace.viewControllers.append(controller)
    }
    
    private func applyConstraint(to view: UIView, maxLength: CGFloat, size mode: SizeMode, priority: UILayoutPriority) {
        let size = sizeForMode(mode, maxLength: maxLength)
        
        if axis == .horizontal && size.width != maxLength {
            let constraint = view.widthAnchor.constraint(equalToConstant: size.width)
            // we lower the constraint so that it doesn't conflict with the UIStackView's constraints
            constraint.priority = max(1, priority - 1)
            NSLayoutConstraint.activate([ constraint ])
        }
        
        if axis == .vertical && size.height != maxLength {
            let constraint = view.heightAnchor.constraint(equalToConstant: size.height)
            // we lower the constraint so that it doesn't conflict with the UIStackView's constraints
            constraint.priority = max(1, priority - 1)
            NSLayoutConstraint.activate([ constraint ])
        }
    }
    
}