1.自定义MainTabBarController：
1.1继承自系统的UITabBarController,自定义一些属性例如，title文本颜色、选
择的颜色等属性。

1.2 向其中添加子控制器

 override func viewDidLoad() {
        super.viewDidLoad()
        setUpUI() //初始化自己的子控制器
        setCustomTabBar()   //使用自定义tabbar
    }
    
    private func setUpUI(){
        
        let oneVC = UIViewController()
        oneVC.view.backgroundColor = UIColor.red
        let secVC = UIViewController()
        secVC.view.backgroundColor = UIColor.yellow
        let treVC = UIViewController()
        treVC.view.backgroundColor = UIColor.orange
        let fourVC = UIViewController()
        fourVC.view.backgroundColor = UIColor.gray
        
        
        //添加子控制器
        setUpChildViewController(childVC: oneVC, image: "btn_tabbar_home_normal_25x25_", selectImage: "btn_tabbar_home_selected_25x25_", title: "首页")
        setUpChildViewController(childVC: secVC, image: "btn_tabbar_lanmu_normal_25x25_", selectImage: "btn_tabbar_lanmu_selected_25x25_", title: "栏目")
        setUpChildViewController(childVC: fourVC, image: "btn_tabbar_zhibo_normal_25x25_", selectImage: "btn_tabbar_zhibo_selected_25x25_", title: "直播")
        setUpChildViewController(childVC: treVC, image: "btn_tabbar_wode_normal_25x25_", selectImage: "btn_tabbar_wode_selected_25x25_", title: "我的")

    }

1.3以下为添加自控制的方法

    //添加子控制器
    func setUpChildViewController(childVC : UIViewController,image:String,selectImage:String,title:String) {
        childVC.title = title
        childVC.tabBarItem.image = UIImage(named: image)
        childVC.tabBarItem.selectedImage = UIImage(named:selectImage)
        let nav = UINavigationController(rootViewController: childVC)
        addChildViewController(nav)
    }

1.4 添加自定义tabBar

    private func setCustomTabBar(){
     
        let tab = QRTabBar()
        tab.qrDelegate = self
        self.setValue(tab, forKeyPath: "tabBar")
    }

1.5 最重要的部分来了，实现tabBar的自定义布局
1.5.1 创建一个自己的QRTabBar 继承与UITabBar
1.5.2 重写layoutSubviews方法 ，实现tabbar Item的自由布局

import UIKit

protocol QRTabBarDelegate {
    func addButtonClickCall()
}

class QRTabBar: UITabBar {
    
    var qrDelegate : QRTabBarDelegate?
    
    
    override func layoutSubviews() {
        super.layoutSubviews()
        
        let buttonW:CGFloat = self.frame.size.width / 5
        let buttonH:CGFloat = self.frame.size.height
        let buttonY:CGFloat = 0
        // 按钮索引
        var buttonIndex:Int = 0;
        for (_,sub) in self.subviews.enumerated() {
            
            if sub.self.isKind(of: NSClassFromString("UITabBarButton")!) {
                // 设置frame
                var buttonX:CGFloat = CGFloat(buttonIndex) * buttonW
                if buttonIndex >= 2 { // 右边的2个UITabBarButton
                    buttonX += buttonW
                }
                sub.frame = CGRect(x: buttonX, y: buttonY, width: buttonW, height: buttonH)
                // 增加索引
                buttonIndex += 1
            }
        }
        //设置中间的发布按钮的frame
        addButton.frame = CGRect(x: 0, y: 0, width: buttonW, height: buttonH)
        addButton.center = CGPoint(x: self.frame.size.width * 0.5, y: self.frame.size.height * 0.5)
    }
    
    // MARK: - 懒加载
    private lazy var addButton: UIButton = {
        () -> UIButton
        in
        let btn = UIButton(type: UIButtonType.custom)
        btn.setImage(UIImage(named: "addButton"), for: UIControlState.normal)
        btn.setImage(UIImage(named: "addButton"), for: UIControlState.highlighted)
        // 2.监听按钮点击
        btn.addTarget(self, action:#selector(addButtonClick), for: UIControlEvents.touchUpInside)
        self.addSubview(btn)
        return btn
    }()
    //点击事件
    @objc private func addButtonClick(){
        if (qrDelegate != nil) {
            qrDelegate?.addButtonClickCall()
        }
    }

    
}

最后附上Demo地址