先上一张效果图：

lALPACOG82h0uBDNCKDNBNo_1242_2208.png_620x10000q90g.jpg

下面开始说一下思路：

第一步；先对UIView进行扩展，方便我们获取它子类的一下属性。代码如下

#import <UIKit/UIKit.h>

@interface UIView (Extension)

@property (nonatomic,assign) CGFloat x;
@property (nonatomic,assign) CGFloat y;
@property (nonatomic,assign) CGFloat width;
@property (nonatomic,assign) CGFloat height;
@property (nonatomic,assign) CGFloat centerX;
@property (nonatomic,assign) CGFloat centerY;
@property (nonatomic,assign) CGSize size;

@end

#import "UIView+Extension.h"

@implementation UIView (Extension)

#pragma mark 重写set get 方法
#pragma mark x
- (void)setX:(CGFloat)x
{
    CGRect frame = self.frame;
    frame.origin.x = x;
    self.frame = frame;
}
- (CGFloat)x
{
    return self.frame.origin.x;
}

#pragma mark y
- (void)setY:(CGFloat)y
{
    CGRect frame = self.frame;
    frame.origin.y = y;
    self.frame = frame;
}
- (CGFloat)y
{
    return self.frame.origin.y;
}

#pragma mark width
- (void)setWidth:(CGFloat)width
{
    CGRect frame = self.frame;
    frame.size.width = width;
    self.frame = frame;
}
- (CGFloat)width
{
    return self.frame.size.width;
}

#pragma mark height
- (void)setHeight:(CGFloat)height
{
    CGRect frame = self.frame;
    frame.size.height = height;
}
- (CGFloat)height
{
    return self.frame.size.height;
}

#pragma mark centerX
- (void)setCenterX:(CGFloat)centerX
{
    CGPoint center = self.center;
    center.x = centerX;
    self.center = center;
}
- (CGFloat)centerX
{
    return self.center.x;
}

#pragma mark centerY
- (void)setCenterY:(CGFloat)centerY
{
    CGPoint center = self.center;
    center.y = centerY;
    self.center = center;
}
- (CGFloat)centerY
{
    return self.center.y;
}

#pragma mark size
- (void)setSize:(CGSize)size
{
    CGRect frame = self.frame;
    frame.size = size;
    self.frame = frame;
}
- (CGSize)size
{
    return self.frame.size;
}

@end

第二步，新建一个类ZLTabbar，继承自UITabBar

1、.h文件

#import <UIKit/UIKit.h>

typedef void(^IrrBtnSelectedBlock)();
@interface ZLTabbar : UITabBar

//用于监听自定义tabbarButton的点击事件
@property (nonatomic,copy) IrrBtnSelectedBlock selectedIrrBtn;
- (void)didselectedIrrBtnWithBlock:(IrrBtnSelectedBlock)selectedIrrBtn;

@end

这里解释一下，我习惯用block，所以这里声明了一个block类型。你也可以根据个人习惯，来使用观察者或者代理。
2、.m文件

#import "ZLTabbar.h"
#import "UIView+Extension.h"

@interface ZLTabbar ()

@property (nonatomic,strong) UIButton *irregularItem;//不规则的按钮，可以放在在中间或者其它任意一个地方

@end

@implementation ZLTabbar

- (instancetype)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self)
    {
        //TODO:加载子视图
        [self createChildrenViews];
    }
    return self;
}

#pragma mark 创建子视图
- (void)createChildrenViews
{
    self.backgroundColor = [UIColor whiteColor];

    //TODO:去掉tabbar的分割线
    [self setBackgroundImage:[UIImage new]];
    [self setShadowImage:[UIImage new]];

    self.irregularItem = [UIButton buttonWithType:UIButtonTypeCustom];
    [self.irregularItem setBackgroundImage:[UIImage imageNamed:@"midBtn"] forState:UIControlStateNormal];
    [self.irregularItem setBackgroundImage:[UIImage imageNamed:@"midBtn"] forState:UIControlStateHighlighted];
    self.irregularItem.size = self.irregularItem.currentBackgroundImage.size;
    self.irregularItem.layer.masksToBounds = YES;
    self.irregularItem.layer.cornerRadius = self.irregularItem.width/2;
    [self.irregularItem addTarget:self action:@selector(irregularItemSelcted) forControlEvents:UIControlEventTouchUpInside];
    [self addSubview:self.irregularItem];
}

#pragma mark 执行和监听不规则按钮的点击事件
- (void)irregularItemSelcted
{
    if (self.selectedIrrBtn)
    {
        self.selectedIrrBtn();
    }
}

- (void)didselectedIrrBtnWithBlock:(IrrBtnSelectedBlock)selectedIrrBtn
{
    self.selectedIrrBtn = selectedIrrBtn;
}

#pragma mark layoutSubviews
- (void)layoutSubviews
{
    [super layoutSubviews];

    //TODO:重新排布系统按钮的位置，空出需要自定义按钮的位置，系统按钮的类型是UITabBarButton
    Class class = NSClassFromString(@"UITabBarButton");

    //TODO:这里设置自定义tabbarButton的位置
    self.irregularItem.centerX = (self.width * 0.75 + self.irregularItem.width * 0.75);
    self.irregularItem.centerY = self.height - self.irregularItem.height/2;

    NSInteger btnIndex = 0;
    for (UIView *btn in self.subviews)
    {
        if ([btn isKindOfClass:class])
        {
            //TODO:如果是系统的UITabBarButton，调整子控件位置，空出自定义UITabBarButton的位置
            //TODO:按钮宽度为Tabbar宽度平分4块
            btn.width = self.width/4;
            if (btnIndex < 3)
            {
                //TODO: -3- 在这里为irregularItem的索引值
                btn.x = btn.width * btnIndex;
            }
            else
            {
                btn.x = btn.width * btnIndex + self.irregularItem.width;
            }
            btnIndex ++;

            if (btnIndex == 0)
            {
                btnIndex ++;
            }
        }
    }
    [self bringSubviewToFront:self.irregularItem];
}

#pragma mark 重写hitTest方法，去监听irregularItem的点击，目的是为了让突出在外面的部分 在点击时也有反应
- (UIView *)hitTest:(CGPoint)point withEvent:(UIEvent *)event
{
    /**判断当前手指是否点击到了irregularItem。如果是，则相应按钮点击；如果是其它，则由系统处理**/
    //当前view是否被隐藏？如果隐藏，就不做其他处理了。
    if (self.isHidden == NO)
    {
        //TODO:将当前tabbar的触摸点转换成坐标系，转换到irregularItem的身上，生成一个新的点
        CGPoint new_point = [self convertPoint:point toView:self.irregularItem];

        //TODO:如果这个新的点是在irregularItem上，那么处理点点击事件最合适的view就是irregularItem
        if ([self.irregularItem pointInside:new_point withEvent:event])
        {
            return self.irregularItem;
        }
    }
    return [super hitTest:point withEvent:event];
}

@end

说明：

1.在layoutSubviews方法中，重设自定义以及系统的tabbarButton的位置。
2.如果不重写hitTest方法，点击突出在外面的button会无事件响应。
3.当调整自定义tabbarButton的位置时，还有可能需要调整它的索引值。此时，它的索引值为3；

第三步，创建tabbarController

.m文件

#import "ZLTabbarController.h"
#import "LeftViewController.h"
#import "RightViewController.h"
#import "MidViewController.h"
#import "ResourceViewController.h"
#import "ZLTabbar.h"
@interface ZLTabbarController ()

@end

@implementation ZLTabbarController

- (void)viewDidLoad {
    [super viewDidLoad];
    [self setupTabbar];
}

#pragma mark 设置tabbar
- (void)setupTabbar
{
    LeftViewController *left = [[LeftViewController alloc]init];
    [self createChildrenVC:left
           childrenVCTitle:@"首页"
                     image:[UIImage imageNamed:@"首页"]
             selectedImage:[UIImage imageNamed:@"首页_S"]];
    ResourceViewController *resource = [[ResourceViewController alloc]init];
    [self createChildrenVC:resource
           childrenVCTitle:@"资源"
                     image:[UIImage imageNamed:@"资源"]
             selectedImage:[UIImage imageNamed:@"资源_S"]];

    RightViewController *right = [[RightViewController alloc]init];
    [self createChildrenVC:right
           childrenVCTitle:@"个人中心"
                     image:[UIImage imageNamed:@"个人中心"]
             selectedImage:[UIImage imageNamed:@"个人中心_S"]];

    ZLTabbar *tabbar = [[ZLTabbar alloc]init];
    tabbar.selectedItem = left.tabBarItem;
    MidViewController *mid = [[MidViewController alloc]init];
    [tabbar didselectedIrrBtnWithBlock:^{
        [self presentViewController:mid animated:YES completion:nil];
    }];
    [self setValue:tabbar forKey:@"tabBar"];
}


#pragma mark 添加子控制器
- (void)createChildrenVC:(UIViewController *)childViewController
         childrenVCTitle:(NSString *)title
                   image:(UIImage *)image
           selectedImage:(UIImage *)selectedImg
{
    //设置选中时item的字体颜色
    UIColor *selectedTinColor = [UIColor colorWithRed:60.0/255
                                                green:160.0/255
                                                 blue:220.0/255
                                                alpha:1.0f];
    [[UITabBarItem appearance] setTitleTextAttributes:[NSDictionary dictionaryWithObject:NSForegroundColorAttributeName
                                                                                  forKey:selectedTinColor]
                                             forState:UIControlStateSelected];

    //设置未选中的image
    UIImage *deselectedImage = image;
    deselectedImage = [deselectedImage imageWithRenderingMode:UIImageRenderingModeAlwaysOriginal];
    childViewController.tabBarItem.image = deselectedImage;

    //设置选中的image
    UIImage *selectedImage = selectedImg;
    selectedImage = [selectedImage imageWithRenderingMode:UIImageRenderingModeAlwaysOriginal];
    childViewController.tabBarItem.selectedImage = selectedImage;

    //设置tabbarItem文字和图标 之间的距离
    childViewController.tabBarItem.title = title;
//    childViewController.tabBarItem.titlePositionAdjustment = UIOffsetMake(0, MAXFLOAT);

    //添加控制器对象
    UINavigationController *childNavigationContorller  = [[UINavigationController alloc]initWithRootViewController:childViewController];
    [self addChildViewController:childNavigationContorller];
}
@end

说明：

1.我们可通过tabBarItem.titlePositionAdjustment来设置文字与图表之间的距离，当值为MAXFLOAT时，这个方法也起到了隐藏文字的作用。当然，你可以直接tabBarItem.title设置为空字符串对象。
2.我们可以通过[tabbar didselectedIrrBtnWithBlock:^{ /**在这里做一些操作**/ }];来响应一些操作。

3、最后把tabbarController的对象设置为window的根视图控制器就可以使用了。详细的代码都在上面了，注释也写的比较清晰。如果有不懂的地方可以留言。代码不足的地方，还望各位大神多多指教。