類似iOS簡書中間發布文章用的圓形按鈕
1、在《框架搭建_純代碼》的基礎上稍作修改
2、自定義幾個頁面，用button控制切換

中間按鈕效果圖

比較：第1種法式適合平行切換，缺點就是按鈕超出TabBar范圍的點擊無響應。第2種設置上可能相對更麻煩，適合model頁面，如簡書的彈出頁面，而不是平行切換。

1、在《框架搭建_純代碼》的基礎上稍作修改

添加按鈕的主要代碼如下：

    //缺點：1、按鈕超過bottom的部分點擊無響應    2、點擊除按鈕外的中間部分時也展示中間item的頁面
    //3個item是可能范圍太大，如果調整item數量，缺點2應該影響不大。
    UIButton *btn = [[UIButton alloc] init];
    btn.backgroundColor = [UIColor redColor];
    [btn setFrame:CGRectMake(130, -12, 60, 60)];
    btn.clipsToBounds = YES;
    btn.layer.cornerRadius = 30;
    [btn addTarget:self action:@selector(clickCenterButton) forControlEvents:UIControlEventTouchUpInside];
    
    [tabBarControl.tabBar addSubview:btn];

2、自定義幾個頁面，用button控制切換

整體思路：自定義三個按鈕，中間按鈕設置較大，并且切成圓角，在點擊事件中設置隱藏或展示或彈出頁面即可。代碼如下：

AppDelegate中代碼

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    self.window.backgroundColor = [UIColor whiteColor];
    [self.window makeKeyAndVisible];
    
    ViewController *vc = [[ViewController alloc] init];
    
    UINavigationController *nav = [[UINavigationController alloc] initWithRootViewController:vc];

    //指定應用的跟視圖控制器
    self.window.rootViewController = nav;
    
    return YES;
}

ViewController中主要代碼

#import "ViewController.h"
#import "ModelViewController.h"

#define ScreenHeight [UIScreen mainScreen].bounds.size.height
#define ScreenWidth  [UIScreen mainScreen].bounds.size.width

@interface ViewController ()
{
    UIView *view1;
    UIView *view2;
    UIView *view3;
    
    ModelViewController *modelVC;
}
@end

@implementation ViewController

- (instancetype)init
{
    self = [super init];
    
    if (self)
    {
        [self createUI];
    }
    
    return self;
}

- (void)createUI
{
    view1 = [[UIView alloc] initWithFrame:self.view.frame];
    view1.backgroundColor = [UIColor greenColor];
    [self.view addSubview:view1];
        
    view3 = [[UIView alloc] initWithFrame:self.view.frame];
    view3.backgroundColor = [UIColor lightGrayColor];
    view3.hidden = YES;
    [self.view addSubview:view3];
    
    UIButton *btn1 = [[UIButton alloc] init];
    [btn1 setFrame:CGRectMake(20, ScreenHeight-44, 44, 44)];
    btn1.backgroundColor = [UIColor greenColor];
    [btn1 addTarget:self action:@selector(tapFirstButton) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:btn1];
    
    UIButton *btn2 = [[UIButton alloc] init];
    [btn2 setFrame:CGRectMake(ScreenWidth/2-30, ScreenHeight-44-16, 60, 60)];
    btn2.clipsToBounds = YES;
    btn2.layer.cornerRadius = 30;
    btn2.backgroundColor = [UIColor redColor];
    [btn2 addTarget:self action:@selector(tapSecondButton) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:btn2];
    
    UIButton *btn3 = [[UIButton alloc] init];
    btn3.backgroundColor = [UIColor grayColor];
    [btn3 setFrame:CGRectMake(ScreenWidth-44-20, ScreenHeight-44, 44, 44)];
    [btn3 addTarget:self action:@selector(tapThirdButton) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:btn3];
}

- (void)tapFirstButton
{
    view1.hidden = NO;
    view2.hidden = YES;
    view3.hidden = YES;
    self.title = @"first";
}

- (void)tapSecondButton
{
    if (!modelVC)
    {
        modelVC = [[ModelViewController alloc] init];
    }
    
    [self presentViewController:modelVC animated:YES completion:nil];
}

- (void)tapThirdButton
{
    view1.hidden = YES;
    view2.hidden = YES;
    view3.hidden = NO;
    
    self.title = @"third";
}

以上基本完成了項目需求，button的效果可以通過賦值的圖片控制。