iOS 开发中，定制 tabbar 的需求比较常见。这里总结自定义的实现。分如下几步：

自定义 DockView

• 重写 initWithFrame;
• 添加 item;
• 调整 item 的 frame;
• 监听 item 的点击事件；
• 重写 setSelectedIndex.

DockView.h

#import <UIKit/UIKit.h>

@interface DockView : UIView

@property (nonatomic, copy) void (^itemClickBlock)(int index);
@property (nonatomic, assign) int selectedIndex;

- (void) addDockItemWithIcon:(NSString *)icon selectedIcon:(NSString *)icon_selecte title:(NSString *) title;

@end

DockView.m

#import "DockView.h"
#import "DockItem.h"

@interface DockView()
{
    DockItem *_currentItem;
}
@end

@implementation DockView

- (id) initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if(self) {
        self.backgroundColor = [UIColor colorWithPatternImage:[UIImage imageNamed:@"tabbar_background.png"]];
    }
    return self;
}

#pragma mark 添加item
- (void) addDockItemWithIcon:(NSString *)icon selectedIcon:(NSString *)icon_selected title:(NSString *) title
{
    DockItem *item = [DockItem buttonWithType:UIButtonTypeCustom];
    [self addSubview:item];
    [item setTitle:title forState:UIControlStateNormal];
    [item setImage:[UIImage imageNamed:icon] forState:UIControlStateNormal];
    [item setImage:[UIImage imageNamed:icon_selected] forState:UIControlStateSelected];
    [item addTarget:self action:@selector(itemClick:) forControlEvents:UIControlEventTouchDown];
    [self adjustDockItemsFrame];
}

- (void) itemClick:(DockItem *) item
{
    _currentItem.selected = NO;
    item.selected = YES;
    _currentItem = item;
    if (_itemClickBlock) {
        _itemClickBlock(item.tag);
    }

}

- (void) adjustDockItemsFrame
{
    int count = self.subviews.count;
    CGFloat itemWidth = self.frame.size.width / count;
    CGFloat itemHeight = self.frame.size.height;

    for (int i = 0; i < count; i ++) {
        DockItem *item = self.subviews[i];
        item.frame = CGRectMake(i * itemWidth, 0, itemWidth, itemHeight);
        if (i == 0) {
            item.selected = YES;
            _currentItem = item;
        } 
        item.tag = i;
    }

}

#pragma mark  重写设置选中的索引方法
- (void) setSelectedIndex:(int)selectedIndex
{
    if (selectedIndex < 0 || selectedIndex >= self.subviews.count) {
         return;
    }
    _selectedIndex = selectedIndex;
    DockItem *item = self.subviews[selectedIndex];
    [self itemClick:item];
}

@end

自定义 DockItem 继承 UIButton

• 重写initWithFrame;
• 重写 titleRectForContentRect，调整title 的 CGRect;
• 重写 imageRectForContentRect, 调整 image 的 CGRect.

DockItem.m

#define kImageRatio 0.6

#import "DockItem.h"

@implementation DockItem

- (id) initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        // 居中
        self.titleLabel.textAlignment = NSTextAlignmentCenter;
        self.titleLabel.font = [UIFont systemFontOfSize:12];
        self.imageView.contentMode = UIViewContentModeScaleAspectFit;
        self.adjustsImageWhenHighlighted = NO;
        [self setBackgroundImage:[UIImage imageNamed:@"tabbar_slider.png"] forState:UIControlStateSelected];
    }
    return self;

}

#pragma mark 返回按钮内部UIlabel 的边框(重写)
- (CGRect) titleRectForContentRect:(CGRect) contentRect
{
    CGFloat titleY = contentRect.size.height * kImageRatio - 5;
    CGFloat titleHeight = contentRect.size.height - titleY;
    return CGRectMake(0, titleY, contentRect.size.width, titleHeight);
}

#pragma mark 返回按钮内部UIImageView 的边框（重写）
- (CGRect) imageRectForContentRect:(CGRect)contentRect
{
    return CGRectMake(0, 0, contentRect.size.width, contentRect.size.height * kImageRatio);
}

@end

使用

• 初始化一个 DockView，并设置 frame;
• 添加 DockItem;
• 设置 itemClickBlock .

DockView *dock = [[DockView alloc] init];
dock.frame = CGRectMake(0, self.view.frame.size.height - kDockHeight, self.view.frame.size.width, kDockHeight);

[self.view addSubview:dock];

[dock addDockItemWithIcon:@"tabbar_home.png" selectedIcon:@"tabbar_home_selected.png" title:@"首页"];
[dock addDockItemWithIcon:@"tabbar_discover.png" selectedIcon:@"tabbar_discover_selected.png" title:@"消息"];
dock.itemClickBlock = ^(int index) {
    NSLog(@"选中了%d", index);
};

效果图

tabbar.png