在MainTabBarController.m文件中写入下面代码：
// 重写tabbar高度
- (void)viewDidLayoutSubviews {
[super viewDidLayoutSubviews];
CGRect frame = self.tabBar.frame;
frame.size.height = 55;
frame.origin.y = self.view.frame.size.height - frame.size.height;
self.tabBar.frame = frame;
self.tabBar.backgroundColor = [UIColor blackColor];
//此处需要设置barStyle，否则颜色会分成上下两层
self.tabBar.barStyle = UIBarStyleBlack;
}
