传送门：http://stackoverflow.com/questions/6325457/getting-the-frame-of-a-particular-tab-bar-item

我稍微改了一下：

- (CGRect)frameForTabInTabBarWithIndex:(NSUInteger)index
{
    NSMutableArray *tabBarItems = [NSMutableArray arrayWithCapacity:[self.tabBar.items count]];
    for (UIView *view in self.tabBar.subviews) {
        if ([view isKindOfClass:[UIControl class]] && [view respondsToSelector:@selector(frame)]) {
            // check for the selector -frame to prevent crashes in the very unlikely case that in the future
            // objects thar don't implement -frame can be subViews of an UIView
            [tabBarItems addObject:view];
        }
    }
    if ([tabBarItems count] == 0) {
        // no tabBarItems means either no UITabBarButtons were in the subView, or none responded to -frame
        // return CGRectZero to indicate that we couldn't figure out the frame
        return CGRectZero;
    }

    // sort by origin.x of the frame because the items are not necessarily in the correct order
    __block BOOL sthWrong = NO;
    [tabBarItems sortUsingComparator:^NSComparisonResult(UIView *view1, UIView *view2) {
        if (view1.frame.origin.x < view2.frame.origin.x) {
            return NSOrderedAscending;
        }
        if (view1.frame.origin.x > view2.frame.origin.x) {
            return NSOrderedDescending;
        }
        /// should't be here
        sthWrong = YES;
        return NSOrderedAscending;
    }];

    if (sthWrong) {
        return CGRectZero;
    }

    CGRect frame = CGRectZero;
    if (index < [tabBarItems count]) {
        // viewController is in a regular tab
        UIView *tabView = tabBarItems[index];
        if ([tabView respondsToSelector:@selector(frame)]) {
            frame = tabView.frame;
        }
    }
    return frame;
}

有了这些frame后，就可以来实现打红点之类的事情了~