Getting the frame of a particular tab bar item

Question

Is there a way to find the frame of a particular UITabBarItem in a UITabBar?

Specifically, I want to create an animation of an image "falling" into one of the tabs, similar to e.g. deleting an email in the Mail, or buying a track in the iTunes app. So I need the target coordinates for the animation.

As far as I can tell, there's no public API to get the coordinates, but would love to be wrong about that. Short of that, I'll have to guesstimate the coordinates using the index of the given tab relative to the tab bar frame.

Answer 1

Imre's implementation is missing a couple of imho important details.

1. The UITabBarButton views are not necessarily in order. For example, if you have more than 5 tabs on iPhone and rearranged tabs, the views might be out of order.
2. If you use more than 5 tabs the out of bounds index only means that the tab is behind the "more" tab. In this case there is no reason to fail with an assert, just use the frame of the last tab.

So I changed his code a little bit and I came up with this:

+ (CGRect)frameForTabInTabBar:(UITabBar*)tabBar withIndex:(NSUInteger)index
{
    NSMutableArray *tabBarItems = [NSMutableArray arrayWithCapacity:[tabBar.items count]];
    for (UIView *view in tabBar.subviews) {
        if ([view isKindOfClass:NSClassFromString(@"UITabBarButton")] && [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
    [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;
        }
        NSAssert(NO, @"%@ and %@ share the same origin.x. This should never happen and indicates a substantial change in the framework that renders this method useless.", view1, view2);
        return NSOrderedSame;
    }];

    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;
        }
    }
    else {
        // our target viewController is inside the "more" tab
        UIView *tabView = [tabBarItems lastObject];
        if ([tabView respondsToSelector:@selector(frame)]) {
            frame = tabView.frame;
        }
    }
    return frame;
}

Answer 2

Another possible but sloppy solution would be the following:

guard let view = self.tabBarVC?.tabBar.items?[0].valueForKey("view") as? UIView else 
{
    return
}
let frame = view.frame

Answer 3

The subviews associated with the tab bar items in a UITabBar are of class UITabBarButton. By logging the subviews of a UITabBar with two tabs:

for (UIView* view in self.tabBar.subviews)
{
    NSLog(view.description);
}

you get:

<_UITabBarBackgroundView: 0x6a91e00; frame = (0 0; 320 49); opaque = NO; autoresize = W; userInteractionEnabled = NO; layer = <CALayer: 0x6a91e90>> - (null)
<UITabBarButton: 0x6a8d900; frame = (2 1; 156 48); opaque = NO; layer = <CALayer: 0x6a8db10>>
<UITabBarButton: 0x6a91b70; frame = (162 1; 156 48); opaque = NO; layer = <CALayer: 0x6a8db40>>

Based on this the solution is kind of trivial. The method I wrote for trying this out is as follows:

+ (CGRect)frameForTabInTabBar:(UITabBar*)tabBar withIndex:(NSUInteger)index
{
    NSUInteger currentTabIndex = 0;

    for (UIView* subView in tabBar.subviews)
    {
        if ([subView isKindOfClass:NSClassFromString(@"UITabBarButton")])
        {
            if (currentTabIndex == index)
                return subView.frame;
            else
                currentTabIndex++;
        }
    }

    NSAssert(NO, @"Index is out of bounds");
    return CGRectNull;
}

It should be noted that the structure (subviews) of UITabBar and the class UITabBarButton itself are not part of the public API, so in theory it can change in any new iOS version without prior notification. Nevertheless it is unlikely that they would change such detail, and it works fine with iOS 5-6 and prior versions.