Tab Bar Controller and class UITabBarController

We have seen that any view controller can have a view underneath it, i.e., under its supervision and control. A special kind of view controller called a UITabBarController can have another view controller underneath it. In fact, a UITabBarController has a whole array of view controllers underneath it. See Tab Bars in the UIKit User Interface Catalog, and Tab Bar Controllers.

A UITabBarController lets you switch back and forth between the view controllers beneath it. Each of these other view controllers has a view of its own. Our example has five view controllers under the tab bar controller, one for each borough.

The drawRect: of our class View displays the view and its complete ancestry all the way back to the enclosing window. The font is monospace font to align the columns of numbers. The translucent tab bar is 49 pairs of pixels high, but this fact is nowhee in Apple’s documentation and may therefore change without warning.

Source code in

  1. main.m
  2. Class TabAppDelegate
  3. Class ViewController
  4. Class View
  5. Png files:
    1. bronx@2x.png: 38 × 48 pixels
    2. brooklyn@2x.png: 64 × 47 pixels
    3. manhattan@2x.png 55 × 52 pixels
    4. queens@2x.png 50 × 50 pixels
    5. statenisland@2x.png: 49 × 49 pixels

Create the project

TabAppDelegate.m must import ViewController.h, since it creates a ViewController. ViewController.m must import View.h, since it creates a View.

Add the five .png files to the project. See America for an example of adding an image file to a project.

The tab bar icons

I should have created my own icons for the five boroughs. But I’m not an artist, so I used Apple’s standard icons (see below).

The names of the png files contain @2x. I should have added five additional png files to the project, without the @2x in their names (bronx.png, etc). These would be smaller files in case the app is run on a non-retina device.

The @2x is not mentioned in TabAppDelegate.m, but the calls to imageNamed: opens them anyway. This method chooses the correct file to open based on whether the device is retina or non-retina.

Apple’s standard icons

See Tab bar icons in the iOS Human Interface Guidelines.

  1. Pass a UITabBarSystemItem enumeration to the initWithTabBarSystemItem:tag: method of class UITabBarItem. Store the return value of this method in the tabBarItem property of the view controller.
  2. Unfortunately, you will have no control of the title in the tab. With UITabBarSystemItemBookmarks, it will always be "Bookmarks".

Other icons

  1. Get icons from Glyphish.
  2. One students creted his or her own icons.
  3. Icons you have to pay for.

Create the .png files with Adobe Photoshop

The tab bar is 49 pixels high on an iPhone. The image in each tab bar item must be a 30 × 30 pixel png (see tab bar icons) drawn with the alpha channel only. I created one with the Adobe Photoshop CS4 Extended version 11.0.1 at NYU.

File → New… Name: bronx
Width: 30 pixels
Height: 30 pixels
Color Mode: RGB Color
Background Contents: Transparent

You now have a square file with a white and gray checkered background. It should be displayed in two places: in a little bronx @ 100% (Layer 1, RGB/8) window, and in the Navigator tab.

Select the pencil tool. Resize the pencil if necessary. The lines you draw will appear in blue against a gray background when the item is selected, and in gray against a black background when the item is not selected.

File → Save As…
Format: PNG
PNG Options Interlace None

Then add the png file to your Xcode project.

Project → Add to Project…

Create the png files using ImageMagick

Here are instructions for getting ImageMagick and using it to create a black and transparent image for iPhone/iPad icons.

  1. If you don’t already have MacPorts, get it from here and install it.
  2. In, run
    sudo port install ImageMagick
    This step may take a while. Alternative instructions are available here.
  3. In, run
    convert image1.jpg -threshold 60% image2.jpg
    This will make a black and white image from a color image. Play around with the percentage to get the desired results.
  4. In, run
    convert image2.jpg -transparent white image3.jpg
    This sets all white pixels to transparent. Use +transparent color to set everything but color to transparent. See the -fill option for the color definitions.

Create the png files using a form

The following form changes the selected color to transparent in an image file. It runs the Perl gateway /home1/m/mm64/public_html/cgi-bin/maketab on

Choose a .gif, .jpg, .png, or .tiff file on your hard disk.

Choose the color that will become transparent (e.g., FF0000 for red, FF00FF for magenta, FFFFFF for white).

Things to try

  1. How wide can a tab bar image be before it is truncated? Does the maximum width depend on the orientation of the iPhone? How tall can an image be? Will a tall image be hidden by the title?

  2. The tab bar has room for only five tabs. If Yonkers becomes the sixth borough of New York City, where will it go? What about a seventh borough?

  3. How could we make a 30 × 30 png alpha file of a Unicode character? ℞ ∫ ℵ

  4. The output reveals three hitherto unknown subclasses of class UIView: UIViewControllerWrapperView, UITransitionView, and UILayoutContainerView. Can you find these classes anywhere in Apple’s online documentation?

  5. Should we create the project in Xcode as a “Tabbed Application” instead of an “Empty Application”?