[iPhone] Simple UIView question

Question

Hi,
I am trying to figure out why iPhone is behaving this way and I can not figure it out.
For testing, I am trying to display 3 views one on top of the other (using UIViewControllers). The problem is that except for the first one that goes directly on top of the window object, the rest of the views display about 20 pixels bellow the top of its superview
My very-very simple application, my delegate code is like this:
- (void)applicationDidFinishLaunching:(UIApplication *)application {
// Override point for customization after application launch
UIViewController *viewController;
UIViewController *viewController2;
UIViewController *viewController3;
// Create the first ViewController and assign green to its view background color
viewController = [[UIViewController alloc] init];
[[viewController view] setBackgroundColor:[UIColor greenColor]];
// Create the second ViewController and assign purple to its view background color
viewController2 = [[UIViewController alloc] init];
[[viewController2 view] setBackgroundColor:[UIColor purpleColor]];
// Create the third ViewController and assign cyan to its view background color
viewController3 = [[UIViewController alloc] init];
[[viewController3 view] setBackgroundColor:[UIColor cyanColor]];
// Add view2 to view and view3 to view2
[[viewController view] addSubview:[viewController2 view]];
[[viewController2 view] addSubview:[viewController3 view]];
// Set background window color to red
[window setBackgroundColor:[UIColor redColor]];
// Add view to window
[window addSubview:[viewController view]];
[window makeKeyAndVisible];
This displays about 20px of green at the top (from view), below that about 20px of purple (from view2) and the rest of the screen is cyan.
The question is why it is behaving like this and not display the views directly one on top of the other so at the end to show the screen in cyan (from view3 that is the last view added).
I do understand that there are different ways to do this but this is not the point here. I am try to understand why using this way, the iPhone behaves like this.
Thanks for your help.

Answer 1

The window frame is normally set to the bounds of the screen, (0, 0, 320, 480), regardless of whether the status bar is visible. I may have over-simplified my last reply, since this thread is really all about the status bar, which is 20 high. When the status bar is visible, a view controller created in code will have a view with a frame of (0, 20, 320, 460). However, when the status bar is hidden, the very same code will create a view with a frame of (0, 0, 320, 480). The taller view would also work with a visible status bar (since the bar is always on top), but the system is just trying not to waste the upper 320 x 20 area of the view.
Anyway, the coordinate system of the window is the same as that of the screen. So when the origin of the root view frame is (0, 20) there's no offset. I.e. the point (0, 20) relative to the window is also (0, 20) relative to the screen.
But when a second view with a (0, 20) frame origin is added to the first, the point (0, 20) is relative to the root view rather than the window. The point (0, 0) in the root view's system is the point (0, 20) in the window's system, so the point (0, 20) in the root view's system is (0, 40) in the window's system.
It might help to remember that every view object has both a frame property and a bounds property. The frame is defined in the coordinate system of the superview. The bounds is in the object's own coordinate system. So we can expect the origin of the bounds to always be (0, 0). I.e. the upper left corner is always in the same place when you're inside the view. But from outside the view, in the superview, you see the object's frame. The origin of the frame could be anywhere relative to the superview.
When a subview is added, its frame is given in the superview's coordinate system. So if the origin of the subview's frame is (0, 0), that means its upper left corner is in the same place as the superview's upper left corner. But the superview's upper left corner could be at the point (100, 200) on the screen.
Not sure if it will confuse you even more than the above, but try adding this line at the top of the code you posted:
[[UIApplication sharedApplication] setStatusBarHidden:YES animated:NO];
- Ray