Navigation bar show/hide

First read the the section in the View Controller Programming Guide for iOS about 'Adopting a Full-Screen Layout for Navigation Views' and the section about the same for Custom Views. If you are trying to do something like the Photos.app then you are probably using a scroll view. Note the comment that Navigation bars automatically add a scroll content inset to your scroll view to account for the height of the navigation bar (and status bar) so you have to reset the contentInset property of your scroll view back to zero (UIEdgeInsetsZero) right after setting up the initial state of the navigationBar and before the view appears.

Then if you have a single tap that toggles the navigationBar and/or status bar to show or hide, you need to do two things in you toggling method. The first seems to be to save the scroll view's contentOffset property before changing the NavigationBar hidden property and restore your saved value to contentOffset right afterward. And second to again zero out the contentInset property to UIEdgeInsetsZero after changing the navigationBarHidden property. Also, if you are toggling the status bar, you need to change its state before you change the navigationBar's state.

This code will help you.

UITapGestureRecognizer *tapGesture = [[UITapGestureRecognizer alloc] 
initWithTarget:self action:@selector(showHideNavbar:)];
[self.view addGestureRecognizer:tapGesture];

-(void) showHideNavbar:(id) sender 
{ 
// write code to show/hide nav bar here 
// check if the Navigation Bar is shown
if (self.navigationController.navigationBar.hidden == NO)
{
// hide the Navigation Bar
[self.navigationController setNavigationBarHidden:YES animated:YES];
}
// if Navigation Bar is already hidden
else if (self.navigationController.navigationBar.hidden == YES)
{
// Show the Navigation Bar
[self.navigationController setNavigationBarHidden:NO animated:YES];
}
}

This isn't something that can fit into a few lines of code, but this is one approach that might work for you.

To hide the navigation bar:

[[self navigationController] setNavigationBarHidden:YES animated:YES];

To show it:

[[self navigationController] setNavigationBarHidden:NO animated:YES];

Documentation for this method is available here.

To listen for a "double click" or double-tap, subclass UIView and make an instance of that subclass your view controller's view property.

In the view subclass, override its -touchesEnded:withEvent: method and count how many touches you get in a duration of time, by measuring the time between two consecutive taps, perhaps with CACurrentMediaTime(). Or test the result from [touch tapCount].

If you get two taps, your subclassed view issues an NSNotification that your view controller has registered to listen for.

When your view controller hears the notification, it fires a selector that either hides or shows the navigation bar using the aforementioned code, depending on the navigation bar's current visible state, accessed through reading the navigation bar's isHidden property.

EDIT

The part of my answer for handling tap events is probably useful back before iOS 3.1. The UIGestureRecognizer class is probably a better approach for handling double-taps, these days.

EDIT 2

The Swift way to hide the navigation bar is:

navigationController?.setNavigationBarHidden(true, animated: true)

To show it:

navigationController?.setNavigationBarHidden(false, animated: true)