问题
1) What is the difference between the three lines of code below?
2) Also why does only the third line of code work if I'm returning to a viewController and the previous viewController has set the navigationBar to hidden through the same approach [self.navigationController setNavigationBarHidden:NO] my assumption was that all three lines do the same thing?
self.navigationController.navigationBar.hidden = NO;
[self.navigationController.navigationBar setHidden:NO];
[self.navigationController setNavigationBarHidden:NO];
Follow up:
Why when I need to run this code:
[self.navigationController.navigationBar setBackgroundImage:incorrectAnswerNavigationBarBackgroundImage forBarMetrics:UIBarMetricsDefault];
It only works, working being setting the background image, otherwise the nav bar is just white.
if I have both these lines:
[self.navigationController setNavigationBarHidden:NO];
self.navigationController.navigationBar.hidden = NO;
If I leave out self.navigationController.navigationBar.hidden = NO; the space for the nav bar pops down but it's just white, there is no background image. If I have both lines it works and there is a background image.
回答1:
The first two are functionally identical; the difference being one uses the dot notation while the other doesn't. These two methods both fire - (void) setHidden:(BOOL)hide on the navigationBar property on the navigation controller.
Now the third one is a completely different method. It's - (void) setNavigationBarHidden:(BOOL)hide and is defined on UINavigationController. The reason why this one works is this method is informing the navigation controller that you wish the navigation bar to be hidden while the first two manually set the navigation bar to be hidden. The first two's changes are undone if UINavigationController calls any methods that modify the hidden property of the navigation bar, hence why the setNavigationBarHidden: method was created so you'd have a way of informing UINavigationController that no matter what it does, it should hide the navigation bar and not change it to be showing.
EDIT: For the second part of this question, you actually need to be calling - (void)setNavigationBarHidden:(BOOL)hidden animated:(BOOL)animated on UINavigationController. That's the proper documented method for UINavigationController.
来源：https://stackoverflow.com/questions/16270616/hide-navigationbar