iPhone-属性，ivars等

Question

On this tutorial , the author has these declarations:

on .h

UIViewController *presentingViewController;
...

@property (retain) UIViewController *presentingViewController;

on .m

@synthesize presentingViewController;

at some point in code, inside a block , he does:

self.presentingViewController = viewController;

and then

[presentingViewController dismissModalViewControllerAnimated:NO];

I find this very strange. If is is assigning the viewController to self.presentingViewController, should't it be calling

    [self.presentingViewController dismissModalViewControllerAnimated:NO];

?

I have changed his code to

.h

@property (retain) UIViewController *presentingViewController;

.m

@synthesize presentingViewController = _presentingViewController;

and what I do is:

self.presentingViewController = viewController;

and then

    [self.presentingViewController dismissModalViewControllerAnimated:NO];

the problem is that, self.presentingViewController is nil at this line, even being declared as retain and never being released.

any clues?

thanks

Answer 1

You are right that it should be:

[self.presentingViewController dismissModalViewControllerAnimated:NO];

However, reinstate this:

@synthesize presentingViewController;

And remove the instance decoration of a similar looking variable:

UIViewController *presentingViewController;

Answer 2

Did you remove the instance variable UIViewController *presentingViewController when you added the instance variable UIViewController *_presentingViewController ? If not i would put money on you accidentally using the wrong one at some point.

Stepping through.. originally you had an instance variable

UIViewController *presentingViewController;

And from the property syntactic sugar you have the two accessor methods for setting and getting the variable

- (UIViewController *)presentingViewController;
- (void)setPresentingViewController:(UIViewController *)val;

at some point in code, inside a block, he does:

[self setPresentingViewController: viewController]; // uses setter

and then

[presentingViewController dismissModalViewControllerAnimated:NO]; // uses instance variable

which is fine, but you think it should be

UIViewController *localPresentingViewController = [self presentingViewController]; // uses getter
[localPresentingViewController dismissModalViewControllerAnimated:NO];

which is also fine but somewhat unnecessary.

Then you added a new instance variable:-

@synthesize presentingViewController = _presentingViewController;

so now you have

UIViewController *presentingViewController;
UIViewController *_presentingViewController;

// These now get / set _presentingViewController
- (UIViewController *)presentingViewController;
- (void)setPresentingViewController:(UIViewController *)val;

But almost certainly, somewhere you are confusing the two ivars (or you aren't aware you have two ivars) presentingViewController / _presentingViewController and still have a reference to the presentingViewController ivar, leading you to think your property is nil.