notification for a view x position

Question

in a slide menu I'm developing for my project i would like to add a black view over the content view when it's slide out. To do this i need to create a method that check continuously the view x-position and darken or brighten up the black layer. The position of this view is the same as the content view. I thought i can use a NSNotificationCenter like this:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(incomingNotification:) name:@"notification" object: darkViewController.view.frame.origin.x]];    

and a method:

- (void) incomingNotification:(NSNotification *)notification{
// the dark layer alpha will be 0 at origin=0 and 0.8 at bounds.size.width
float alphaToUse =  (darkViewController.view.frame.origin.x / self.view.bounds.size.width) * 0.8;
[darkViewController.view setAlpha:alphaToUse];
}

The problem is that i must use an object as parameter. I'm new to notifications so i'm asking: is it wrong to use them for this kind of things? Is it better to solve this in another way?

EDIT: Following Denis advice i'm now trying to use the key-value-observe solution. My app is structured like this: MenuViewController-->ContainerViewController-->DarkViewController

In MenuViewController.m :

@interface MenuViewController ()

@property (strong,nonatomic) ContainerViewController *containerViewController;
@property (strong,nonatomic) DarkViewController *darkViewController;

@end

@implementation MenuViewController

@synthesize containerViewController,darkViewController;

# pragma mark - Views

- (void)viewDidLoad{
[super viewDidLoad];    
containerViewController = [[ContainerViewController alloc]init];
[self addChildViewController:containerViewController];
[self.view addSubview:containerViewController.view];
[containerViewController didMoveToParentViewController:self];

darkViewController = [[DarkViewController alloc]init];
[containerViewController addChildViewController:darkViewController];
[containerViewController.view addSubview:darkViewController.view];
[darkViewController didMoveToParentViewController:containerViewController];

[UIView animateWithDuration:slideDuration delay:0 options:UIViewAnimationOptionBeginFromCurrentState animations:^{
    [darkViewController.view setAlpha:0.7];
    containerViewController.view.frame = CGRectMake(self.view.frame.size.width - slideWidth, 0, self.view.frame.size.width, self.view.frame.size.height);
}
                 completion:^(BOOL finished) {
                     if (finished) {
                     }
                 }];

[darkViewController addObserver:self forKeyPath:@"darkViewController.view.frame.origin.x" options:NSKeyValueObservingOptionNew context:nil];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:    (NSDictionary *)change context:(void *)context
{
NSLog(@"x is changed");
}

When i run this i get this exception:

*** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<DarkViewController 0x10962d280> addObserver:<MenuViewController 0x10922c890> forKeyPath:@"darkViewController.view.frame.origin.x" options:1 context:0x0] was sent to an object that is not KVC-compliant for the "darkViewController" property.'

Ok, it seems that i found a solution following this example Notificationsin IOS

I just added this in the viewDidLoad of my ContainerViewController

    [self addObserver:self forKeyPath:@"view.frame" options:0 context:nil];

and implemented the observer method with a for cycle to find my DarkViewController view

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
for (UIViewController * vc in self.childViewControllers) {
    if ([vc isKindOfClass:[DarkViewController class]]) {
        float alphaToUse =  (self.view.frame.origin.x / self.view.bounds.size.width)  * 0.8;
        [vc.view setAlpha:alphaToUse];
    }
}
}

Now i just have to understand where to put the removeObserver method, since my ContainerViewController will be always loaded...

Answer 1

There is another machanism in iOS for such kind of things called Key value coding and Key value observing.

From Notification Center documentation:

As you design your application, do not simply assume that you should send a notification to communicate with interested parties. You should also consider alternatives such as key-value observing, key-value binding, and delegation.

Key-value binding and key-value observing were introduced in OS X version 10.3 and provide a way of loosely coupling data. With key-value observing, you can request to be notified when the properties of another object change. Unlike regular notifications, there is no performance penalty for unobserved changes. There is also no need for the observed object to post a notification because the key-value observing system can do it for you automatically, although you can still choose do it manually.

So if you'll have another notification observers while making slide menu animation it may reduce its handling performance.

And the best solution would be to invoke incomingNotification: method inside the animation block (the method where animation performs).

Apple docs again:

Though key-value coding is efficient, it adds a level of indirection that is slightly slower than direct method invocations. You should use key-value coding only when you can benefit from the flexibility that it provides.

ANSWERING EDITED QUESTION:

This answer describes exactly what you're trying to do. When add the observer on some object's property object's name shouldn't be included in the property key path. So in you case adding an observer looks like this:

[darkViewController addObserver:self forKeyPath:@"view.frame" options:NSKeyValueObservingOptionNew context:nil];

When trying to observe some object property don't forget to ensure the object's class is KVC compliant for that property!

And also don't forget to remove the observers after job is done.