Add Observer to BOOL variable

Question

Is it possible to add observers to simple variables such as BOOLs or NSIntegers and see when they change?

Thanks!

Answer 1

You observe keys to be notified when their value changes. The data type can be anything. For anything defined as an Objective-C property (with @property in the .h file) this is ready to go so if you want to observe a BOOL property you add to a view controller you do it as follows:

in myViewController.h:

@interface myViewController : UIViewController {
    BOOL      mySetting;
}

@property (nonatomic)    BOOL    mySetting;

in myViewController.m

@implementation myViewController

@synthesize mySetting;

// rest of myViewController implementation

@end

in otherViewController.m:

// assumes myVC is a defined property of otherViewController

- (void)presentMyViewController {
    self.myVC = [[[MyViewController alloc] init] autorelease];
    // note: remove self as an observer before myVC is released/dealloced
    [self.myVC addObserver:self forKeyPath:@"mySetting" options:0 context:nil];
    // present myVC modally or with navigation controller here
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if (object == self.myVC && [keyPath isEqualToString:@"mySetting"]) {
        NSLog(@"OtherVC: The value of self.myVC.mySetting has changed");
    }
}

Answer 2

If they are properties of objects then yes.

If they are not properties then no.

Answer 3

Yes; the only requirement is that the object in which those variables occur are key-value compliant for those properties.

Answer 4

I believe what you meant was: How to get INT or BOOL value from the 'change' dictionary if the property has changed.

You can simply do it this way:

- (void)observeValueForKeyPath:(NSString *)keyPath
                      ofObject:(id)object
                        change:(NSDictionary *)change
                       context:(void *)context
{
    if ([keyPath isEqualToString:@"mySetting"])
    {
        NSNumber *mySettingNum = [change objectForKey:NSKeyValueChangeNewKey];
        BOOL newSetting = [mySettingNum boolValue];
        NSLog(@"mySetting is %s", (newSetting ? "true" : "false")); 
        return;
    }

    [super observeValueForKeyPath:keyPath ofObject:object change:change context:context];
}