关键值察

设置键值观察。

在这种情况下，我们想要观察我们观察者拥有的对象上的 contentOffset

//
// Class to observe
//
@interface XYZScrollView: NSObject
@property (nonatomic, assign) CGPoint contentOffset;
@end

@implementation XYZScrollView
@end

//
// Class that will observe changes
//
@interface XYZObserver: NSObject
@property (nonatomic, strong) XYZScrollView *scrollView;
@end

@implementation XYZObserver

// simple way to create a KVO context
static void *XYZObserverContext = &XYZObserverContext;

// Helper method to add self as an observer to 
// the scrollView's contentOffset property
- (void)addObserver {

    // NSKeyValueObservingOptions
    //
    // - NSKeyValueObservingOptionNew
    // - NSKeyValueObservingOptionOld
    // - NSKeyValueObservingOptionInitial
    // - NSKeyValueObservingOptionPrior
    //
    // can be combined:
    // (NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld)
    
    NSString *keyPath = NSStringFromSelector(@selector(contentOffset));
    NSKeyValueObservingOptions options = NSKeyValueObservingOptionNew;    

    [self.scrollView addObserver: self 
                      forKeyPath: keyPath 
                         options: options
                         context: XYZObserverContext];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSString *,id> *)change context:(void *)context {
    
    if (context == XYZObserverContext) { // check the context

        // check the keyPath to see if it's any of the desired keyPath's.
        // You can observe multiple keyPath's
        if ([keyPath isEqualToString: NSStringFromSelector(@selector(contentOffset))]) {

            // change dictionary keys:
            // - NSKeyValueChangeKindKey
            // - NSKeyValueChangeNewKey
            // - NSKeyValueChangeOldKey
            // - NSKeyValueChangeIndexesKey
            // - NSKeyValueChangeNotificationIsPriorKey
            
            // the change dictionary here for a CGPoint observation will
            // return an NSPoint, so we can take the CGPointValue of it.
            CGPoint point = [change[NSKeyValueChangeNewKey] CGPointValue];
            
            // handle point
        }
        
    } else {

        // if the context doesn't match our current object's context
        // we want to pass the observation parameters to super
        [super observeValueForKeyPath: keyPath
                             ofObject: object
                               change: change
                              context: context];
    }
}

// The program can crash if an object is not removed as observer 
// before it is dealloc'd
//
// Helper method to remove self as an observer of the scrollView's
// contentOffset property
- (void)removeObserver {
    NSString *keyPath = NSStringFromSelector(@selector(contentOffset));
    [self.scrollView removeObserver: self forKeyPath: keyPath];
}

@end