目标C中的关键值编码和关键值观察是什么?

id someValue = [myObject valueForKeyPath:@"foo.bar.baz"];

id someValue = [[[myObject foo] bar] baz];

[myObject addObserver:self forKeyPath:@"foo.bar.baz" options:0 context:NULL];

// Here is a new instance of an object
Foo *foo = [[Foo alloc] init];
// Accessing a property called someValue with literal syntax:
[foo someValue];
// Accessing the same property with dot notation
foo.someValue;
// Accessing the same property with Key-Value coding:
[foo valueForKey:@"someValue"];

NSNumber* foo = [myPopup valueForKey: @"selectedItemIndex"];
[myPopup setValue: @15 forKey: @"selectedItemIndex"];

void*    gMyKVOContext = &gMyKVOContext; // global variable somewhere that guarantees us a unique address that doesn't collide with a subclass's registration for observing the same property

...

[interestingObject addObserver: interestedObject forKeyPath: @"interestingProperty" options: 0 context: gMyKVOContext];

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if( context == gMyKVOContext && [keyPath isEqualToString: @"interestingProperty"] )
    {
        // Update UI that shows interestingProperty
    }
    else
        [super observeValueForKeyPath: keyPath ofObject: object change: change context: context];
}

-(void)  setFoo: (int)inFoo
{
    [self willChangeValueForKey: @"foo"];
    _foo = inFoo;
    [self didChangeValueForKey: @"foo"];
}

-(void) setFoo: (int)inFoo bar: (int)inBar
{
    [self willChangeValueForKey: @"foo"];
    [self willChangeValueForKey: @"bar"];
    _foo = inFoo;
    _bar = inBar;
    [self didChangeValueForKey: @"bar"];
    [self didChangeValueForKey: @"foo"];
}

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