KVO: Apparently the same "effective" keyPath produce different results

Question

I have struggled with the setup of KVO for a few hours and I managed to get it to work:

This works:

 [self addObserver:self forKeyPath:@"session.loginState" options:0 context:nil];

This doesn't:

 [self addObserver:self.session forKeyPath:@"loginState" options:0 context:nil];

Please note that self.session lazily creates an empty Session object so self.session is never nil. However, it seems that:

1. the keypath session.loginState of self is not the same as...
2. the keypath loginState of self.session from a KVO perspective

Why is this the case?

Answer 1

You have your observer and observee backwards. Try

[self.session addObserver:self forKeyPath:@"loginState" options:0 context:nil];

Answer 2

With this code:

 [self addObserver:self forKeyPath:@"session.loginState" options:0 context:nil];

You are adding self as an observer of self's keypath session.loginState, that is effectively self.session.loginState.

In this code:

 [self addObserver:self.session forKeyPath:@"loginState" options:0 context:nil];

You are adding self.session as an observer of self's keypath loginState, that is effectively self.loginState.