cocoa - KVO's add & remove observer methods – counted or idempotent? -

in kvo subscribe receive change notifications potential sender with:

[potentialsender addobserver: self                    forkeypath: nsstringfromselector(@selector(aproperty))                      options: 0                      context: mykvohandle]; 

and can unsubscribe with:

[potentialsender removeobserver: self                      forkeypath: nsstringfromselector(@selector(aproperty))                         context: mykvohandle]; 

where kvo handles might created this c idiom.

are calls these counted – every add must directly paired , precede remove – pre-arc retain , release?

alternatively, idempotent – number of add calls can stopped single remove, several remove operations without matching add safe?

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i believe idempotent approach used [acontrol addtarget:action:forcontrolevents:] and [anotificationcentre addobserver:selector:name:object:]. wrong notification centre addobserver calls. not idempotent. however, removes are. rob correcting me on this.

kvo calls must balanced. every addobserver:... must balanced equivalent removeobserver:.... , there no mechanism "unsubscribe all" there nsnotificationcenter. remove not balance add error. kvo picky.

note nsnotificationcenter absolutely not idempotent. multiple, identical addobserver:... calls result in multiple notifications. it's possible global removeobserver:self without balancing every addobserver:..., , safe remove when have not added.