The difference between KVC and KVO

VC is a kind of indirect manipulation of object properties through strings of the mechanism, Accessing an object property we can person.age also can be KVC by the way [person valueforkey:@ ' age '] KeyPath is attribute-chained access such as person.address.street a bit like the Pojo OGNL expression in Java If the given string does not have an object's property to access the Valueforundefinekey method the default implementation is raise an exception but you can rewrite this method, SetValue is also the same reason Key path Accounts.transactions.payee would return of the payee objects, for all the transactions, in all th E accounts. Throws an exception when you set an nil property to be an object, but you can also override the method KVO is an observer pattern that is implemented at the language framework level to actively notify the observer when modifying the attribute in a KVC manner Example Person class @implementation person @synthesize Name,age; -(void) changename{ name=@ "ChangeName directly"; } @end The Personmonitor class monitors the Name property @implementation Personmonitor -(void) Observevalueforkeypath: (NSString *) keypath Ofobject: (ID) object Change: (nsdictionary *) change Context: (void *) context { if ([KeyPath isequal:@ "name"]) { NSLog (@ "Change happen, old:%@ new:%@", [Change Objectforkey:nskeyvaluechangeoldkey],[change Objectforkey: Nskeyvaluechangenewkey]); } } @end Test code Person *p =[[person alloc] init]; Personmonitor *pm= [[Personmonitor alloc]init]; [P addobserver:pm forkeypath:@ ' name ' options: (Nskeyvalueobservingoptionnew | Nskeyvalueobservingoptionold) Context:nil]; NSLog (@ "P.name is%@", p.name); [P setvalue:@ "name KVC" forkey:@ "name"]; NSLog (@ "p name Get by KVC is%@", [P valueforkey:@ "name"]); p.name=@ "Name change by. Name="; [P changename]; Can ' t be notify Output 2011-07-03 16:35:57.406 cocoa[13970:903] P.name is name 2011-07-03 16:35:57.418 cocoa[13970:903] change happen, Old:name New:name KVC 2011-07-03 16:35:57.420 cocoa[13970:903] p name Get by KVC is name KVC 2011-07-03 16:35:57.421 cocoa[13970:903] Change happen old:name KVC. new:name The last modification was a direct modification so no notification could be generated.