Key-value Observation

If you want to be a key observer, add the following code.

[Theappdelegate addobserver:self forkeypath:@ "Fido" Options:nskeyvalueobservingoptionold Context:nil];

The above method is defined in NSObject, which is actually similar to saying, "Send me a message whenever Fido changes", and the options and context decide what additional data will be sent with the message when the Fido changes. The triggering method procedure is as follows

-(void) Observevalueforkeypath: (NSString *) KeyPath Ofobject: (ID) object change: (nsdictionary *) Change context: (void *) Context

In this example, the keypath should be @ "Fido", object should be self-represented, and context is the parameter change of the context in the Add key observation is the dictionary that holds the old and new values of the Fido (a collection of key-value pairs)

Removing observers

[Theappdelegate removeobserver:self forkeypath:@ "Fido"];

Key-value Observation