oc kvc的模式：匹配搜索模式（模式匹配）、装包解包

按照一定规则使用匹配模式在目标空间进行搜索，然后执行相应操作；

运行时系统将kvc的运行机制解释为模式匹配，将值的兼容性问题解释为装包解包问题

一、模式匹配

The default implementation of the NSKeyValueCoding protocol provided by NSObject maps key-based accessor calls to an object’s underlying properties using a clearly defined set of rules. These protocol methods use a key parameter to search their own object instance for accessors, instance variables, and related methods that follow certain naming conventions. Although you rarely modify this default search, it can be helpful to understand how it works, both for tracing the behavior of key-value coded objects, and for making your own objects compliant.

Search Pattern for the Basic Setter

The default implementation of setValue:forKey:, given key and value parameters as input, attempts to set a property named key to value (or, for non-object properties, the unwrapped version of value, as described in Representing Non-Object Values) inside the object receiving the call, using the following procedure:

1. Look for the first accessor named set<Key>: or _set<Key>, in that order. If found, invoke it with the input value (or unwrapped value, as needed) and finish. 
2. If no simple accessor is found, and if the class method accessInstanceVariablesDirectly returns YES, look for an instance variable with a name like _<key>, _is<Key>, <key>, or is<Key>, in that order. If found, set the variable directly with the input value (or unwrapped value) and finish. 
3. Upon finding no accessor or instance variable, invoke setValue:forUndefinedKey:. This raises an exception by default, but a subclass of NSObject may provide key-specific behavior. 

Search Pattern for the Basic Getter

The default implementation of valueForKey:, given a key parameter as input, carries out the following procedure, operating from within the class instance receiving the valueForKey: call.

1. Search the instance for the first accessor method found with a name like get<Key>, <key>, is<Key>, or _<key>, in that order. If found, invoke it and proceed to step 5 with the result. Otherwise proceed to the next step. 
2. If no simple accessor method is found, search the instance for methods whose names match the patterns countOf<Key> and objectIn<Key>AtIndex: (corresponding to the primitive methods defined by the NSArray class) and <key>AtIndexes: (corresponding to the NSArray method objectsAtIndexes:).
   If the first of these and at least one of the other two is found, create a collection proxy object that responds to all NSArray methods and return that. Otherwise, proceed to step 3.
   The proxy object subsequently converts any NSArray messages it receives to some combination of countOf<Key>, objectIn<Key>AtIndex:, and <key>AtIndexes: messages to the key-value coding compliant object that created it. If the original object also implements an optional method with a name like get<Key>:range:, the proxy object uses that as well, when appropriate. In effect, the proxy object working together with the key-value coding compliant object allows the underlying property to behave as if it were an NSArray, even if it is not. 
3. If no simple accessor method or group of array access methods is found, look for a triple of methods named countOf<Key>, enumeratorOf<Key>, and memberOf<Key>: (corresponding to the primitive methods defined by the NSSet class).
   If all three methods are found, create a collection proxy object that responds to all NSSet methods and return that. Otherwise, proceed to step 4.
   This proxy object subsequently converts any NSSet message it receives into some combination of countOf<Key>, enumeratorOf<Key>, and memberOf<Key>: messages to the object that created it. In effect, the proxy object working together with the key-value coding compliant object allows the underlying property to behave as if it were an NSSet, even if it is not. 
4. If no simple accessor method or group of collection access methods is found, and if the receiver's class method accessInstanceVariablesDirectly returns YES, search for an instance variable named _<key>, _is<Key>, <key>, or is<Key>, in that order. If found, directly obtain the value of the instance variable and proceed to step 5. Otherwise, proceed to step 6. 
5. If the retrieved property value is an object pointer, simply return the result.
   If the value is a scalar type supported by NSNumber, store it in an NSNumber instance and return that.
   If the result is a scalar type not supported by NSNumber, convert to an NSValue object and return that. 
6. If all else fails, invoke valueForUndefinedKey:. This raises an exception by default, but a subclass of NSObject may provide key-specific behavior. 

二、装包解包

setValue:forKey: - Sets the value of the specified key relative to the object receiving the message to the given value. The default implementation of setValue:forKey: automatically unwraps NSNumber and NSValue objects that represent scalars and structs and assigns them to the property. See Representing Non-Object Values for details on the wrapping and unwrapping semantics.

When you invoke one of the protocol’s getters, such as valueForKey:, the default implementation determines the particular accessor method or instance variable that supplies the value for the specified key according to the rules described in Accessor Search Patterns. If the return value is not an object, the getter uses this value to initialize an NSNumber object (for scalars) or NSValue object (for structures) and returns that instead.