Introspection

參考:http://psvsps2.blogspot.tw/2009/07/cocoa-fundamental-iphone-part2_31.html

動態型別(Dynamically type)和靜態型別(Statically Type)的優劣爭論持續了好一陣子， 動態型別程式語言帶來的靈活性，靜態型別的嚴謹各有千秋，動態型別表現傑出，幾個重要的程式語言如 python ， ruby 都有相當亮眼的表現，Objective-C 也是一種動態型別的程式語言，通常動態型別程式語言會提供比較多的自省(introspection)能力，如 runtime 檢查type，檢查method等等。

 Objective-C’s runtime support allows you to discover various properties of objects at
execution time. This process is called introspection. You can find out an object’s class and
superclass using the methods class and superclass, as follows:

Class objectsClass;
Class objectsSuperClass;
id anObject;
objectsClass = [anObject class];
objectsSuperClass = [anObject superclass];

蘋果原始資料
http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocObjectsClasses.html

Introspection refers to the inherent ability of an object to divulge, upon request, its essential characteristics at runtime. By sending objects certain messages, you can ask them questions about themselves as objects and the Objective-C runtime provides you with answers. Introspection is an important coding tool because it makes your programs more efficient and robust.

因為所有的class都繼承自NSObject，因此有三個方便來辨認現在的狀態

All objects that inherit from NSObject know these methods
isKindOfClass: returns whether an object is that kind of class (inheritance included)
isMemberOfClass: returns whether an object is that kind of class (no inheritance)
respondsToSelector: returns whether an object responds to a given method

例如
You get a Class by sending the class method class to a class :)
if ([obj isKindOfClass:[NSString class]]) {
    NSString *s = [(NSString *)obj stringByAppendingString:@”xyzzy”];
}

Types of Introspection Information

The NSObject protocol, which is adopted by the NSObject class, defines introspection methods that yield the following kinds of information about an object:

• Class membership. To determine if an object inherits, directly or indirectly, from a particular class, send it an isKindOfClass: message and evaluate the result. This method tells you if the object is a direct instance of the given class. You can also use the class and superclass methods to obtain the class or superclass of an object and then use that result in comparison operations.
• Messages responded to. To find out if an object’s class or superclass implements a method, send the object a respondsToSelector: message. The parameter is a SEL-typed value constructed from the signature of the method using the @selector directive. For example:
  

  BOOL doesRespond = [anObject respondsToSelector:@selector(writeToFile:atomically:)];

• Protocol conformance. If a class conforms to a formal protocol, you can expect it to implement the required methods of that protocol and send messages to it accordingly. Use the conformsToProtocol: method to obtain this information. You specify the argument of this method using the @protocol directive.