Objective-C Fundamentals

Runtime type information
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The if statement is designed to avoid the length method call when someString is
nil, indicating no string is available:
NSString *someString = ...;
int numberOfCharacters = [someString length];
If a message is sent to nil, or if a target object decides to completely ignore a message,
the message in most cases will return the equivalent of zero. This means the code snippet
just outlined would set numberOfCharacters to zero when you attempted to send
the length message to the someString object.
8.4 Runtime type information
Objective-C provides rich type information at runtime about the objects and classes
that live in your application. At its simplest, you can query an object to determine its
type by sending it an isKindOfClass: message, as shown here:
id someObject = ...;
if ([someObject isKindOfClass:[CTRentalProperty class]]) {
NSLog(@"This object is a kind of CTRentalProperty (or a subclass)");
}
The isKindOfClass: message returns true if the object is an instance of the specified
class or is compatible with it (the object is of a subclass).
8.4.1 Determining if a message will respond to a message
If an object receives a message and determines it can’t process it, an error typically
results. In some scenarios, especially with protocols with @optional messages, it can
be helpful to determine if a particular message send will successfully execute or result
in an error.
Therefore, the NSObject class provides a message named respondsToSelector:
that can be used to determine if an object will respond to (if it’ll handle or process) a
given selector. You can use the respondsToSelector: message as follows:
if ([anObject respondsToSelector:@selector(setAddress:)])
[anObject setAddress:@"13 Adamson Crescent"];
else
NSLog(@"This object does not respond to the setAddress: selector");
The respondsToSelector: test is especially important when sending messages to
objects that you don’t have control over at compile time, particularly if the object is
typed as id. For example, if you write code that sends a message to an object represented
by a variable that others can set, you should make sure the receiver implements
a method that can respond to the message.
8.4.2 Sending a message generated at runtime
Sometimes you need to send a message to an object but don’t know the name or set of
arguments that must be included until runtime. NSObject comes to the rescue again by
providing messages named performSelector:, performSelector:withObject:, and