Message Encapsulation

This section describes the classes used by the distributed objects system to encapsulate messages passed over a connection. Unless you are getting involved with the low-level details of distributed objects, you should never need to use these classes directly. NSInvocation and NSMethodSignature, however, have uses outside of distributed objects, so you may encounter them in other situations.


An NSInvocation object is an Objective-C message rendered static, an action turned into an object. NSInvocation objects are used to store and forward messages between objects and between applications, primarily by NSTimer and the distributed objects system. An NSInvocation object contains all the elements of an Objective-C message: a target, a selector, arguments, and the return value. Each of these elements can be set directly, and the return value is set automatically when the invocation is dispatched.

An NSInvocation object can be repeatedly dispatched to different targets; its arguments can be modified between dispatch for varying results; even its selector can be changed to another with the same method signature (argument and return types). This makes it useful for repeating messages with many arguments and variations; rather than retyping a slightly different expression for each message, you modify the NSInvocation object as needed each time before dispatching it to a new target.

For examples of how NSInvocation is used, see Using NSInvocation.


An NSMethodSignature object records type information for the arguments and return value of a method. It is used to forward messages that the receiving object does not respond to—most notably in the case of distributed objects. An NSMethodSignature object is typically created using NSObject's methodSignatureForSelector: instance method. It is then used to create an NSInvocation object, which is passed as the argument to a forwardInvocation: message to send the invocation on to whatever other object can handle the message. In the default case, NSObject invokes doesNotRecognizeSelector:, which raises an exception. For distributed objects, the NSInvocation object is encoded using the information in the NSMethodSignature object and sent to the real object represented by the receiver of the message.


NSPortCoder is a concrete subclass of NSCoder used in the distributed objects system to transmit object proxies (and sometimes objects themselves) between NSConnection objects. An NSPortCoder object is always created and used by an NSConnection object; your code should never need to explicitly create or use one.


An NSPortMessage object defines a low level, operating-system independent type for interapplication (and interthread) messages. NSPortMessage objects are used primarily by the distributed objects system. You should implement interapplication communication using distributed objects whenever possible, and use NSPortMessage objects only when necessary.

An NSPortMessage object has three major parts: the send and receive ports, which are NSPorts that link the sender of the message to the receiver, and the components, which form the body of the message. The components are held as an NSArray object containing NSData and NSPort objects. NSPortMessage’s sendBeforeDate: message sends the components out through the send port; any replies to the message arrive on the receive port. See the NSPort class specification for information on handling incoming messages.

An NSPortMessage object can be initialized with a pair of NSPort objects and an NSArray instance containing components. An NSPortMessage object’s body can contain only NSPort objects or NSData objects. In the distributed objects system the byte/character arrays are usually encoded NSInvocation objects that are being forwarded from a proxy to the corresponding real object.

An NSPortMessage object also maintains a message identifier, which can be used to indicate the class of a message, such as an Objective-C method invocation, a connection request, an error, and so on. Use the setMsgid: and msgid methods to access the identifier.


NSDistantObjectRequest objects are used by the Distributed Objects system to help handle invocations between different processes. You should never create NSDistantObjectRequest objects directly. Unless you are getting involved with the low-level details of Distributed Objects, there should never be a need to access an NSDistantObjectRequest object. The distant object request for an incoming message is sent to the connection’s delegate if it implements connection:handleRequest:.