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For interprocess communication, you should use XPC instead; see XPC Services API Reference for more information.
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.
NSInvocation
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.
NSMethodSignature
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
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.
NSPortMessage
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
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:
.
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