NSMethodSignature Class Reference

Inherits from
Conforms to
Framework
/System/Library/Frameworks/Foundation.framework
Availability
Available in iOS 2.0 and later.
Declared in
NSMethodSignature.h

Overview

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. You typically create an NSMethodSignature object using NSObject’s methodSignatureForSelector: instance method (on OS X v10.5 and later you can also use signatureWithObjCTypes:). 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.

An NSMethodSignature object presents its argument types by index with the getArgumentTypeAtIndex: method. The hidden arguments for every method, self and _cmd, are at indices 0 and 1, respectively. The arguments normally specified in a message invocation follow these. In addition to the argument types, an NSMethodSignature object offers the total number of arguments with numberOfArguments, the total stack frame length occupied by all arguments with frameLength (this varies with hardware architecture), and the length and type of the return value with methodReturnLength and methodReturnType. Finally, applications using distributed objects can determine if the method is asynchronous with the isOneway method.

Tasks

Creating a Method Signature Object

Getting Information on Argument Types

Getting Information on Return Types

Determining Synchronous Status

Class Methods

signatureWithObjCTypes:

Returns an NSMethodSignature object for the given Objective C method type string.

+ (NSMethodSignature *)signatureWithObjCTypes:(const char *)types
Parameters
types

An array of characters containing the type encodings for the method arguments.

Indices begin with 0. The hidden arguments self (of type id) and _cmd (of type SEL) are at indices 0 and 1; method-specific arguments begin at index 2.

Return Value

An NSMethodSignature object for the given Objective C method type string in types.

Discussion

Special Considerations

This method, available since OS X v10.0, is exposed in OS X v10.5. Only type encoding strings of the style of the runtime that the application is running against are supported. In exposing this method there is no commitment to binary compatibily supporting any "old-style" type encoding strings after such changes occur.

It is your responsibility to pass in type strings which are either from the current runtime data or match the style of type string in use by the runtime that the application is running on.

Availability
  • Available in iOS 2.0 and later.
Declared In
NSMethodSignature.h

Instance Methods

frameLength

Returns the number of bytes that the arguments, taken together, occupy on the stack.

- (NSUInteger)frameLength
Return Value

The number of bytes that the arguments, taken together, occupy on the stack.

Discussion

This number varies with the hardware architecture the application runs on.

Availability
  • Available in iOS 2.0 and later.
Declared In
NSMethodSignature.h

getArgumentTypeAtIndex:

Returns the type encoding for the argument at a given index.

- (const char *)getArgumentTypeAtIndex:(NSUInteger)index
Parameters
index

The index of the argument to get.

Return Value

The type encoding for the argument at index.

Discussion

Indices begin with 0. The hidden arguments self (of type id) and _cmd (of type SEL) are at indices 0 and 1; method-specific arguments begin at index 2. Raises NSInvalidArgumentException if index is too large for the actual number of arguments.

Argument types are given as C strings with Objective-C type encoding. This encoding is implementation-specific, so applications should use it with caution.

Availability
  • Available in iOS 2.0 and later.
Declared In
NSMethodSignature.h

isOneway

Returns a Boolean value that indicates whether the receiver is asynchronous when invoked through distributed objects.

- (BOOL)isOneway
Return Value

YES if the receiver is asynchronous when invoked through distributed objects, otherwise NO.

Discussion

If the method is oneway, the sender of the remote message doesn’t block awaiting a reply.

Availability
  • Available in iOS 2.0 and later.
Declared In
NSMethodSignature.h

methodReturnLength

Returns the number of bytes required for the return value.

- (NSUInteger)methodReturnLength
Return Value

The number of bytes required for the return value.

Availability
  • Available in iOS 2.0 and later.
Declared In
NSMethodSignature.h

methodReturnType

Returns a C string encoding the return type of the method in Objective-C type encoding.

- (const char *)methodReturnType
Return Value

A C string encoding the return type of the method in Objective-C type encoding.

Discussion

This encoding is implementation-specific, so applications should use it with caution.

Availability
  • Available in iOS 2.0 and later.
Declared In
NSMethodSignature.h

numberOfArguments

Returns the number of arguments recorded in the receiver.

- (NSUInteger)numberOfArguments
Return Value

The number of arguments recorded in the receiver.

Discussion

There are always at least 2 arguments, because an NSMethodSignature object includes the hidden arguments self and _cmd, which are the first two arguments passed to every method implementation.

Availability
  • Available in iOS 2.0 and later.
Declared In
NSMethodSignature.h