NSInvocation.md

NSInvocation

Encapsulates an Objective-C message (method call) that can be stored and invoked later. Supports variable arguments and MetaABI.

Base Class

NSObject

Instance Variables

NSMethodSignature   *_methodSignature;    // Method type information
char               *_storage;             // Argument storage
char               *_sentinel;            // Storage bounds check
char                _argumentsRetained;   // Arguments retention flag
char                _returnValueRetained; // Return value retention flag

Methods

Creation

• +invocationWithMethodSignature: - Creates invocation from signature
• +mulleInvocationWithTarget:selector: - Creates with target/selector
• +mulleInvocationWithTarget:selector:object: - Creates with single argument
• +mulleInvocationWithTarget:selector:metaABIFrame: - Creates with MetaABI frame

Arguments

• -getArgument:atIndex: - Gets argument value
• -setArgument:atIndex: - Sets argument value
• -retainArguments - Retains object arguments
• -argumentsRetained - Checks if arguments retained

Target/Selector

• -target - Gets target object
• -setTarget: - Sets target object
• -selector - Gets selector
• -setSelector: - Sets selector

Invocation

• -invoke - Invokes on current target
• -invokeWithTarget: - Invokes on specified target

Return Value

• -getReturnValue: - Gets return value
• -setReturnValue: - Sets return value
• -mulleRetainReturnValue - Retains return value
• -mulleReturnValueRetained - Checks if return value retained
• -mulleIntReturnValue - Gets integer return value

Usage Example

// Create invocation
NSMethodSignature *sig = [target methodSignatureForSelector:@selector(doWork:)];
NSInvocation *inv = [NSInvocation invocationWithMethodSignature:sig];
[inv setTarget:target];
[inv setSelector:@selector(doWork:)];

// Set arguments
id arg = @"data";
[inv setArgument:&arg atIndex:2];  // 0=self, 1=_cmd, 2=first arg

// Retain arguments if needed
[inv retainArguments];

// Invoke
[inv invoke];

// Get return value
id result;
[inv getReturnValue:&result];

Important Notes

1. Memory Management

   • Arguments not retained by default
   • Use retainArguments for object arguments
   • Return values not retained by default
   • Clean up retained values properly

2. Argument Indexing

   • Index 0 is self (target)
   • Index 1 is _cmd (selector)
   • User arguments start at index 2

3. MetaABI Support

   • Supports mulle-objc MetaABI
   • Efficient parameter passing
   • Frame-based invocation

4. Thread Safety

   • Not thread-safe by default
   • Synchronize access when shared
   • Consider thread affinity

5. Performance

   • More overhead than direct calls
   • Cache invocations when possible
   • Use for dynamic dispatch needs