An implentation of a TrampolineObject as described in HigherOrderMessaging. Notes follow below.

---

BSTrampoline.h

#import <Foundation/Foundation.h>

//It’d probably be nicer to use an enum here #define kDoMode 0 #define kCollectMode 1 #define kSelectMode 2 #define kRejectMode 3

@interface BSTrampoline : NSProxy { NSEnumerator *enumerator; int mode; NSArray *temp; //For returning from collect, select, reject }

• (id)initWithEnumerator:(NSEnumerator *)inEnumerator mode:(int)operationMode;
• (NSArray *)fakeInvocationReturningTempArray; //Like the name says @end

---

BSTrampoline.m

#import “BSTrampoline.h”

@implementation BSTrampoline

• (id)initWithEnumerator:(NSEnumerator *)inEnumerator mode:(int)operationMode { if (operationMode < kDoMode | operationMode > kRejectMode) [NSException raise:@”InvalidArgumentException” format:@”operationMode argument of initWithEnumerator:mode: was %d, outside %d-%d range.”, operationMode, kDoMode, kRejectMode]; enumerator = [inEnumerator retain]; mode = operationMode; return self; }

• (void)dealloc { [enumerator release]; [super dealloc]; }

• (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector { if (mode > kCollectMode) //For select&reject, return BOOL return [NSMethodSignature signatureWithObjCTypes:”c^v^c”]; else //For do&collect, return id (lowest common denominator) return [NSMethodSignature signatureWithObjCTypes:”@^v^c”]; }

• (void)forwardInvocation:(NSInvocation *)anInvocation { id index; //Next line unnecessary in kDoMode but the preprocessor doesn’t like it if I add a test up here:/ NSMutableArray *array = [[[NSMutableArray alloc] init] autorelease]; //The array to be returned id objRetVal; //Temporaries for reading off the return value of the bounced method BOOL boolRetVal;

  /*

  • This is the core of the trampoline. invokeWithTarget: bounces the method to a member of the array,
  • which is fine for -do, but for -collect, -select and -reject there’s some additional logic
  • needed. What we do is this:
  • COLLECT MODE
  • Get return value (making sure it’s an object)
  • Add to return array
  • SELECT/REJECT MODE
  • Get return value
  • Test whether to add receiving object to array

  • Do so if necessary */ while (index = [enumerator nextObject]) { [anInvocation invokeWithTarget:index];

    switch (mode) { case kDoMode: break; case kCollectMode: if (strcmp(anInvocation methodSignature] methodReturnType], “@”)) //Decoded: id [[[NSException raise:@”InvalidArgumentException” format:@”All array items must return objects for the given selector to use -collect. Return type is %s”, anInvocation methodSignature] methodReturnType; [anInvocation getReturnValue:&objRetVal]; [array addObject:objRetVal]; break; case kSelectMode: case kRejectMode: if (strcmp(anInvocation methodSignature] methodReturnType], “c”)) { //Decoded: char/BOOL [[[NSException raise:@”InvalidArgumentException” format:@”All array items must return bool for the given selector to use -select or -reject. Return type is %s”, anInvocation methodSignature] methodReturnType; } [anInvocation getReturnValue:&boolRetVal]; if (boolRetVal && mode == kSelectMode || !boolRetVal && mode == kRejectMode) [array addObject:index]; break; } }

  /*

  • Here’s our rather silly method of returning the array: we fix up the invocation to point
  • to our own getter method, and invoke that - so that when the flow of control resumes,
  • it’s the last valid invocation. NSProxy is weird. Note that first we make sure that the
  • method signature is @^v^c to return an NSArray* - if we’re in select or reject mode, the
  • default will be c^v^c, returning a BOOL. That would cause a segfault. */ if (mode != kDoMode) { temp = array; [anInvocation initWithMethodSignature:[NSMethodSignature signatureWithObjCTypes:”@^v^c”]]; [anInvocation setSelector:@selector(fakeInvocationReturningTempArray)]; [anInvocation invokeWithTarget:self]; } }

• (NSArray *)fakeInvocationReturningTempArray { return temp; }

@end

---

Feel free to use this code for anything you want, although it’d be nice if you credited the original author (ThomasCastiglione).

BSTrampoline differs slightly from RobRix’s description of a trampoline in that it makes use of NSEnumerator, making the implementation of categories on collection objects to use it trivial. For example, categories on NSArray:

#import <Foundation/Foundation.h> #import “BSTrampoline.h”

@interface NSArray(HigherOrderMessaging)

• (id)do;
• (id)collect;
• (id)select;
• (id)reject;

@end

@implementation NSArray(HigherOrderMessaging)

• (id)do { return [[[BSTrampoline alloc] initWithEnumerator:[self objectEnumerator] mode:kDoMode] autorelease]; }

• (id)collect { return [[[BSTrampoline alloc] initWithEnumerator:[self objectEnumerator] mode:kCollectMode] autorelease]; }

• (id)select { return [[[BSTrampoline alloc] initWithEnumerator:[self objectEnumerator] mode:kSelectMode] autorelease]; }

• (id)reject { return [[[BSTrampoline alloc] initWithEnumerator:[self objectEnumerator] mode:kRejectMode] autorelease]; }

@end

If anybody has any suggestions for ways to improve the code or finds bugs in it, please say so. A simple test rig can be found at BSTrampolineTestRig - it will work with the provided code if NSArray(HigherOrderMessaging) is split into header and implementation files.

Regarding undocumented APIs: BSTrampoline makes use of two method calls for which Apple provides no documentation or public headers. Specifically, NSMethodSignature + signatureWithObjCTypes and NSInvocation -initWithMethodSignature. The former is necessary to create method signatures at all, and the latter to reuse a forwarded invocation, fooling the calling code into accepting a different return value. Both of these methods are documented in the OpenStep and GNUStep specifications, and hopefully at some point Apple will also follow this path.

Finally, the formatting of encoded method signatures is defined at http://www.toodarkpark.org/computers/objc/moreobjc.html#1037 . This is technically implementation-dependant, although all current objective-c runtimes use the same formatting. For greater portability, “@^v^c” could, for example, be replaced with “@encode(id ) @encode(void) @encode(char)”.

–ThomasCastiglione