OOWeakReference.m

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/*
 
OOWeakReference.m
 
Written by Jens Ayton in 2007-2013 for Oolite.
This code is hereby placed in the public domain.
 
*/
 
#import "OOWeakReference.h"
 
 
@interface OOWeakReferenceTemplates: NSObject
 
+ (void)weakRefDrop;
+ (id)weakRefUnderlyingObject;
+ (id)nilMethod;
 
@end
 
 
@implementation OOWeakReference
 
// *** Core functionality.
 
+ (id)weakRefWithObject:(id<OOWeakReferenceSupport>)object
{
    if (object == nil)  return nil;
    
    OOWeakReference *result = [OOWeakReference alloc];
    // No init for proxies.
    result->_object = object;
    return [result autorelease];
}
 
 
- (void)dealloc
{
    [_object weakRefDied:self];
    
    [super dealloc];
}
 
 
- (NSString *)description
{
    if (_object != nil)  return [_object description];
    else  return [NSString stringWithFormat:@"<Dead %@ %p>", [self class], self];
}
 
 
- (id)weakRefUnderlyingObject
{
    return _object;
}
 
 
- (id)weakRetain
{
    return [self retain];
}
 
 
- (void)weakRefDrop
{
    _object = nil;
}
 
 
// *** Proxy evilness beyond this point.
 
- (Class) class
{
    return [_object class];
}
 
 
- (BOOL) isProxy
{
    return YES;
}
 
 
- (void)forwardInvocation:(NSInvocation *)invocation
{
    // Does the right thing even with nil _object.
    [invocation invokeWithTarget:_object];
}
 
 
- (NSMethodSignature *)methodSignatureForSelector:(SEL)selector
{
    NSMethodSignature       *result = nil;
    
    if (__builtin_expect(
        selector != @selector(weakRefDrop) &&
        selector != @selector(weakRefUnderlyingObject), 1))
    {
        // Not a proxy method; get signature from _object if it exists, otherwise generic signature for nil calls.
        if (__builtin_expect(_object != nil, 1))  result = [(id)_object methodSignatureForSelector:selector];
        else  result = [OOWeakReferenceTemplates methodSignatureForSelector:@selector(nilMethod)];
    }
    else
    {
        // One of OOWeakReference's own methods.
        result = [OOWeakReferenceTemplates methodSignatureForSelector:selector];
    }
    
    return result;
}
 
 
- (BOOL)respondsToSelector:(SEL)selector
{
    if (__builtin_expect(_object != nil &&
        selector != @selector(weakRefDrop) &&
        selector != @selector(weakRefUnderlyingObject), 1))
    {
        // _object exists and it's not one of our methods, ask _object.
        return [_object respondsToSelector:selector];
    }
    else
    {
        // Selector we responds to, or _object is nil and therefore responds to everything.
        return YES;
    }
}
 
 
// New fast forwarding mechanism introduced in Mac OS X 10.5.
// Note that -forwardInvocation: is still called if _object is nil.
- (id)forwardingTargetForSelector:(SEL)sel
{
    return _object;
}
 
@end
 
 
@implementation NSObject (OOWeakReference)
 
- (id)weakRefUnderlyingObject
{
    return self;
}
 
@end
 
 
@implementation OOWeakRefObject
 
- (id)weakRetain
{
    if (weakSelf == nil)  weakSelf = [OOWeakReference weakRefWithObject:self];
    return [weakSelf retain];   // Each caller releases this, as -weakRetain must be balanced with -release.
}
 
 
- (void)weakRefDied:(OOWeakReference *)weakRef
{
    if (weakRef == weakSelf)  weakSelf = nil;
}
 
 
- (void)dealloc
{
    [weakSelf weakRefDrop]; // Very important!
    [super dealloc];
}
 
 
- (id)weakSelf
{
    return [[self weakRetain] autorelease];
}
 
@end
 
 
@implementation OOWeakReferenceTemplates
 
// These are never called, but an implementation must exist so that -methodSignatureForSelector: works.
+ (void)weakRefDrop  {}
+ (id)weakRefUnderlyingObject  { return nil; }
+ (id)nilMethod { return nil; }
 
@end