Geek asks and answers

ForwardInvocation: return value is lost

I want to use message forwarding in my SZNUnmanagedReference class. It has the following properties:

@property (nonatomic, strong) NSSet *authors;
@property (nonatomic, strong) SZNReferenceDescriptor *referenceDescriptor;

Basically, when an UnmanagedReference instance receives a message authorsString, it should forward it to referenceDescriptorthat which has a method with a name - (NSString *)authorsStringWithSet:(NSSet *)authors.

So, I wrote this in SZNUnmanagedReference.m:

- (void)forwardInvocation:(NSInvocation *)anInvocation {

    SEL aSelector = anInvocation.selector;

    if ([NSStringFromSelector(aSelector) isEqualToString:NSStringFromSelector(@selector(authorsString))]) {
        NSMethodSignature *signature = [self.referenceDescriptor methodSignatureForSelector:@selector(authorsStringWithSet:)];
        NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:signature];
        NSSet *authors = [NSSet setWithSet:self.authors];
        [invocation setSelector:@selector(authorsStringWithSet:)];
        [invocation setArgument:&authors atIndex:2];
        [invocation setTarget:self.referenceDescriptor];

        [invocation invoke];
    } else {
        [self doesNotRecognizeSelector:aSelector];
    }
}

- (BOOL)respondsToSelector:(SEL)aSelector {
    if ([super respondsToSelector:aSelector]) {
        return YES;
    } else if ([NSStringFromSelector(aSelector) isEqualToString:NSStringFromSelector(@selector(authorsString))] && [self.referenceDescriptor respondsToSelector:@selector(authorsStringWithSet:)]) {
        return YES;
    } else {
        return NO;
    }
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    NSMethodSignature *signature = [super methodSignatureForSelector:aSelector];
    if (!signature) {  
        signature = [self.referenceDescriptor methodSignatureForSelector:@selector(authorsStringWithSet:)];
    }
    return signature;
}

Everything works, the code in the class SZNReferenceDescriptoris executed. However, I do not know how to get it authorsStringback. If I understand the documentation correctly, I think I referenceDescriptorshould send the result back to the original sender of the message. But it does not seem to work. In my test class [unmanagedReference authorsString]returns nil.

+5
source share
1

, NSInvocation, , ( "" "" ). , ( forwardInvocation:), , . , , :

- (void)forwardInvocation:(NSInvocation *)anInvocation {

    if (anInvocation.selector == @selector(authorsString)) {
        id retVal = [self.referenceDescriptor authorsStringWithSet:self.authors];

        [anInvocation setReturnValue:&retVal];   // ARC may require some memory-qualification casting here; I'm compiling this by brain at the moment
    } else {
        [super forwardInvocation:anInvocation];
    }
}

, ; , - , ( , authorsString SZNUnmanagedReference, ).

, - SEL .

+8

More articles:


All Articles