Detect if a selector belongs to a protocol

selector_belongsToProtocol.m

/*
 * Copyright © 2012 Scott Perry (http://numist.net)
 *
 * Released under the MIT License; its terms are at the end of this file.
 */

#import <objc/runtime.h>

/**
 * `selector_belongsToProtocol` solves a common problem in proxy objects for delegates where selectors that are not part of the protocol may be unintentionally forwarded to the actual delegate.
 */
BOOL selector_belongsToProtocol(SEL selector, Protocol *protocol)
{
    // protocol_getMethodDescription(protocol, selector, required, instance)
    for (int optionbits = 0; optionbits < (1 << 2); optionbits++) {
        // Check required methods first, then optional
        BOOL required = optionbits & 1;
        // Check instance methods first, then class
        BOOL instance = !(optionbits & (1 << 1));
        
        struct objc_method_description hasMethod = protocol_getMethodDescription(protocol, selector, required, instance);
        if (hasMethod.name || hasMethod.types) {
            // NSLog(@"%@ selector %@ is %@ for %s", instance ? @"Instance" : @"Class", NSStringFromSelector(selector), required ? @"required" : @"optional", protocol_getName(protocol));
            return YES;
        }
    }
    
    // NSLog(@"Selector %@ is not part of protocol %s", NSStringFromSelector(selector), protocol_getName(protocol));
    return NO;
}

// Example usage:
#if 0

- (BOOL)respondsToSelector:(SEL)aSelector;
{
    // Does this object respond to the selector?
    if ([super respondsToSelector:aSelector]) {
        return YES;
    }
    
    // Is it a delegate method, and does the delegate respond to it?
    if (selector_belongsToProtocol(aSelector, @protocol(<#protocol#>)) && [self.actualDelegate instancesRespondToSelector:aSelector]) {
        return YES;
    }
    
    // Welp
    return NO;
}

-(id)forwardingTargetForSelector:(SEL)aSelector;
{
    // Protocol methods always get forwarded, regardless of if the actual delegate supports them.
    // The message sender should have checked -respondsToSelector: for optional methods, and unimplemented required methods should blow up as if there was no interposing receiver.
    if(selector_belongsToProtocol(aSelector, @protocol(<#protocol#>))) {
        return self.actualDelegate;
    }

    return nil;
}

#endif

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 *
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 *
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 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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