Delegate proxying (`protocol_methodForEach()` is a custom function that does just what its name implies)

gistfile1.txt

@interface DDDelegateProxy : NSProxy

+ (id)proxyForDelegate:(id)delegate conformingToProtocol:(Protocol *)protocol;

@property (weak, readonly) id delegate;
@property (strong, readonly) Protocol *protocol;

/*!
 * Set a default return value for a method on the delegate's protocol.
 * \param value This must be a block that returns the default value.  Bad Things will happen if it's not.
 * \param selector The selector to which the default value is applied.
 * This may be any selector in the delegate protocol \em or any of its conformed protocols.
 */
- (void)setDefaultReturnValue:(id<NSCopying>)value forSelector:(SEL)selector;


@end



@implementation DDDelegateProxy {
    id _defaultStub;
    NSDictionary *_signatures;
    NSDictionary *_types;
}

+ (id)proxyForDelegate:(id)delegate conformingToProtocol:(Protocol *)protocol {
    return [[self alloc] initWithDelegate:delegate protocol:protocol];
}

- (id)initWithDelegate:(id)delegate protocol:(Protocol *)protocol {
    // create a new Class on which we'll hang the default values for this delegate
    NSString *defaultStubName = [NSString stringWithFormat:@"DDDelegateDefaultStub_%s_%p", protocol_getName(protocol), delegate];
    Class defaultStubClass = NSClassFromString(defaultStubName);
    if (defaultStubClass == nil) {
        defaultStubClass = objc_allocateClassPair([NSObject class], [defaultStubName UTF8String], 0);
        objc_registerClassPair(defaultStubClass);
    }
    
    _delegate = delegate;
    _defaultStub = [[defaultStubClass alloc] init];
    _protocol = protocol;
    
    // retrieve the instance methods for this protocol and save their info
    NSMutableDictionary *sigs = [NSMutableDictionary dictionary];
    NSMutableDictionary *types = [NSMutableDictionary dictionary];
    protocol_methodForEach(protocol, YES, ^(SEL name, const char *signature, BOOL isInstanceMethod, BOOL isRequired, BOOL *stop) {
        if (isInstanceMethod) {
            NSString *n = NSStringFromSelector(name);
            // we need to keep the signature ourselves because NSMethodSignature doesn't expose it. #15422834
            [types setObject:@(signature) forKey:n];
            [sigs setObject:[NSMethodSignature signatureWithObjCTypes:signature] forKey:n];
        }
    });
    _signatures = [sigs copy];
    _types = [types copy];
    
    return self;
}

- (void)setDefaultReturnValue:(id<NSCopying>)value forSelector:(SEL)selector {
    // turn the default value block into an IMP
    // and hang the IMP off the default stub
    NSString *name = NSStringFromSelector(selector);
    NSString *type = [_types objectForKey:name];
    
    NSParameterAssert(value);
    NSAssert(type != nil, @"Unable to find type information for selector: %@", name);
    
    IMP i = imp_implementationWithBlock(value);
    
    class_replaceMethod(object_getClass(_defaultStub), selector, i, [type UTF8String]);
}

- (id)forwardingTargetForSelector:(SEL)aSelector {
    // fast-path method forwarding
    
    id delegate = _delegate;
    
    // prefer the delegate, obviously
    if ([delegate respondsToSelector:aSelector]) {
        return delegate;
    }
    // provide a default value
    if ([_defaultStub respondsToSelector:aSelector]) {
        return _defaultStub;
    }
    // fallback to slow path forwarding
    return self;
}

- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
    return [_signatures objectForKey:NSStringFromSelector(sel)];
}

- (void)forwardInvocation:(NSInvocation *)invocation {
    // we'll simply return all zeros
    NSMethodSignature *signature = [invocation methodSignature];
    NSUInteger length = [signature methodReturnLength];
    int8_t *zeroedReturnValue = calloc(length, sizeof(int8_t));
    [invocation setReturnValue:zeroedReturnValue];
    free(zeroedReturnValue);
}

@end