Implementation of `isEqual:` http://nshipster.com/equality/

UserModel.h

@interface UserModel : NSObject

@property (copy, nonatomic) NSString *name;
@property (copy, nonatomic) NSNumber *age;

@end

UserModel.m

@implementation UserModel

/********************************
* NSHipster: Equality
* http://nshipster.com/equality/
********************************/
- (BOOL)isEqual:(id)otherId
{
    if (!otherId) {
        return NO;
    }

    if (otherId == self) {
        return YES;
    }

    if (![otherId isKindOfClass:self.class]) {
        return NO;
    }

    UserModel *other = (UserModel *)otherId;

    BOOL namesAreEqual = (!self.name && !other.name) || [self.name isEqualToString:other.name];
    if (!namesAreEqual) { return NO; }
    
    BOOL agesAreEqual = (!self.age && !other.age) || [self.age isEqualToNumber:other.age];
    if (!agesAreEqual) { return NO; }

    return YES;
}

@end