First working attempt on property observers with Objective-C

NSObject+PropertyObservers.m

#import "NSObject+PropertyObservers.h"
#import <objc/runtime.h>

@implementation NSObject (PropertyObservers)

+ (void)enablePropertyObservers
{
    uint count;
    Class class = [self class];
    objc_property_t* properties = class_copyPropertyList(class, &count);
        
    for (int i = 0; i < count; ++i)
    {
        const char* propertyName = property_getName(properties[i]);
        NSString *nameString = [NSString stringWithCString:propertyName encoding:NSUTF8StringEncoding];
        
        const char *attributes = property_getAttributes(properties[i]);
        NSString *attributeData = [NSString  stringWithCString:attributes encoding:NSUTF8StringEncoding];
                
        NSArray *attributeArray = [attributeData componentsSeparatedByString:@","];
        if ([attributeArray containsObject:@"R"]) {
            continue;
        }
        
        const char* getterName = property_copyAttributeValue( properties[i], "G" );
        const char* setterName = property_copyAttributeValue( properties[i], "S" );
        
        if (getterName != NULL || setterName != NULL) {
            NSLog(@"Cannot create property observers for %@. It has renamed setter or getter.", nameString);
        }
        
        NSString *capitalizedPropertyName = [[[nameString substringToIndex:1] uppercaseString] stringByAppendingString:[nameString substringFromIndex:1]];
        SEL setter = NSSelectorFromString([NSString stringWithFormat:@"set%@:", capitalizedPropertyName]);
                
        NSString *willSetMethodName = [NSString stringWithFormat:@"willSet%@:oldValue:", capitalizedPropertyName];
        NSString *didSetMethodName = [NSString stringWithFormat:@"didSet%@:oldValue:", capitalizedPropertyName];
        
        BOOL willSetImplemented = [class instancesRespondToSelector:NSSelectorFromString(willSetMethodName)];
        BOOL didSetImplemented = [class instancesRespondToSelector:NSSelectorFromString(didSetMethodName)];
        
        SEL poSelector;
        
        if (willSetImplemented && didSetImplemented) {
            poSelector = @selector(po_set_id_willset_didset:);
        } else if (willSetImplemented) {
            poSelector = @selector(po_set_id_willset:);
        } else if (didSetImplemented) {
            poSelector = @selector(po_set_id_didset:);
        } else {
            continue;
        }
        
        SEL newSetterSelector = NSSelectorFromString([NSString stringWithFormat:@"po_replacement_set%@:", capitalizedPropertyName]);
                
        Method originalMethod = class_getInstanceMethod(class, setter);
        Method poMethod = class_getInstanceMethod(class, poSelector);
        
        BOOL didAddReplacementSetter = class_addMethod(class,
                                                       newSetterSelector,
                                                       method_getImplementation(originalMethod),
                                                       method_getTypeEncoding(originalMethod));
        
        BOOL didAddPoSetter = class_replaceMethod(class,
                                                  setter,
                                                  method_getImplementation(poMethod),
                                                  method_getTypeEncoding(poMethod));
        
        if (!didAddPoSetter || !didAddReplacementSetter) {
            NSLog(@"*ERROR %d %d", didAddPoSetter, didAddReplacementSetter);
        }
    }
}

- (void)po_set_id_willset_didset:(id)value
{
    NSString *tempProp = [NSStringFromSelector(_cmd) substringFromIndex:3];
    NSString * const propertyName = [tempProp substringToIndex:tempProp.length - 1];
    SEL getterSel = NSSelectorFromString([[[propertyName substringToIndex:1] lowercaseString] stringByAppendingString:[propertyName substringFromIndex:1]]);
    SEL setterSel = NSSelectorFromString([NSString stringWithFormat:@"po_replacement_set%@:", propertyName]);
    SEL willSetSel = NSSelectorFromString([NSString stringWithFormat:@"willSet%@:oldValue:", propertyName]);
    SEL didSetSel = NSSelectorFromString([NSString stringWithFormat:@"didSet%@:oldValue:", propertyName]);
    
    IMP getterImp = [self methodForSelector:getterSel];
    id (*getter)(id, SEL) = (void *)getterImp;
    id oldValue = getter(self, getterSel);
    
    IMP willSetImp = [self methodForSelector:willSetSel];
    void (*willSet)(id, SEL, id ,id) = (void *)willSetImp;
    willSet(self, willSetSel, value, oldValue);
    
    IMP setterImp = [self methodForSelector:setterSel];
    void (*setter)(id, SEL, id) = (void *)setterImp;
    setter(self, setterSel, value);
    
    IMP didSetImp = [self methodForSelector:didSetSel];
    void (*didSet)(id, SEL, id ,id) = (void *)didSetImp;
    didSet(self, didSetSel, value, oldValue);
}

- (void)po_set_id_didset:(id)value
{
    NSString *propertyName = [NSStringFromSelector(_cmd) substringFromIndex:3];
    propertyName = [propertyName substringToIndex:propertyName.length - 1];
    SEL getterSel = NSSelectorFromString([[[propertyName substringToIndex:1] lowercaseString] stringByAppendingString:[propertyName substringFromIndex:1]]);
    SEL setterSel = NSSelectorFromString([NSString stringWithFormat:@"po_replacement_set%@:", propertyName]);
    SEL didSetSel = NSSelectorFromString([NSString stringWithFormat:@"didSet%@:oldValue:", propertyName]);
    
    IMP getterImp = [self methodForSelector:getterSel];
    id (*getter)(id, SEL) = (void *)getterImp;
    id oldValue = getter(self, getterSel);
    
    IMP setterImp = [self methodForSelector:setterSel];
    void (*setter)(id, SEL, id) = (void *)setterImp;
    setter(self, setterSel, value);
    
    IMP didSetImp = [self methodForSelector:didSetSel];
    void (*didSet)(id, SEL, id ,id) = (void *)didSetImp;
    didSet(self, didSetSel, value, oldValue);
}

- (void)po_set_id_willset:(id)value
{
    NSString *propertyName = [NSStringFromSelector(_cmd) substringFromIndex:3];
    propertyName = [propertyName substringToIndex:propertyName.length - 1];
    SEL getterSel = NSSelectorFromString([[[propertyName substringToIndex:1] lowercaseString] stringByAppendingString:[propertyName substringFromIndex:1]]);
    SEL setterSel = NSSelectorFromString([NSString stringWithFormat:@"po_replacement_set%@:", propertyName]);
    SEL willSetSel = NSSelectorFromString([NSString stringWithFormat:@"willSet%@:oldValue:", propertyName]);
    
    IMP getterImp = [self methodForSelector:getterSel];
    id (*getter)(id, SEL) = (void *)getterImp;
    id oldValue = getter(self, getterSel);
    
    IMP willSetImp = [self methodForSelector:willSetSel];
    void (*willSet)(id, SEL, id ,id) = (void *)willSetImp;
    willSet(self, willSetSel, value, oldValue);
    
    IMP setterImp = [self methodForSelector:setterSel];
    void (*setter)(id, SEL, id) = (void *)setterImp;
    setter(self, setterSel, value);
}

@end