- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector { 
  if(aSelector == @selector(testMethod)) 
  { 
    return [NSMethodSignature signatureWithObjCTypes:"v@:"]; 
  } 
  return nil; 
} 
 
 
-(void)forwardInvocation:(NSInvocation *)anInvocation 
{ 
  if (anInvocation.selector == @selector(testMethod)) 
  { 
    TestModelHelper1 *h1 = [[TestModelHelper1 alloc] init]; 
    TestModelHelper2 *h2 = [[TestModelHelper2 alloc] init]; 
    [anInvocation invokeWithTarget:h1]; 
    [anInvocation invokeWithTarget:h2]; 
  } 
} 

-(void)testMethod 
{ 
  NSLog(@"i am TestModelHelper1"); 
} 
TestModelHelper2
[objc] view plain copy
-(void)testMethod 
{ 
  NSLog(@"i am TestModelHelper2"); 
} 

TestModel *model = [[TestModel alloc] init]; 
[model testMethod]; 

-(void)testMethod; 

-(NSString *)testMethod2:(NSString *)str;

-(void)testMethod 
{ 
  NSLog(@"i am TestModelHelper1"); 
  Method method = class_getInstanceMethod(self.class, @selector(testMethod)); 
  const char *des = method_getTypeEncoding(method); 
  NSString *desStr = [NSString stringWithCString:des encoding:NSUTF8StringEncoding]; 
  NSLog(@"%@",desStr); 
} 

-(NSString *)testMethod2:(NSString *)str 
{ 
  Method method = class_getInstanceMethod(self.class, @selector(testMethod2:)); 
  const charchar *des = method_getTypeEncoding(method); 
  NSString *desStr = [NSString stringWithCString:des encoding:NSUTF8StringEncoding]; 
  NSLog(@"%@",desStr); 
  return @""; 
}