unrecognized selector给接盘侠的两次机会

@implementation Person
-(void)sayHello{
    [self performSelector:@selector(xx)];
}
+(BOOL)resolveInstanceMethod:(SEL)sel{
    if ([NSStringFromSelector(sel) isEqualToString:@"xx"]) {
        return NO;
    }
    return true;
}
@end

/**
 将为未被实现的转移给了一个备用的类，加入能够响应了错误就不再往下传递
 @param aSelector 未被实现的函数
 @return 能够响应这个未被实现的函数的实例
 */
-(id)forwardingTargetForSelector:(SEL)aSelector{
    if ([NSStringFromSelector(aSelector) isEqualToString:@"xx"]) {
        Helper *helper = [[Helper alloc] init];
       return helper;
    }
    return nil;
}

//下面两方法是一个整体，为啥呢？因为只有有了方法签名才会走forwardInvocation
-(void)forwardInvocation:(NSInvocation *)anInvocation{
    if ([Helper instancesRespondToSelector:anInvocation.selector]) {
        [anInvocation invokeWithTarget:[[Helper alloc]init] ];
    }
}
- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector{
    return nil;
    NSMethodSignature * signature = [super methodSignatureForSelector:aSelector];
    if (signature == nil) {
        //检测是否能响应函数，然后将方法签名给对应class
        //Returns an NSMethodSignature object that contains a description of the instance method identified by a given selector.

        if([Helper instancesRespondToSelector:aSelector]){
            
            signature =  [Helper instanceMethodSignatureForSelector:aSelector];
        }
    }
    
    return signature;
}

@interface Helper:NSObject
    
@end
@implementation Helper

-(void)xx{
    NSLog(@"接盘侠");
}
@end