ARC与报文转发

Question

我尝试实现消息转发。Xcode5，ARC开启，新的默认iPhone项目。I read a documentation here

我的项目中有两个自定义类：Hello和World。

#import <Foundation/Foundation.h>
@interface Hello : NSObject
    - (void) say;
@end

#import "Hello.h"
#import "World.h"

@implementation Hello

- (void) say {
    NSLog(@"hello!");
}

-(void)forwardInvocation:(NSInvocation *)invocation {
    NSLog(@"forward invocation");
    World *w = [[World alloc] init];
    if ([w respondsToSelector:[invocation selector]]) {
        [invocation invokeWithTarget:w];
    } else {
        [self doesNotRecognizeSelector: [invocation selector]];
    }
}

-(NSMethodSignature*)methodSignatureForSelector:(SEL)selector {
    NSLog(@"method signature");    
    NSMethodSignature *signature = [super methodSignatureForSelector:selector];
    if (! signature) {
        World *w = [[World alloc] init];
        signature = [w methodSignatureForSelector:selector];
    }
    return signature;
}

@end

世界很简单：

#import <Foundation/Foundation.h>
@interface World : NSObject
    - (void) spin;
@end

#import "World.h"
@implementation World

- (void) spin {
    NSLog(@"spin around");
}

@end

在我的AppDelegate中，我写了三行简单的代码：

Hello  *me = [[Hello alloc] init];
[me say];
[me spin];

编译器给了我一个错误：AppDelegate.m:23:9: No visible @interface for 'Hello' declares the selector 'spin'，并且没有构建项目。当我重新输入它时：[me performSelector:@selector(spin)]; -它工作得很好。

仅当ARC关闭时，Code [me spin]才起作用(但编译器会生成警告AppDelegate.m:23:9: 'Hello' may not respond to 'spin')。

我的问题:为什么？如何使用ARC进行消息转发？

Answer 1

尝试将我声明为id：

 id me = [[Hello alloc] init];
 [me say];
 [me spin];