NSDictionary <FBGraphUser>*用户语法说明

Question

在Facebook iOS SDK中,使用以下处理程序返回请求:

 ^(FBRequestConnection *connection, 
       NSDictionary<FBGraphUser> *user, 
       NSError *error) { }

然后可以使用这些调用来访问用户变量......

   self.userNameLabel.text = user.name;
   self.userProfileImage.profileID = user.id;

这种语法有点类似于id <protocolDelegate> object公共属性声明的语法语法,除了NSDictionary是明确的id对象,并且该字典符合协议？但点语法来自何处以及如何声明任意NSFoundation对象与协议相对应而没有对对象本身进行子类化并使其符合？

我做了一些关于点符号和NSDictionary的额外研究,看起来如果没有在NSDictionary中添加类别,就不可能在字典上使用点符号.但是,我没有在Apple文档中看到任何<>语法的引用,表明NSDictionary的这个特定实例符合该表示法.

关于这种包装的工作方式,Facebook文档有点稀疏:

FBGraphUser协议表示Facebook用户对象最常用的属性.它可用于访问已使用FBGraphObject外观包装的NSDictionary对象.

如果跟随这个引导到FBGraphObject文档,那么有一些方法返回符合这个"外观......"的字典,但没有关于如何包装字典的进一步解释.

所以我想我的问题有几点:

1. 底层代码看起来会使这种语法有效吗？
2. 它为什么存在？
3. 为什么facebook会以这种方式实现它,而不是只创建一个可以将数据转换成对象的对象？

任何解释或见解将非常感谢!

Answer 1

基本上,NSDictionary<FBGraphUser> *user意味着一个继承自的对象NSDictionary,添加FBGraphUser协议声明的功能(特别是类型化访问).

这种方法背后的原因在FBGraphObject文档中详细描述(FBGraphUser协议扩展了FBGraphObject协议).你可能会感到困惑的是,这FBGraphObject是一个协议(在这里描述)和一个继承自的类(在这里描述)NSMutableDictionary.

在内部实现方面,它是一些非常先进的Objective-C动态魔法,你可能不想担心.您需要知道的是,如果您愿意,可以将对象视为字典,或者使用协议中的其他方法.如果您真的想知道详细信息,可以查看FBGraphObject的源代码,特别是这些方法:

#pragma mark -
#pragma mark NSObject overrides

// make the respondsToSelector method do the right thing for the selectors we handle
- (BOOL)respondsToSelector:(SEL)sel
{
    return  [super respondsToSelector:sel] ||
    ([FBGraphObject inferredImplTypeForSelector:sel] != SelectorInferredImplTypeNone);
}

- (BOOL)conformsToProtocol:(Protocol *)protocol {
    return  [super conformsToProtocol:protocol] ||
    ([FBGraphObject isProtocolImplementationInferable:protocol 
                           checkFBGraphObjectAdoption:YES]);
}

// returns the signature for the method that we will actually invoke
- (NSMethodSignature *)methodSignatureForSelector:(SEL)sel {
    SEL alternateSelector = sel;

    // if we should forward, to where?
    switch ([FBGraphObject inferredImplTypeForSelector:sel]) {
        case SelectorInferredImplTypeGet:
            alternateSelector = @selector(objectForKey:);
            break;
        case SelectorInferredImplTypeSet:
            alternateSelector = @selector(setObject:forKey:);
            break;
        case SelectorInferredImplTypeNone:
        default:
            break;
    }

    return [super methodSignatureForSelector:alternateSelector];
}

// forwards otherwise missing selectors that match the FBGraphObject convention
- (void)forwardInvocation:(NSInvocation *)invocation {
    // if we should forward, to where?
    switch ([FBGraphObject inferredImplTypeForSelector:[invocation selector]]) {
        case SelectorInferredImplTypeGet: {
            // property getter impl uses the selector name as an argument...
            NSString *propertyName = NSStringFromSelector([invocation selector]);
            [invocation setArgument:&propertyName atIndex:2];
            //... to the replacement method objectForKey:
            invocation.selector = @selector(objectForKey:);
            [invocation invokeWithTarget:self];
            break;
        }
        case SelectorInferredImplTypeSet: {
            // property setter impl uses the selector name as an argument...
            NSMutableString *propertyName = [NSMutableString stringWithString:NSStringFromSelector([invocation selector])];
            // remove 'set' and trailing ':', and lowercase the new first character
            [propertyName deleteCharactersInRange:NSMakeRange(0, 3)];                       // "set"
            [propertyName deleteCharactersInRange:NSMakeRange(propertyName.length - 1, 1)]; // ":"

            NSString *firstChar = [[propertyName substringWithRange:NSMakeRange(0,1)] lowercaseString];
            [propertyName replaceCharactersInRange:NSMakeRange(0, 1) withString:firstChar];
            // the object argument is already in the right place (2), but we need to set the key argument
            [invocation setArgument:&propertyName atIndex:3];
            // and replace the missing method with setObject:forKey:
            invocation.selector = @selector(setObject:forKey:);
            [invocation invokeWithTarget:self]; 
            break;
        } 
        case SelectorInferredImplTypeNone:
        default: 
            [super forwardInvocation:invocation];
            return;
    }
}