keypath

一个参数时， 取.后面的子字符串。如果没有.就取字符串本身
二个参数时， 取的第二个参数的字符串。 两个参数，第一个参数的作用在于 自动提示补全OBJ.PATH，实现了代码自动补全的功能

define keypath(...) \

metamacro_if_eq(1, metamacro_argcount(VA_ARGS))(keypath1(VA_ARGS))(keypath2(VA_ARGS))

define keypath1(PATH) \

(((void)(NO && ((void)PATH, NO)), strchr(# PATH, '.') + 1))

define keypath2(OBJ, PATH) \

(((void)(NO && ((void)OBJ.PATH, NO)), # PATH))

keypath(self);      //(((void)(__objc_no && ((void)self, __objc_no)), strchr("self", '.') + 1)) ;
keypath(self.view);     // (((void)(__objc_no && ((void)self.view, __objc_no)), strchr("self.view", '.') + 1)) ;
keypath(self.view.backgroundColor);  //(((void)(__objc_no && ((void)self.view.backgroundColor, __objc_no)), strchr("self.view.backgroundColor", '.') + 1)) ;


keypath(self, view.backgroundColor);     //   (((void)(__objc_no && ((void)self.view.backgroundColor, __objc_no)), "view.backgroundColor"));
keypath(self.view,backgroundColor);     //    (((void)(__objc_no && ((void)self.view.backgroundColor, __objc_no)), "backgroundColor"));
keypath(self.view.backgroundColor,CGColor); //(((void)(__objc_no && ((void)self.view.backgroundColor.CGColor, __objc_no)), "CGColor"));

1、 RACObserve

RACObserve(self, kvoValue);

({ __attribute__((objc_ownership(weak))) id target_ = (self);

[target_ rac_valuesForKeyPath:@(((void)(__objc_no && ((void)self.kvoValue, __objc_no)), "kvoValue")) observer:self]; });

• (RACSignal *)rac_valuesForKeyPath:(NSString *)keyPath observer:(__weak NSObject *)observer {
  return [[[self
  rac_valuesAndChangesForKeyPath:keyPath options:NSKeyValueObservingOptionInitial observer:observer]
  map:^(RACTuple *value) {
  // -map: because it doesn't require the block trampoline that -reduceEach: uses
  return value[0];
  }]
  setNameWithFormat:@"RACObserve(%@, %@)", RACDescription(self), keyPath];
  }

2、 RACChannelTo
RACChannelTo(self.view, backgroundColor) = RACChannelTo(self,sscolor);

[[RACKVOChannel alloc] initWithTarget:(self.view) keyPath:

@(((void)(__objc_no && ((void)self.view.backgroundColor, __objc_no)), "backgroundColor"))
nilValue:(((void *)0))]
[@(((void)(__objc_no && ((void)RACKVOChannel.new.followingTerminal, __objc_no)), "followingTerminal"))]

= [[RACKVOChannel alloc] initWithTarget:(self) keyPath:
@(((void)(__objc_no && ((void)self.sscolor, __objc_no)), "sscolor"))
nilValue:(((void *)0))]

[@(((void)(__objc_no && ((void)RACKVOChannel.new.followingTerminal, __objc_no)), "followingTerminal"))] ;

// 下标操作

@implementation RACKVOChannel (RACChannelTo)

• (RACChannelTerminal *)objectForKeyedSubscript:(NSString *)key {
  NSCParameterAssert(key != nil);

  RACChannelTerminal *terminal = [self valueForKey:key];
  NSCAssert([terminal isKindOfClass:RACChannelTerminal.class], @"Key "%@" does not identify a channel terminal", key);

  return terminal;
  }

• (void)setObject:(RACChannelTerminal *)otherTerminal forKeyedSubscript:(NSString *)key {
  NSCParameterAssert(otherTerminal != nil);

  RACChannelTerminal *selfTerminal = [self objectForKeyedSubscript:key];
  [otherTerminal subscribe:selfTerminal];
  [[selfTerminal skip:1] subscribe:otherTerminal];
  }

3、 RAC

[[RACSubscriptingAssignmentTrampoline alloc] initWithTarget:(TARGET) nilValue:(NILVALUE)][@keypath(TARGET, KEYPATH)]



 [[RACSubscriptingAssignmentTrampoline alloc] initWithTarget:(self.view) nilValue:([UIColor redColor])]

[@(((void)(__objc_no && ((void)self.view.backgroundColor, __objc_no)),

"backgroundColor"))]=a;

// 下标操作

• (void)setObject:(RACSignal *)signal forKeyedSubscript:(NSString *)keyPath {
  [signal setKeyPath:keyPath onObject:self.target nilValue:self.nilValue];
  }