Array insertion/deletion signal

mutableArray-KVO-RACSignal.m

RACSignal *s = [self rac_valuesAndChangesForKeyPath:@keypath(self, mutableArray)
                                            options:NSKeyValueObservingOptionNew|
                                                    NSKeyValueObservingOptionOld
                                           observer:nil];
[s subscribeNext:^(RACTuple *x){
    NSLog(@"x:%@", x);
}];

self.mutableArray = [NSMutableArray new];
NSMutableArray *proxyArray = [self mutableArrayValueForKey:@keypath(self, mutableArray)];
    
[proxyArray addObject:@"1"];
[proxyArray addObject:@"2"];
[proxyArray addObject:@"3"];
[proxyArray addObject:@"4"];
[proxyArray removeLastObject];