rac中有个私有的方法：
+ (instancetype)join:(id<NSFastEnumeration>)streams block:(RACStream * (^)(id, id))block;
虽然是私有的方法，但对理解racstream比较重要，zipwith：最后都会走到join：block：
+ (instancetype)zip:(id<NSFastEnumeration>)streams {
return [[self join:streams block:^(RACStream *left, RACStream *right) {
return [left zipWith:right];//两个信号打包
}] setNameWithFormat:@"+zip: %@", streams];
}
+ (instancetype)join:(id<NSFastEnumeration>)streams block:(RACStream * (^)(id, id))block {
RACStream *current = nil;
//遍历streams
// Creates streams of successively larger tuples by combining the input
// streams one-by-one.
for (RACStream *stream in streams) {
// For the first stream, just wrap its values in a RACTuple. That way,
// if only one stream is given, the result is still a stream of tuples.
if (current == nil) {
//第一个值，直接封装为ractuple
current = [stream map:^(id x) {
return RACTuplePack(x);
}];
continue;
}
// 除了第一个stream，后续的stream和current打包组成新的current
current = block(current, stream);
}
if (current == nil) return [self empty];
return [current map:^(RACTuple *xs) {
// Right now, each value is contained in its own tuple, sorta like:
//
// (((1), 2), 3)
//
// We need to unwrap all the layers and create a tuple out of the result.
//将(((1), 2), 3)数据类型转换为数组
NSMutableArray *values = [[NSMutableArray alloc] init];
while (xs != nil) {
[values insertObject:xs.last ?: RACTupleNil.tupleNil atIndex:0];
xs = (xs.count > 1 ? xs.first : nil);
}
return [RACTuple tupleWithObjectsFromArray:values];
}];
}
