[[[[client
logInUser]
flattenMap:^(User *user) {
// Return a signal that loads cached messages for the user.
return [client loadCachedMessagesForUser:user];
}]
flattenMap:^(NSArray *messages) {
// Return a signal that fetches any remaining messages.
return [client fetchMessagesAfterMessage:messages.lastObject];
}]
subscribeNext:^(NSArray *newMessages) {
NSLog(@"New messages: %@", newMessages);
} completed:^{
NSLog(@"Fetched all messages.");
}];

这个例子还比较经典，其实说到底就是封装吧