初次看到这个函数时候，疑惑reduce的意思。查字典发现reduce除了“减少”的意思还有“简化”的意思。之前的疑惑终于释然。
RACSignal *formValid = [RACSignal combineLatest:
@[ self.username.rac_textSignal,
self.emailField.rac_textSignal,
self.passwordField.rac_textSignal,
self.passwordVerificationField.rac_textSignal ]
reduce:^(NSString *username, NSString *email, NSString *password, NSString *passwordVerification) {
return @([username length] > 0 && [email length] > 0 &&
[password length] > 8 &&
[password isEqual:passwordVerification]);
}];
RAC(self.createButton.enabled) = formValid;
