ruby variable scoping across classes

问题:

RuNubie here. I've got a class Login that logs into gmail using the net/IMAP library. What is happening is that I create a new instance of that class, such as:

a = Login.new("username", "gmail.com", "passw")

Then, I'm working on other classes that will do some "stuff" with the mailbox. The problem is that the @imap variable I've defined in Login seems to have disappeared (due to scoping I assume).

This is how @imap is declared in Login class: @imap = Net::IMAP.new('imap.gmail.com',993,true,nil,false)

So this:

  @today = Date.today
  @received_today = imap.search(["SINCE", @today.strftime("%d-%b-%Y")]).count.to_s

...returns an error. These are the two errors I've gotten while playing around with this. The first one is when I use imap, the second one is when I try @imap:

NameError: undefined local variable or method `imap' for #<Object:0x10718d2a8>
NoMethodError: undefined method `search' for nil:NilClass

What are the best practices for dealing with a situation like this? Is the only solution to define my methods that do "stuff" in the same class where I'm creating the new instance of Net::IMAP? Is declaring @imap as a global variable $imap a bad practice? So confused, I bet the answer is very simple and obvious too, but I'm just not seeing it. Thanks!

回答1:

This:

@received_today = imap.search(["SINCE", @today.strftime("%d-%b-%Y")]).count.to_s

won't work because, well, there is no imap in scope at that point and so you get a NameError. When you try it like this:

@received_today = @imap.search(["SINCE", @today.strftime("%d-%b-%Y")]).count.to_s

You get a NoMethodError because instance variables, such as @imap, are automatically created at first use and initialized as nil. Your real @imap is in another object so you can't refer to it as @imap anywhere else.

I think you want a structure more like this:

class User
    def imap
        if(!@imap)
            @imap = Net::IMAP.new('imap.gmail.com', 993, true, nil, false)
            # and presumably an @imap.authenticate too...
        end
        @imap
    end
end

class OtherOne
    def some_method(user)
        @today = Date.today
        @received_today = user.imap.search(["SINCE", @today.strftime("%d-%b-%Y")]).count.to_s
    end
end

Keep your Net::IMAP localized inside your User and let other objects use it by providing a simple accessor method.

Oh and that global $imap idea, I'll just pretend I didn't see that as globals are almost always a really bad idea.

回答2:

a shorter way to define the imap variable in the User class, which is pretty much the same as what mu posted:

class User
   def imap
      @imap ||= Net::IMAP.new...
   end
end