[Newbies] Check wheater an instance of a class is actually there or not

Ralph Johnson johnson at cs.uiuc.edu
Wed Dec 23 12:47:13 UTC 2009

Whenever you create MyFileLibrary, you need to store it in a
particular variable "aFileLibrary".  That means you need an object
that owns the variable.  Let's call the class of this object
MyApplication (since I have no idea what your application is; this is
actually a horrible name and I'm sure you can do better).  This object
might be the container of the morphs.

Hans> I need this, because wenn an Instance of MyFileLibrary exists at the
Hans> checking time, I want to proceed with an other method than I want to
Hans> proceed with when an instance of MyFileLibrary does not exists.

This is probably a method of MyApplication.  It checks if aFileLibrary
s nil and does the appropriate action.  If it is the container of the
morphs then the morphs can just send to their container (or parent, I
forget what it is called).  In general, morphs should be as decoupled
from each other as possible, so it is usually not a good idea for them
all to have varibles that reference each other.  Even sending
specialized messages to their parent is a little odd, but it is a good

Of course, you have told us very little.  The odds are good that there
is something simpler that would work better.

-Ralph Johnson

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