Smalltalk and Object Orientation: an Introduction - Free

dependents
ObjectA
ObjectB
ObjectC
Figure 25.2: An object and its dependents
The message dependents can be used to obtain a list of all the dependents of an object (no matter
what the class of the object). For example, to obtain a list of the dependents on ObjectA we could use:
ObjectA dependents.
From the point of view o f the actual implementation, the dependents are sometimes held in an
instance variable inherited from above (so you can see it in an inspector). This is the case for most of
the user interface classes. In other cases the dependents list is held as part of
an
IdentityDictionary in a class variable (this is the default mechanism inherited from Object).
25.3.3 Constructing dependencies
The addDependent: message is used to add an object to another object’s dependency list. For
example, we can construct the above dependencies using:
ObjectA addDependent: ObjectB.
ObjectA addDependent: ObjectC.
Note that an object holds (or can access) a list of objects which depend on it. Whereas an object
cannot access information about the objects on which it depends. For example, there are no references
from ObjectB or ObjectC back to ObjectA in Figure 25.2. Thus an object does not hold a list of
objects on which it depends. This may seem a bit strange at first, however, o nce you gain an
understanding of how the dependency mechanism works, hopefully you will see why things are this
way round.
It is also possible to remove dependencies once they have been created. This can be done using the
removeDependent: message. For example:
ObjectA removeDependent: ObjectB.
This removes ObjectB from the dependency list of ObjectA. It should be noted that as an
IdentityDictionary is used (at least in objects which are instances of classes below Object),
ObjectB must be the same object as was used to create the dependency. It cannot be merely
equivalent.
Figure 25.3: The dependents message defined in Object
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