Question

I have some event handler on a boundary class that manages a persistence mechanism for a given generic transaction:

void MyBoundaryClass::MyEventHandler(...) {   //retrieve stuff from the UI   //...   //declare and initialize trasaction to persist   SimpleTransaction myTransaction(.../*pass down stuff*/);   //do some other checks   //...   //declare transaction persistor   TransactionPersistor myPersistor(myTransaction, .../*pass down connection to DB and other stuff*/);   //persist transaction   try   {     myPersistor.Persist();   }   catch(...)   {     //handle errors   } } 

Would it be better to have some kind of TransactionManager to wrap SimpleTransaction and TransactionPErsistor objects?

Is there any useful rule of thumb to understand if I need a further level of encapsulation?

At the moment the rule of thumb I follow is ‘if the method gets too big – do something about it’. It is hard sometimes to find the right balance between procedural and object oriented when dealing with boundary event handlers.

Any opinion?

Cheers

Answer 1

Considering that:

• the concept of encapsulation is about defining a container, and
• object-oriented design is based on the concept of message passing (invocation of methods)

I would argue that the API is a good indication about the pertinence of a new high-level encapsulation (I.e. the definition of a new object)

If the services (i.e the API) offered by this new object are coherent, and are better exposed to the rest of the program when regrouped in one special object, then by all means, use a new object.

Otherwise, it is probable an overkill.

Since you expose a public API by creating a new object, the notion of test may be easier to do within that new object (and a few other mock objects), rather than create many legacy objects in order to test those same operations.

In your case, if you want to test the transaction, you must actually test MyEventHandler of MyBoundaryClass, in order to retrieve data from the UI.

But if you define a TransactionManager, that gives you the opportunity to lower coupling of different architecture levels (GUI vs. data) present in MyBoundaryClass, and to export data management into a dedicated class.
Then, you can test data persistence in independent test scenario, focusing especially on limit values, and database failure, and not-nominal conditions, and so on.

Testing scenario can help you refine the cohesion (great point mentioned by Daok) of your different objects. If your tests are simple and coherent, chances are that your objects have a well-define service boundary.

Since it can be argued that Coupling and Cohesion are two cornerstones of OO Programming, the cohesion of a new class like TransactionManager can be evaluated in term of the set of actions it will perform.

Cohesive means that a certain class performs a set of closely related actions. A lack of cohesion, on the other hand, means that a class is performing several unrelated tasks. […] the application software will eventually become unmanageable as more and more behaviors become scattered and end up in wrong places.

If you regroup behaviors otherwise implemented in several different places into your TransactionManager, it should be fine, provided that its public API represent clear steps of what a transaction involves and not ‘stuff about transaction’ like various utility functions. A name in itself is not enough to judge the cohesiveness of a class. The combination of the name and its public API is needed.

For instance, one interesting aspect of a TransactionManager would be to completely encapsulate the notion of Transaction, which would :

• become virtually unkown by the rest f the system, and would lower coupling between the other classes and ‘Transaction’
• reinforce the cohesiveness of TransactionManager by centering its API around transaction steps (like initTransaction(), persistTransaction(), …), avoiding any getter or setter for any Transaction instance.