Abstract design / patterns question

Question

I had a bunch of objects which were responsible for their own construction (get properties from network message, then build). By construction I mean setting frame sizes, colours, that sort of thing, not literal object construction.

The code got really bloated and messy when I started adding conditions to control the building algorithm, so I decided to separate the algorithm to into a "Builder" class, which essentially gets the properties of the object, works out what needs to be done and then applies the changes to the object.

The advantage to having the builder algorithm separate is that I can wrap/decorate it, or override it completely. The object itself doesn't need to worry about how it is built, it just creates a builder and 'decorates' the builder with extra the functionality that it needs to get the job done.

I am quite happy with this approach except for one thing... Because my Builder does not inherit from the object itself (object is large and I want run-time customisation), I have to expose a lot of internal properties of the object.

It's like employing a builder to rebuild your house. He isn't a house himself but he needs access to the internal details, he can't do anything by looking through the windows. I don't want to open my house up to everyone, just the builder.

I know objects are supposed to look after themselves, and in an ideal world my object (house) would build itself, but I am refactoring the build portion of this object only, and I need a way to apply building algorithms dynamically, and I hate opening up my objects with getters and setters just for the sake of the Builder.

I should mention I'm working in Obj-C++ so lack friend classes or internal classes. If the explanation was too abstract I'd be happy to clarify with something a little more concrete. Mostly just looking for ideas or advice about what to do in this kind of situation.

Cheers folks, Sam

EDIT: is it a good approach to declare a

interface House(StuffTheBuilderNeedsAccessTo)

category inside Builder.h ? That way I suppose I could declare the properties the builder needs and put synthesizers inside House.mm. Nobody would have access to the properties unless they included the Builder header....

That's all I can think of!

Answer 1

I would suggest using Factory pattern to build the object.
You can search for "Factory" on SO and you'll a get a no. of questions related to it.

Also see the Builder pattern.

Answer 2

You might want to consider using a delegate. Add a delegate method (and a protocol for the supported methods) to your class. The objects of the Builder class can be used as delegates. The delegate can implement methods like calculateFrameSize (which returns a frame size) etc. The returned value of the delegate can be stored as an ivar. This way the implementation details of your class remain hidden. You are just outsourcing part the logic.

Answer 3

There is in fact a design pattern called, suitable enough, Builder which does tries to solve the problem with creating different configurations for a certain class. Check that out. Maybe it can give you some ideas?

But the underlying problem is still there; the builder needs to have access to the properties of the object it is building.

Answer 4

What about the other way around, using multiple inheritance, so your class is also a Builder? That would mean that the bulk of the algorithms could be in the base class, and be extended to fit the neads of you specific House. It is not very beautiful, but it should let you abstract most of the functionality.

Answer 5

I don't know Obj-C++, so I don't know if this is possible, but this sounds like a problem for Categories. Expose only the necessary methods to your house in the declaration of the house itself, create a category that contains all the private methods you want to keep hidden.