Monday, October 20, 2008


Q: What is an object-oriented framework?

A: The short answer is: A framework is a set of related classes which you specialize and/or instantiate to implement an application or subsystem.

Q: What is the difference between an object-oriented framework and a class library?

A: An object-oriented framework is a kind of class library. The distinguishing feature is that when the library is a framework the flow of control is bi-directional between the application and the library. This feature is achieved by the dynamic binding in object-oriented languages where an operation can be defined in a library class but implemented in a subclass in the application.

Q: Are frameworks a kind of GUI technology?

A: No. The first frameworks, such as Model/View/Controller, were developed for graphical user interfaces, but the technique is general and you can now find frameworks for many different domains.

Q: What is so good about frameworks?

A: Two main reasons. The framework is not just a collection of classes but also defines a generic design. When you use the framework your re-use that design and save time and effort. Secondly, because of the bidirectional flow of control the framework can contain much more functionality than a traditional library regradless if it is a procedural or class library. Having to write less code also saves you time and effort.

Q: If those are the good news, which are the bad ones?

A: The main obstacle to frameworks is the learning curve. To reuse a design you have to learn and understand it. This is never easy if the domain is non-trivial. Furthermore, no one has yet come up with a real good way to document a framework.

Q: Can I develop an object-oriented framework in C?

A: Not easily, since frameworks rely heavily on dynamic binding. In C you would have to implement that by hand and combine it with strict programming conventions.

Q: What is the first step towards framework based development?

A: Before you even start to think about develop a framework you must learn how to use one. Get a commercially or publicly available framework and start to develop applications from it.

Q: What is the most difficult thing to learn when starting to use frameworks?

A: That you have to turn the way you think about programming inside out. Frameworks follow the Hollywood principle: "Don't call us, we'll call you". The library code is in charge and as an application programmer you just provide various bits and pieces and is no longer in control. It is like programming event driven applications, but one step further.

Q: Can I use more than one framework in same application?

A: Historically frameworks were monolithic and were not easy to combine. But the current trend is to make smaller frameworks, one framework for each part or aspect of an application rather than a single framework for the whole application.

Q: Which of the available OO methods should I choose for developing a framework?

A: None. All of the established methods build on the traditional one way flow of control. They support inheritance between classes but are very weak on dynamic binding. You can use the notation to describe different parts of the framework but the method itself can do more harm than good.

Q: What is the relationship between frameworks and design patterns?

A: Frameworks are software whereas patterns are knowledge or information about software. Patterns can be used to describe frameworks and some part or aspect of a framework can be a realization of a pattern.

Q: When should I develop a framework of my own?

A: Developing a framework is somewhat like developing a scientific theory for some area. Developing a framework is hard and you should do that only for your own special area of competence.

Q: Is the development process different when you use a framework?

A: Sure. The problem has to be cast into the generic template imposed by the framework. That may at first appear to be a limitation, but trying to express the problems in terms of a previously successful design is often the best way to understand the user's real needs.

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