The visitor pattern, a well known Gang of Four design pattern, provides a suitable way to add operational behaviour to models. However, as soon as the number of metamodels and visitor implementations start to grow, some of the pattern shortcomings make its usage less convenient. This paper presents how the synergy between the Visitor pattern and MDE has been addressed by two open source projects: Eclipse OCL and QVTd. As a result, a visitors framework generator is proposed to alleviate some of the visitor pattern shortcomings.
The visitor pattern is one of the behavioural patterns described in the Gang of
Four software design patterns [
Whereas applying the visitor pattern to models might be considered
straightforward in the Eclipse Modeling Framework (EMF) [
Provided the potential bene ts that the visitor pattern can bring to models, this paper focuses on the challenge presented by the limitations of using the visitor pattern in MDE, which we propose to overcome by reducing the amount of manual intervention required to realize the visitor pattern. Speci cally, this challenge has been tackled in the context of the Eclipse OCL1 and Eclipse QVTd2 projects by the means of a visitors framework generator. 1 https://projects.eclipse.org/projects/modeling.mdt.ocl 2 https://projects.eclipse.org/projects/modeling.mmt.qvtd
Eclipse OCL and Eclipse QVTd are two projects conceived to support MDE by providing model management languages and tools, in which a substantial number of EMF based metamodels are involved. The reasons of introducing model visitors come from the original visitor pattern: the operational behaviour of a model is centralised in a visitor class; and the metamodel doesn't need to be changed every time a new behaviour is needed, which is convenient to third party consumers who can not alter the metamodel at all.
However, given the substantial number of metamodels and comprised
metaclasses, introducing the pattern is rather a tedious task. Additionally, evolving
the visitors along with any change to the underlying metamodel is troublesome,
as identi ed in the original design pattern [
With the proposed generator we not only leverage the barrier of model
visitors creation, but also we can alleviate some of the shortcomings of the visitor
pattern in some metamodel evolution scenarios [
In Eclipse OCL there is a MDE-based tool which facilitates the generation of metamodel-speci c visitors framework. The main features to highlight:
High degree of automation: For a given metamodel, the visitor pattern is weaved into the generated metamodel implementation, so that for every nonabstract X class an accept method is generated, and whose implementation will delegate to the corresponding visitX method of a Visitor interface, as depicted by Figure 1.
Additionally, a framework of abstract visitors with various purposes3 is also generated. Figure 2 gives a brief overview of the framework for a particular OCL metamodel. Support for derived metamodels: QVT languages reuse OCL as their expression languages. Therefore, the tooling provides generation of visitors which can extend the visitors generated for a di erent metamodel they might extend and/or use. Figure 3 shows a snippet of a particular QVTd visitor of the generated visitors framework. 3 Getting into details of the framework is beyond the scope of this paper
As the reader might understand, we can't get into details in this short paper about the generated visitors framework or provide implementation details about the own generator. However, in this section we give a brief overview of the developed tooling as well as mentioning some of the involved third party tools.
Figure 4 depicts the overall approach. The input Ê to the generator is an
EMF [
With all of this, creating speci c behaviours for the model can be achieved by implementing manual visitors Î that extend the appropriate default visitor of the generated framework.
Alternatively, by the means of additional MDE-based tools Ï, we could also generate more speci c visitors Ð that comprise a particular modelled behaviour.
To conclude, we provide an overview about how the visitors framework generator is extensively used within Eclipse OCL and QVTd projects. Table 1 provides detailed measurements about the generated visitors usage. The table variables are de ned as follows: { Vn: Denotes the number of involved metamodels on which the visitor pattern has been applied. { Va: Denotes the number of visitors classes which are automatically generated as part of the visitors framework4. { Vm: Denotes the number of visitors classes which are manually implemented, and which rely on the automatically generated visitors framework.
Project Vn Va Vm OCL 7 48 67
QVTd 10 89 76
Table 1. Measurements of visitors usage in Eclipse OCL & QVTd
Given the high amount of visitor implementations, and the substantial autogenerated infrastructure that the model speci c visitors frameworks provide, it can be concluded that both the visitor pattern and the framework generator have turned to be fundamental within the Eclipse OCL and QVTd projects.
Acknowledgement. I gratefully acknowledge the support of the Engineering and Physical Sciences Research Council (UK) via the LSCITS initiative.