Grid Designer's Blog

The open-source movement plays a pretty big role in everyday life here at Grid Dynamics. We always try to do our best to improve the software we're working with and contribute back into the community.

While developing an RIA project we faced a need to display complex graphs within an Adobe Flex application. It turns out that there are not many solutions to this problem existing at the moment. Actually AFAIK there are only Flare toolkit, yFiles Flex, SpringGraph and Flex Visual Graph Library (further on FVG). Having done the comparison between the libraries we have chosen Flex Visual Graph Library as our current graphing solution.

Good Things and Bad Things

1. It is an open-source project so everyone can use it for free and modify it in the way he likes;
2. It can render not only trees but also (relatively) complex graphs;
3. It contains a number of algorithms for performing layout;
4. Laying out the graph is performed on the client-side and no server resources are involved in the process.
   

The library has some documentation and samples, although sometimes (and maybe too often) one has to dig into its source code to figure out the way it works.

However, it is a young project so it is expected to be imperfect. Here are the main drawbacks that sometimes annoy me:

1. A somewhat monolithic library design, god-classes anti-pattern here and there; favor is given to state changes instead of the parameter passing. I would also mention here coding conventions, although it is surely my personal impression;
   
2. Lack of documentation on layout algorithms; incomplete design diagrams, tutorials and examples.
   

Our humble help

So here is a list of the things we improved:

1. Fixed and successfully harnessed functionality for adding and removing of nodes. Before the fix it was impossible to do that after the graph was initialized. It seems that the developers of the library just did not need it so they haven't got it into shape.
   
2. Fixed support for directed graphs. It was one of the most controversial changes to the library. Before, the layouting algorithms were tested only on nondirected graphs so the spanning tree in the case of a directed graph was computed in a wrong way. This led to the graph being laid out inappropriately. Now the user can choose if he wants to get the new behavior or emulate the old one.
3. Enhanced hierarchical layouter. It now supports interleaving of nodes. Sibling nodes (the nodes on the same level) are placed now in a checkerboard formation so every even node is put a bit higher than an odd one. This enhances the readability of the graph and the overall appearance if nodes have text labels under them.
4. Added possibility to sort sibling nodes in a specified order. This again enhances readability. Suppose that you have some text on your nodes. You can sort the siblings in the lexicographical order so you can more easily find the node you want with your eyes. Or you may want to display a certain type of nodes first. Plugging in your own sorting logic is tremendously easy.
5. Added a very cool feature: graph zooming. There was nothing like that in the library before. One could only try to tune some parameters of layouters in order to achieve zooming effect but the real zooming for every layouter was impossible.
   

In addition, we've been implementing numerous small fixes and improvements — and we are definitely going to continue this effort in the future.

Conclusion

In general, supporting the open-source library has the following noticeable peculiarity: as with other source management models, it is always needed to keep compatibility with those who already use your library in their applications. But when a need to make significant and incompatible changes to the library appears, one can always commit the changes into a separate branch. It turns out to be a rather common practice in the open-sourced world. Although this is not as convenient as just committing the changes directly to the trunk, the approach has advantages. It allows you to not break the compatibility and let other developers test and review the new functionality. We've been maintaining a separate branch for quite a long time, partly because of tensions with the maintainers, partly because of a lack of time to perform all the necessary merges and testing. However, not long ago we decided that this will likely cause the branches to diverge in some point of time — definitely that will not add value to the library, but rather subtract from it. So we've consolidated our efforts with the maintainer of the library and presented a demo of the new features. The guys were very collaborative and accepted our changes into the trunk with just a couple of minor changes.

Labels: flex, graph, open source, RIA, visualization, ~Ivan Bulanov