Drawing on the slate -- Constraints

The grapplers of the previous section ``stick to'' a picture when it is moved. This is an example of the use of constraints in VL Toolkit. A constraint from one picture to another is, in effect, a relation between pictures such that the first continues to propagate interaction commands onto a selected set of other picture, and in a selected way.

In the case of the grapplers, the set of grapplers is a composite object containing them. The constraint is set up from the grappled picture to the set of grapplers say, in effect: if the picture is moved, propagate a move operation to the grappler composite.

This section explores constraints, showing how they can be used to keep pictures aligned, and (more interestingly for many visual languages) to keep arrowed lines connected to pictures.