The DragDropper Interactor

The Slate uses objects called interactors to abstract user interface events into concrete objects. One of these, the DragDropper, was used in the prototype editor. However, DragDropper does not map directly to the dynamic model for making connections. Here is the dynamic model of DragDropper.

Additional notes on the model:

• DragDropper is a single object that manipulates other objects. The dynamic model for making connections has two state diagrams, for Terminals and Nets. The DragDropper can be viewed as an abstraction of multiple dynamic models. It is activated by clicking on one object, called the source, and deactivated by releasing over another object, called the target.

• DragDropper is parameterised with client callbacks. On the dynamic model, these are shown as the events Init, Moving, Drag, Charge, and Done. The current DragDropper is not quite implemented this way, but it should probably be altered so that it does.

• The DragDropper incorporate the notion of "hysteresis." A condition such as "over target" means that the cursor is within a certain distance from the target object or a suitable part of it. The current DragDropper needs to be extended to allow choices such as:
  • With a certain distance (the halo) of a source or target object's bounding box.
  • A mouse movement of a given amount (for the source object only).
  • Some other criterion accessed via a callback to the client.

• Here is how we could map the DragDropper to the dynamic model for making connections. Each mapping shows the relation of a DragDropper state or event send to a state or action of Net or Terminal:
  • Creating a new Net

    States
    Idle = Terminal.Idle
    Ready = Terminal.Ready
    Primed = Terminal.Primed
    Dragging = Net.Dragging
    Charged = Net.Snapped

    Events
    Init(source) = ()
    Moving(source) = Terminal/create Net in Dragging
    Drag(x,y) = Net/snap to cursor
    Ready(target) = Net/snap to target
    Done()[Net created] = Net/create pinned grapple
    Done()[Net not created] = ()
    Done(target) = Net/create edge, Net/make selected, Net/create grapples

  • Breaking a connection:

    States
    Idle = Net.Selected
    Ready = (?)
    Primed = Net.Resisting
    Dragging = Net.Dragging
    Charged = Net.Snapped

    Events
    Init(source) = ()
    Moving(source) = Net/delete edge, Net/delete grapples
    Drag(x,y) = Net/snap to cursor
    Ready(target) = Net/snap to target
    Done() = Net/create pinned grapple
    Done(target) = Net/create edge, Net/make selected, Net/create grapples

  • Making a Junction. Here Net' is the newly create net.

    States
    Idle = Net.Selected
    Ready = ()
    Primed = Net.Preparing
    Dragging = Net'.Dragging
    Charged = Net'.Snapped

    Events
    Init(source) = ()
    Moving(source) = Net/create new Junction and Vertex, Net/split Net and Edge in two, create Net' in Dragging
    Drag(x,y) = Net'/snap to cursor
    Ready(target) = Net'/snap to target
    Done() = Net'/create pinned grapple
    Done(target) = Net'/create edge, Net/make selected, Net/create grapples

JohnR, 20 Oct 97