Apply this force, optionally observing the specified alpha. Typically, the force is applied to the array of nodes previously passed to force.initialize, however, some forces may apply to a subset of nodes, or behave differently. For example, d3.forceLink applies to the source and target of each link.
Optional initializeSupplies the array of nodes and random source to this force. This method is called when a force is bound to a simulation via simulation.force and when the simulation’s nodes change via simulation.nodes.
A force may perform necessary work during initialization, such as evaluating per-node parameters, to avoid repeatedly performing work during each application of the force.
A force is simply a function that modifies nodes’ positions or velocities; in this context, a force can apply a classical physical force such as electrical charge or gravity, or it can resolve a geometric constraint, such as keeping nodes within a bounding box or keeping linked nodes a fixed distance apart.
Forces typically read the node’s current position [x,y] and then add to (or subtract from) the node’s velocity [vx,vy]. However, forces may also “peek ahead” to the anticipated next position of the node, [x + vx,y + vy]; this is necessary for resolving geometric constraints through iterative relaxation. Forces may also modify the position directly, which is sometimes useful to avoid adding energy to the simulation, such as when recentering the simulation in the viewport.
Forces may optionally implement force.initialize to receive the simulation’s array of nodes.