drag.on()

drag.on(typenames, [listener])

If listener is specified, sets the event listener for the specified typenames and returns the drag behavior. If an event listener was already registered for the same type and name, the existing listener is removed before the new listener is added. If listener is null, removes the current event listeners for the specified typenames, if any. If listener is not specified, returns the first currently-assigned listener matching the specified typenames, if any. When a specified event is dispatched, each listener will be invoked with the same context and arguments as selection.on listeners: the current datum d and index i, with the this context as the current DOM element.

The typenames is a string containing one or more typename separated by whitespace. Each typename is a type, optionally followed by a period (.) and a name, such as drag.foo and drag.bar; the name allows multiple listeners to be registered for the same type. The type must be one of the following:

• start - after a new pointer becomes active (on mousedown or touchstart).
• drag - after an active pointer moves (on mousemove or touchmove).
• end - after an active pointer becomes inactive (on mouseup, touchend or touchcancel).

See dispatch.on for more.

Changes to registered listeners via drag.on during a drag gesture do not affect the current drag gesture. Instead, you must use event.on, which also allows you to register temporary event listeners for the current drag gesture. Separate events are dispatched for each active pointer during a drag gesture. For example, if simultaneously dragging multiple subjects with multiple fingers, a start event is dispatched for each finger, even if both fingers start touching simultaneously. See Drag Events for more.