// TODO Track touch points by identifier.

d3.behavior.drag = function() {

var event = d3_eventDispatch(drag, "drag", "dragstart", "dragend"),

origin = null;

function drag() {

this.on("mousedown.drag", mousedown)

.on("touchstart.drag", mousedown);

}

function mousedown() {

var target = this,

event_ = event.of(target, arguments),

eventTarget = d3.event.target,

offset,

origin_ = point(),

moved = 0;

var w = d3.select(window)

.on("mousemove.drag", dragmove)

.on("touchmove.drag", dragmove)

.on("mouseup.drag", dragend, true)

.on("touchend.drag", dragend, true);

if (origin) {

offset = origin.apply(target, arguments);

offset = [offset.x - origin_[0], offset.y - origin_[1]];

} else {

offset = [0, 0];

}

d3_eventCancel();

event_({type: "dragstart"});

function point() {

var p = target.parentNode,

t = d3.event.changedTouches;

return t ? d3.touches(p, t)[0] : d3.mouse(p);

}

function dragmove() {

if (!target.parentNode) return dragend(); // target removed from DOM

var p = point(),

dx = p[0] - origin_[0],

dy = p[1] - origin_[1];

moved |= dx | dy;

origin_ = p;

d3_eventCancel();

event_({type: "drag", x: p[0] + offset[0], y: p[1] + offset[1], dx: dx, dy: dy});

}

function dragend() {

event_({type: "dragend"});

// if moved, prevent the mouseup (and possibly click) from propagating

if (moved) {

d3_eventCancel();

if (d3.event.target === eventTarget) w.on("click.drag", click, true);

}

w .on("mousemove.drag", null)

.on("touchmove.drag", null)

.on("mouseup.drag", null)

.on("touchend.drag", null);

}

// prevent the subsequent click from propagating (e.g., for anchors)

function click() {

d3_eventCancel();

w.on("click.drag", null);

}

}

drag.origin = function(x) {

if (!arguments.length) return origin;

origin = x;

return drag;

};

return d3.rebind(drag, event, "on");

};