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What does brush.event do when animating brush events?

Question:

When checking out the Brush Snapping function hosted on bl.ocks.org, I was confused by this block of code:

var gBrush = svg.append("g")
        .attr("class", "brush")
        .call(brush)
        .call(brush.event);

To be clear, I understand what I'm doing .call(brush.event), but I don't understand why it is in this particular block of code. I see that the same call is being made in the brushended event handler, but why do the transitions in the brushend event depend on the call copied above?

Previous research:

I checked the docs API which says the following, but I have to admit that I really don't understand this explanation.

If the selection is a transition, logs the appropriate teens so that the brush sends out events during the transition

, nitty gritty brush.event?

(, ):

Meteor.js. , , , . , . , , .

: , "d3" . d3.js: d3, .

:

<!DOCTYPE html>
<meta charset="utf-8">
<style>

.axis text {
  font: 11px sans-serif;
}

.axis path {
  display: none;
}

.axis line {
  fill: none;
  stroke: #000;
  shape-rendering: crispEdges;
}

.grid-background {
  fill: #ddd;
}

.grid line,
.grid path {
  fill: none;
  stroke: #fff;
  shape-rendering: crispEdges;
}

.grid .minor.tick line {
  stroke-opacity: .5;
}

.brush .extent {
  stroke: #000;
  fill-opacity: .125;
  shape-rendering: crispEdges;
}

</style>
<body>
<script src="http://d3js.org/d3.v3.min.js"></script>
<script>

var margin = {top: 200, right: 40, bottom: 200, left: 40},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

var x = d3.time.scale()
    .domain([new Date(2013, 7, 1), new Date(2013, 7, 15) - 1])
    .range([0, width]);

var brush = d3.svg.brush()
    .x(x)
    .extent([new Date(2013, 7, 2), new Date(2013, 7, 3)])
    .on("brushend", brushended);

var svg = d3.select("body").append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

svg.append("rect")
    .attr("class", "grid-background")
    .attr("width", width)
    .attr("height", height);

svg.append("g")
    .attr("class", "x grid")
    .attr("transform", "translate(0," + height + ")")
    .call(d3.svg.axis()
        .scale(x)
        .orient("bottom")
        .ticks(d3.time.hours, 12)
        .tickSize(-height)
        .tickFormat(""))
  .selectAll(".tick")
    .classed("minor", function(d) { return d.getHours(); });

svg.append("g")
    .attr("class", "x axis")
    .attr("transform", "translate(0," + height + ")")
    .call(d3.svg.axis()
      .scale(x)
      .orient("bottom")
      .ticks(d3.time.days)
      .tickPadding(0))
  .selectAll("text")
    .attr("x", 6)
    .style("text-anchor", null);

var gBrush = svg.append("g")
    .attr("class", "brush")
    .call(brush)
    .call(brush.event);

gBrush.selectAll("rect")
    .attr("height", height);

function brushended() {
  if (!d3.event.sourceEvent) return; // only transition after input
  var extent0 = brush.extent(),
      extent1 = extent0.map(d3.time.day.round);

  // if empty when rounded, use floor & ceil instead
  if (extent1[0] >= extent1[1]) {
    extent1[0] = d3.time.day.floor(extent0[0]);
    extent1[1] = d3.time.day.ceil(extent0[1]);
  }

  d3.select(this).transition()
      .call(brush.extent(extent1))
      .call(brush.event);
}

</script>
+4
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