Expose transition id.

d3.js

@@ -1776,16 +1776,17 @@ function d3_selection_transition() {
     }
   }
 
-  return d3_transition(subgroups);
+  return d3_transition(subgroups, d3_transitionInheritId || ++d3_transitionId);
 }
-function d3_transition(groups) {
+function d3_transition(groups, id) {
   d3_arraySubclass(groups, d3_transitionPrototype);
 
-  var id = d3_transitionInheritId || ++d3_transitionId,
-      tweens = {},
+  var tweens = {},
       event = d3.dispatch("start", "end"),
       ease = d3_transitionEase;
 
+  groups.id = id;
+
   groups.tween = function(name, tween) {
     if (arguments.length < 2) return tweens[name];
     if (tween == null) delete tweens[name];
@@ -1914,7 +1915,7 @@ function d3_transition_select(selector) {
     }
   }
 
-  return d3_transition(subgroups).ease(this.ease());
+  return d3_transition(subgroups, this.id).ease(this.ease());
 }
 function d3_transition_selectAll(selector) {
   var subgroups = [],
@@ -1934,7 +1935,7 @@ function d3_transition_selectAll(selector) {
     }
   }
 
-  return d3_transition(subgroups).ease(this.ease());
+  return d3_transition(subgroups, this.id).ease(this.ease());
 }
 function d3_transition_attr(name, value) {
   return this.attrTween(name, d3_transitionTween(value));

src/core/selection-transition.js

@@ -10,5 +10,5 @@ function d3_selection_transition() {
     }
   }
 
-  return d3_transition(subgroups);
+  return d3_transition(subgroups, d3_transitionInheritId || ++d3_transitionId);
 }

src/core/transition-select.js

@@ -18,5 +18,5 @@ function d3_transition_select(selector) {
     }
   }
 
-  return d3_transition(subgroups).ease(this.ease());
+  return d3_transition(subgroups, this.id).ease(this.ease());
 }

src/core/transition-selectAll.js

@@ -16,5 +16,5 @@ function d3_transition_selectAll(selector) {
     }
   }
 
-  return d3_transition(subgroups).ease(this.ease());
+  return d3_transition(subgroups, this.id).ease(this.ease());
 }

src/core/transition.js

@@ -1,11 +1,12 @@
-function d3_transition(groups) {
+function d3_transition(groups, id) {
   d3_arraySubclass(groups, d3_transitionPrototype);
 
-  var id = d3_transitionInheritId || ++d3_transitionId,
-      tweens = {},
+  var tweens = {},
       event = d3.dispatch("start", "end"),
       ease = d3_transitionEase;
 
+  groups.id = id;
+
   groups.tween = function(name, tween) {
     if (arguments.length < 2) return tweens[name];
     if (tween == null) delete tweens[name];

test/core/transition-test-each.js

@@ -36,7 +36,8 @@ module.exports = {
           context: context,
           data: data,
           index: index,
-          count: count
+          count: count,
+          id: transition.id
         });
       });
     },
@@ -66,7 +67,7 @@ module.exports = {
     },
 
     "sets an exclusive lock on transitioning nodes": function(result) {
-      var id = result.selection[0][0].__transition__.owner;
+      var id = result.id;
       assert.isTrue(id > 0);
       assert.equal(result.selection[0][0].__transition__.owner, id);
       assert.equal(result.selection[0][1].__transition__.owner, id);
@@ -87,13 +88,7 @@ module.exports = {
           data = [],
           index = [],
           count = [],
-          delay = [],
-          id;
-
-      // A callback to grab the transition id.
-      transition.each("start", function() {
-        id = this.__transition__.active;
-      });
+          delay = [];
 
       // A callback to verify that multiple callbacks are allowed.
       transition.each("end", function() {
@@ -114,7 +109,7 @@ module.exports = {
           data: data,
           index: index,
           count: count,
-          id: id
+          id: transition.id
         });
       });
     },

test/core/transition-test-id.js

@@ -0,0 +1,20 @@
+require("../env");
+require("../../d3");
+
+var assert = require("assert");
+
+module.exports = {
+  topic: function() {
+    return d3.select("body").append("div").transition();
+  },
+  "has a positive integer id": function(transition) {
+    var id = transition.id;
+    assert.isTrue(id > 0);
+    assert.equal(~~id, id);
+  },
+  "increases monotonically across transitions": function(transition) {
+    var t0 = d3.select("body").append("div").transition(),
+        t1 = d3.select("body").append("div").transition();
+    assert.isTrue(t1.id > t0.id);
+  }
+};

test/core/transition-test-select.js

@@ -52,5 +52,12 @@ module.exports = {
   "returns null if no match is found": function(transition) {
     var t = transition.select("span");
     assert.isNull(t[0][0]);
+  },
+  "inherits transition id": function(transition) {
+    var id = transition.id,
+        t0 = transition.select("span"),
+        t1 = transition.select("span");
+    assert.equal(t0.id, id);
+    assert.equal(t1.id, id);
   }
 };

test/core/transition-test-selectAll.js

@@ -44,5 +44,12 @@ module.exports = {
   "returns empty if no match is found": function(transition) {
     var t = transition.selectAll("span");
     assert.isEmpty(t[0]);
+  },
+  "inherits transition id": function(transition) {
+    var id = transition.id,
+        t0 = transition.selectAll("span"),
+        t1 = transition.selectAll("span");
+    assert.equal(t0.id, id);
+    assert.equal(t1.id, id);
   }
 };

test/core/transition-test.js

@@ -43,7 +43,8 @@ suite.addBatch({
   // Control
   "each": require("./transition-test-each"),
   "call": require("./transition-test-call"),
-  "tween": require("./transition-test-tween")
+  "tween": require("./transition-test-tween"),
+  "id": require("./transition-test-id")
 
 });