flow like the river

BACK_BACK/node_modules/d3-transition/src/transition/end.js

@@ -0,0 +1,29 @@
+import {set} from "./schedule.js";
+
+export default function() {
+  var on0, on1, that = this, id = that._id, size = that.size();
+  return new Promise(function(resolve, reject) {
+    var cancel = {value: reject},
+        end = {value: function() { if (--size === 0) resolve(); }};
+
+    that.each(function() {
+      var schedule = set(this, id),
+          on = schedule.on;
+
+      // If this node shared a dispatch with the previous node,
+      // just assign the updated shared dispatch and we’re done!
+      // Otherwise, copy-on-write.
+      if (on !== on0) {
+        on1 = (on0 = on).copy();
+        on1._.cancel.push(cancel);
+        on1._.interrupt.push(cancel);
+        on1._.end.push(end);
+      }
+
+      schedule.on = on1;
+    });
+
+    // The selection was empty, resolve end immediately
+    if (size === 0) resolve();
+  });
+}