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ui/webui/resources/js/util.js

Index: ui/webui/resources/js/util.js
diff --git a/ui/webui/resources/js/util.js b/ui/webui/resources/js/util.js
index 759bfbbf308ac232374ffec3e12436f68882fbee..f94f8ec833c4a86868b10ae1ae78bce9718c5b86 100644
--- a/ui/webui/resources/js/util.js
+++ b/ui/webui/resources/js/util.js
@@ -393,6 +393,21 @@ function quoteString(str) {
   return str.replace(/([\\\.\+\*\?\[\^\]\$\(\)\{\}\=\!\<\>\|\:])/g, '\\$1');
 }
 
+/**
+ * Runs |callback| first time the |eventName| event is fired on |target|, then
+ * removes the listener.
+ * @param {!EventTarget} target
+ * @param {string} eventName

[Dan Beam, 2016/09/01 01:33:13]

nit: maybe support multiple events?

... Callback is called and listening stops on the first event triggered. ...

@param {!Array<!string>|!string} eventNames An array of event types or a
space-separated list as a string (i.e. 'click mousedown').

var eventNames =
    Array.isArray(eventNames) ? eventNames : eventNames.split(/\s+/);

function removeAllAndCallback() {
  eventNames.forEach(function(eventName) {
    target.removeEventListener(eventName, removeAllAndCallback);
  });
  return callback(event);
}

eventNames.forEach(function(eventName) {
  target.addEventListener(eventName, removeAllAndCallback);
});

listenOnce(window, 'scroll resize', reposition);

[michaelpg, 2016/09/01 02:20:26]

Done. I had briefly considered this but thought it may be overkill.

+ * @param {function(!Event)} callback Called at most once. The
+ *     optional return value is passed on by the listener.
+ */
+function listenOnce(target, eventName, callback) {
+  target.addEventListener(eventName, function runOnce(event) {

[michaelpg, 2016/09/01 01:00:27]

dbeam: your example returned a Promise, but this way is more general-purpose
(and can be used synchronously when necessary)

+    target.removeEventListener(eventName, runOnce, false);

[michaelpg, 2016/09/01 01:00:27]

apparently closure requires this 3rd argument for EventTarget...

+    return callback(event);
+  }, false);

[Dan Beam, 2016/09/01 01:33:13]

we'll probably need some type @param {boolean=} opt_capture eventually, fine to
wait on that if we'd like

[michaelpg, 2016/09/01 02:20:27]

Acknowledged (does anyone actually use that? I've forgotten how it works more
often than I've used it.)

+}
+
 // <if expr="is_ios">
 // Polyfill 'key' in KeyboardEvent for iOS.
 // This function is not intended to be complete but should