New: new technique for “waiting for a variable”

This is better than just using a timer and guessing
how long it will take for some variables to be ready.

Author: bnmnetp

views/peer/peer_question.html

@@ -40,45 +40,69 @@ <h2>Peer Instruction: {{=assignment_name}}</h2>
         <form>
             <div class="form-group">
                 Please provide a confidential rating of <select id="peersel" name="peercel"></select>
-                 peer's explanation<br />
-                <input class="form-check-input ratingradio" type="radio" name="inlineRadioOptions" id="inlineRadio1" value="poor"
-                    onclick="ratePeer(this)">
+                peer's explanation<br />
+                <input class="form-check-input ratingradio" type="radio" name="inlineRadioOptions" id="inlineRadio1" value="poor" onclick="ratePeer(this)">
                 <label class="form-check-label" for="inlineRadio1">Poor</label>
-                <input class="form-check-input ratingradio" type="radio" name="inlineRadioOptions" id="inlineRadio2" value="good"
-                    onclick="ratePeer(this)">
+                <input class="form-check-input ratingradio" type="radio" name="inlineRadioOptions" id="inlineRadio2" value="good" onclick="ratePeer(this)">
                 <label class="form-check-label" for="inlineRadio2">Good</label>
-                <input class="form-check-input ratingradio" type="radio" name="inlineRadioOptions" id="inlineRadio3"
-                    value="excellent" onclick="ratePeer(this)">
+                <input class="form-check-input ratingradio" type="radio" name="inlineRadioOptions" id="inlineRadio3" value="excellent" onclick="ratePeer(this)">
                 <label class="form-check-label" for="inlineRadio3">Excellent</label>
             </div>
         </form>
     </div>
 </div>
-<p>Please do not leave or refresh the page during a peer instruction session.  <strong>Exception:</strong> If you should have seen a new question at this point then you may continue.</p>
+<p>Please do not leave or refresh the page during a peer instruction session. <strong>Exception:</strong> If you should have seen a new question at this point then you may continue.</p>
 
 <script src="/runestone/static/js/peer.js?v={{=request.peer_mtime}}"></script>
 <script>
     var user = '{{=auth.user.username}}';
     var currentQuestion = '{{=XML(current_question.name)}}';
     var vote2done = false;
-    document.addEventListener("DOMContentLoaded", function (event) {
+    document.addEventListener("DOMContentLoaded", function(event) {
         setTimeout(connect, 1000);
-        setTimeout(function () {
-            window.mcList[currentQuestion].submitButton.innerHTML = "Submit";
-            window.mcList[currentQuestion].submitButton.addEventListener(
-                "click",
-                function() {
-                    if (studentVoteCount == 2) {
-                        vote2done = true;
-                    }
+    });
+
+    // This function serves the purpose of making sure that a variable is defined before we
+    // access it.
+    function ensureMcList(timeout) {
+        var start = Date.now();
+        return new Promise(waitForIt); // set the promise object
+        // waitForIt makes the decision whether the condition is met
+        // or not met or the timeout has been exceeded which means
+        // this promise will be rejected
+        // TODO -- this could/should be generalized into a library so it can
+        // take the name of any variable as a string along with a function to do whatever needs to be done.
+        function waitForIt(resolve, reject) {
+            if (window.mcList && Object.keys(window.mcList).length > 0) {
+                console.log("mcList is set, resolving...")
+                resolve(window.mcList[currentQuestion]);
+            }
+            else if (timeout && (Date.now() - start) >= timeout)
+                reject(new Error("timeout"));
+            else {
+                console.log("wiating for mclist")
+                setTimeout(waitForIt.bind(this, resolve, reject), 30);
+            }
+        }
+    }
+
+    // This says what to do when the mcList variable from above is defined.
+    ensureMcList(50000).then(function() {
+        console.log("mcList is set");
+        window.mcList[currentQuestion].submitButton.innerHTML = "Submit";
+        window.mcList[currentQuestion].submitButton.addEventListener(
+            "click",
+            function() {
+                if (studentVoteCount == 2) {
+                    vote2done = true;
                 }
-            );
-        }, 1000);
+            })
     });
+
     var studentVoteCount = 1;
     $(window).on(
         "beforeunload",
-        function (event) {
+        function(event) {
             // this actual value gets ignored by newer browsers
             if (this.done) {
                 return;