Fix "dropped" frames for text updates (#224, #339) using @timthelion's insights in #350

* Change the implementation of `notify` so that it does not have a
return value any more.

* Get rid of the only remaning use of the return value of `notify` in
the `every` function.

* Have a "lock" on each round to ensure that no library or runtime
calls `notify` synchronously, causing an update to happen half way
through another update, ultimately leading to a dropped frame.

libraries/Native/Signal/Time.js

@@ -34,13 +34,11 @@ Elm.Native.Time.make = function(elm) {
     return A3( Signal.lift2, F2(f), isOn, ticker );
   }
 
-  function everyWhen(t, isOn) {
+  function every(t) {
     var clock = Signal.constant(Date.now());
-    var id = setInterval(function tellTime() {
-            if (!elm.notify(clock.id, Date.now())) {
-                clearInterval(id);
-            }
-        }, t);
+    setInterval(function() {
+        elm.notify(clock.id, Date.now());
+    }, t);
     return clock;
   }
 
@@ -56,7 +54,7 @@ Elm.Native.Time.make = function(elm) {
   return elm.Native.Time.values = {
       fpsWhen : F2(fpsWhen),
       fps : function(t) { return fpsWhen(t, Signal.constant(true)); },
-      every : function(t) { return everyWhen(t, Signal.constant(true)) },
+      every : every,
       delay : NS.delay,
       timestamp : NS.timestamp,
       since : F2(since),

libraries/Native/Signal/Window.js

@@ -28,10 +28,21 @@ Elm.Native.Window.make = function(elm) {
   height.defaultNumberOfKids = 0;
 
   function resizeIfNeeded() {
+      // Do not trigger event if the dimensions have not changed.
+      // This should be most of the time.
       var w = getWidth();
       var h = getHeight();
       if (dimensions.value._0 === w && dimensions.value._1 === h) return;
-      elm.notify(dimensions.id, Tuple2(w,h));
+
+      setTimeout(function () {
+          // Check again to see if the dimensions have changed.
+          // It is conceivable that the dimensions have changed
+          // again while some other event was being processed.
+          var w = getWidth();
+          var h = getHeight();
+          if (dimensions.value._0 === w && dimensions.value._1 === h) return;
+          elm.notify(dimensions.id, Tuple2(w,h));
+      }, 0);
   }
   elm.addListener([dimensions.id], window, 'resize', resizeIfNeeded);
 

runtime/Init.js

@@ -31,14 +31,20 @@ function init(display, container, module, moduleToReplace) {
   // defining state needed for an instance of the Elm RTS
   var inputs = [];
 
+  var updateInProgress = false;
   function notify(id, v) {
-      var timestep = Date.now();
-      var changed = false;
-      for (var i = inputs.length; i--; ) {
-          // order is important here to avoid short-circuiting
-          changed = inputs[i].recv(timestep, id, v) || changed;
+      if (updateInProgress) {
+          throw new Error(
+              'The notify function has been called synchronously!\n' +
+              'This can lead to frames being dropped.\n' +
+              'Definitely report this to <https://github.com/evancz/Elm/issues>\n');
       }
-      return changed;
+      updateInProgress = true;
+      var timestep = Date.now();
+      for (var i = inputs.length; i--; ) {
+          inputs[i].recv(timestep, id, v);
+      }
+      updateInProgress = false;
   }
 
   var listeners = [];