Fixes w3c#152. Each Sensor instance fires 'change' event considering …

…its individual

frequency hint. Thus we achieve that  appearance of a new Sensor instance with a
higher frequency hint does not affect the 'onchange' notification of the existing
Sensor instances of the same type.

index.bs

@@ -758,9 +758,9 @@ with the internal slots described in the following table:
         </tr>
         <tr>
             <td><dfn attribute for=Sensor>\[[waitingForUpdate]]</dfn></td>
-            <td>A boolean which indicates wether the observers have been updated
+            <td>A boolean which indicates whether the observers have been updated
                 or whether the object is waiting for a new reading to do so.
-                It is initially `true`.</td>
+                It is initially `false`.</td>
         </tr>
         <tr>
             <td><dfn attribute for=Sensor>\[[identifyingParameters]]</dfn></td>
@@ -826,8 +826,8 @@ The {{Sensor/stop()}} method must run these steps or their [=equivalent=]:
 
 ### Sensor.onchange ### {#sensor-onchange}
 
-{{Sensor/onchange}} is an {{EventHandler}} which is called
-whenever a new [=sensor reading|reading=] is available.
+{{Sensor/onchange}} is an {{EventHandler}} which is called to notify that new [=sensor reading|reading=] is available.
+
 
 Issue: Should this be renamed `onreading`?
 Should we instead add an `ondata` {{EventHandler}} for continuous data
@@ -1175,6 +1175,9 @@ Gets the {{Error}} object passed to {{SensorErrorEventInit}}.
         value of |reading| to see if there's a change
         that needs to be propagated.
     1.  Unset |sensor|’s [=reporting flag=].
+    1.  [=set/For each=] |s| of |activated_sensors|,
+        1.  Invoke the [=Report Latest Reading updated=] abstract operation,
+            passing it |s| as argument.
 </div>
 
 
@@ -1203,6 +1206,74 @@ Gets the {{Error}} object passed to {{SensorErrorEventInit}}.
 </div>
 
 
+<h3 dfn>Find the reporting frequency of a Sensor</h3>
+
+<div algorithm="find the reporting frequency of a sensor">
+
+    : input
+    :: |sensor_instance|, a {{Sensor}} object.
+    : output
+    :: |frequency|, a [=frequency=].
+    1.  Let |frequency| be `null`.
+    1.  Let |f| be |sensor_instance|.{{[[desiredPollingFrequency]]}}.
+        1. if |f| is set,
+            1. set |frequency| to |f| capped by the upper and lower [=current polling frequency=]
+               bounds for the associated [=sensor=].
+        1.  Otherwise,
+            1. user agent can assign |frequency| to an appropriate value.
+    1. return |frequency|.
+</div>
+
+
+<h3 dfn>Report Latest Reading updated</h3>
+
+<div algorithm="report latest reading updated">
+
+    : input
+    :: |sensor_instance|, a {{Sensor}} object.
+    : output
+    :: None
+
+    1.  If |sensor_instance|.{{[[waitingForUpdate]]}} is `true`,
+        1. Abort these steps. 
+    1.  Set |sensor_instance|.{{[[waitingForUpdate]]}} to `true`.
+    1.  Let |lastReportedTimestamp| be the value of |sensor_instance|.{{[[lastEventFiredAt]]}}.
+    1.  If |lastReportedTimestamp| is not set
+        1.  Queue a task to run the [=Notify Sensor Object about new reading=] abstract operation,
+            passing it |sensor_instance| as argument.
+        1.  Abort these steps.
+    1.  Let |reportingFrequency| be result of invoking the [=Find the reporting frequency of a Sensor=]
+        abstract operation.
+    1.  If |reportingFrequency| is `null`
+        1.  Queue a task to run the [=Notify Sensor Object about new reading=] abstract operation,
+            passing it |sensor_instance| as argument.
+        1.  Abort these steps.
+    1.  Let |reportingInterval| be the result of 1 / |reportingFrequency|.
+    1.  Let |timestampDelta| be the result of [=latest reading=]["timestamp"] - |lastReportedTimestamp|.
+    1.  If  |timestampDelta| is greater than or equal to |reportingInterval|
+        1.  Queue a task to run the [=Notify Sensor Object about new reading=] abstract operation,
+            passing it |sensor_instance| as argument.
+        1.  Abort these steps.
+    1.  Let |deferUpdateTime| be the result of |reportingInterval| - |timestampDelta|.
+    1.  User agent must defer queueing a task to run the [=Notify Sensor Object about new reading=] abstract
+        operation for a period of time equal to |deferUpdateTime|.
+</div>
+
+<h3 dfn>Notify Sensor Object about new reading</h3>
+
+<div algorithm="notify sensor object about new reading">
+
+    : input
+    :: |sensor_instance|, a {{Sensor}} object.
+    : output
+    :: None
+
+    1.  Set |sensor_instance|.{{[[waitingForUpdate]]}} to `false`.
+    1.  Set |sensor_instance|.{{[[lastEventFiredAt]]}} to [=latest reading=]["timestamp"].
+    1.  [=Fire an event=] named "change" at |sensor_instance|.
+</div>
+
+
 <h3 dfn>Handle Errors</h3>
 
 <div algorithm="handle errors">