Sensor obects have their own a reading copies.

Fixes w3c#152. Each Sensor instance reads the sensor readings considering its individual
frequency hint, sends 'onchange' and caches the sensor latest reading at this moment.
The Sensor's attributes return values from the cached reading.
Thus we achieve:

- 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.
- consistency between the Sensor's 'onchange' notification and its attribute values.

Fixes w3c#168. A Sensor object returns reading values only in "activated" state and returns null otherwise.

index.bs

@@ -746,7 +746,8 @@ with the internal slots described in the following table:
         </tr>
         <tr>
             <td><dfn attribute for=Sensor>\[[desiredPollingFrequency]]</dfn></td>
-            <td>The requested polling frequency. It is initially unset.</td>
+            <td>The requested polling frequency for the {{Sensor}} object.
+                It is initially unset.</td>
         </tr>
         <tr>
             <td><dfn attribute for=Sensor>\[[lastEventFiredAt]]</dfn></td>
@@ -762,6 +763,21 @@ with the internal slots described in the following table:
                 or whether the object is waiting for a new reading to do so.
                 It is initially `true`.</td>
         </tr>
+        <tr>
+            <td><dfn attribute for=Sensor>\[[reading]]</dfn></td>
+            <td>A deep copy of the the latest [=sensor reading=]
+                for the {{Sensor}} object.
+                Initially, it contains [=map/entries=] with the same
+                [=map/keys=] as the [=map/entries=] in [=latest reading=]
+                [=ordered map|map=] and with all the [=map/values=] set to `null`.
+            </td>
+        </tr>
+        <tr>
+            <td><dfn attribute for=Sensor>\[[readingUpdateRequested]]</dfn></td>
+            <td>A boolean which indicates whether {{[[reading]]}} [=ordered map|map=]
+                needs to be updated after a new [=sensor reading=] was reported.
+                It is initially `false`.</td>
+        </tr>
         <tr>
             <td><dfn attribute for=Sensor>\[[identifyingParameters]]</dfn></td>
             <td>
@@ -787,7 +803,7 @@ with the internal slots described in the following table:
 ### Sensor.timestamp ### {#sensor-timestamp}
 
 The getter of the {{Sensor/timestamp!!attribute}} attribute returns
-[=latest reading=]["timestamp"].
+<emu-val>this</emu-val>.{{[[reading]]}}["timestamp"].
 
 ### Sensor.start() ### {#sensor-start}
 
@@ -827,7 +843,7 @@ 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.
+when <emu-val>this</emu-val>.{{[[reading]]}} is updated with the [=latest reading=] [=map/entries=].
 
 Issue: Should this be renamed `onreading`?
 Should we instead add an `ondata` {{EventHandler}} for continuous data
@@ -986,6 +1002,8 @@ Gets the {{Error}} object passed to {{SensorErrorEventInit}}.
     : output
     :: None
 
+    1.  [=map/For each=] |key| → |value| of |sensor_instance|.{{[[reading]]}}.
+        1.  [=map/Set=] |sensor_instance|.{{[[reading]]}}[|key|] to `null`.
     1.  Let |sensor| be the [=sensor=] associated with |sensor_instance|.
     1.  Remove |sensor_instance| from |sensor|'s set of [=activated Sensor objects=].
     1.  If |sensor|'s set of [=activated Sensor objects=] is empty,
@@ -1175,6 +1193,9 @@ 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 +1224,73 @@ 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|.{{[[readingUpdateRequested]]}} is `true`,
+        1. Abort these steps. 
+    1.  Set |sensor_instance|.{{[[readingUpdateRequested]]}} to `true`.
+    1.  Let |lastReportedTimestamp| be |sensor_instance|.{{[[reading]]}}["timestamp"].
+    1.  If |lastReportedTimestamp| is not set
+        1.  Invoke the [=Update Sensor 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 not set
+        1.  Invoke the [=Update Sensor 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 less than or equal to |reportingInterval|
+        1.  Invoke the [=Update Sensor 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 invoking of the [=Update Sensor Reading=] abstract operation for a period of time
+        equal to |deferUpdateTime|.
+</div>
+
+<h3 dfn>Update Sensor Reading</h3>
+
+<div algorithm="update sensor reading">
+
+    : input
+    :: |sensor_instance|, a {{Sensor}} object.
+    : output
+    :: None
+
+    1.  [=map/For each=] |key| → |value| of [=latest reading=].
+        1.  [=map/Set=] |sensor_instance|.{{[[reading]]}}[|key|] to the corresponding
+            value of [=latest reading=].
+    1.  Set |sensor_instance|.{{[[readingUpdateRequested]]}} to `false`.
+    1.  [=Fire an event=] named "change" at |sensor_instance|.
+</div>
+
+
 <h3 dfn>Handle Errors</h3>
 
 <div algorithm="handle errors">
@@ -1214,6 +1302,8 @@ Gets the {{Error}} object passed to {{SensorErrorEventInit}}.
     :: None
 
     1.  Set |sensor_instance|.{{[[state]]}} to "idle".
+    1.  [=map/For each=] |key| → |value| of |sensor_instance|.{{[[reading]]}}.
+        1.  [=map/Set=] |sensor_instance|.{{[[reading]]}}[|key|] to `null`.
     1.  [=Fire an event=] named "error" at |sensor_instance| using {{SensorErrorEvent}}
         with its {{SensorErrorEvent/error!!attribute}} attribute initialized to |error|.
 </div>