Spec inspect() operator (#168)

spec.bs

@@ -326,7 +326,19 @@ dictionary SubscriptionObserver {
   VoidFunction complete;
 };
 
+callback ObservableInspectorAbortHandler = undefined (any value);
+
+dictionary ObservableInspector {
+  ObservableSubscriptionCallback next;
+  ObservableSubscriptionCallback error;
+  VoidFunction complete;
+
+  VoidFunction subscribe;
+  ObservableInspectorAbortHandler abort;
+};
+
 typedef (ObservableSubscriptionCallback or SubscriptionObserver) ObserverUnion;
+typedef (ObservableSubscriptionCallback or ObservableInspector) ObservableInspectorUnion;
 
 dictionary SubscribeOptions {
   AbortSignal signal;
@@ -362,6 +374,7 @@ interface Observable {
   Observable drop(unsigned long long amount);
   Observable flatMap(Mapper mapper);
   Observable switchMap(Mapper mapper);
+  Observable inspect(optional ObservableInspectorUnion inspect_observer = {});
   Observable finally(VoidFunction callback);
 
   // Promise-returning operators.
@@ -1085,6 +1098,115 @@ For now, see [https://github.com/wicg/observable#operators](https://github.com/w
         |innerObserver| and |innerOptions|.
 </div>
 
+<div algorithm>
+  The <dfn for=Observable method><code>inspect(|inspector_union|)</code></dfn> method steps are:
+
+    1. Let |subscribe callback| be a {{VoidFunction}}-or-null, initially null.
+
+    1. Let |next callback| be a {{ObservableSubscriptionCallback}}-or-null, initially null.
+
+    1. Let |error callback| be a {{ObservableSubscriptionCallback}}-or-null, initially null.
+
+    1. Let |complete callback| be a {{VoidFunction}}-or-null, initially null.
+
+    1. Let |abort callback| be a {{ObservableInspectorAbortHandler}}-or-null, initially null.
+
+    1. Process |inspector_union| as follows:
+         <dl class="switch">
+           <dt>If |inspector_union| is an {{ObservableSubscriptionCallback}}</dt>
+           <dd>
+             1. Set |next callback| to |inspector_union|.
+
+           <dt>If |inspector_union| is an {{ObservableInspector}}</dt>
+           <dd>
+             1. If {{ObservableInspector/subscribe}} [=map/exists=] in |inspector_union|, then set
+                |subscribe callback| to it.
+
+             1. If {{ObservableInspector/next}} [=map/exists=] in |inspector_union|, then set
+                |next callback| to it.
+
+             1. If {{ObservableInspector/error}} [=map/exists=] in |inspector_union|, then set
+                |error callback| to it.
+
+             1. If {{ObservableInspector/complete}} [=map/exists=] in |inspector_union|, then set
+                |complete callback| to it.
+
+             1. If {{ObservableInspector/abort}} [=map/exists=] in |inspector_union|, then set
+                |abort callback| to it.
+           </dd>
+         </dl>
+
+    1. Let |sourceObservable| be [=this=].
+
+    1. Let |observable| be a [=new=] {{Observable}} whose [=Observable/subscribe callback=] is an
+       algorithm that takes a {{Subscriber}} |subscriber| and does the following:
+
+       1. If |subscribe callback| is not null, then [=invoke=] it.
+
+          If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then run
+          |subscriber|'s {{Subscriber/error()}} method, given |E|, and abort these steps.
+
+          Note: The result of this is that |sourceObservable| is never subscribed to.
+
+       1. If |abort callback| is not null, then [=AbortSignal/add|add the following abort
+          algorithm=] to |subscriber|'s [=Subscriber/subscription controller=]'s
+          [=AbortController/signal=]:
+
+          1. [=Invoke=] |abort callback| with |subscriber|'s [=Subscriber/subscription
+             controller=]'s [=AbortController/signal=]'s [=AbortSignal/abort reason=].
+
+             If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then
+             [=report the exception=] |E|.
+
+       1. Let |sourceObserver| be a new [=internal observer=], initialized as follows:
+
+          : [=internal observer/next steps=]
+          :: 1. If |next callback| is not null, then [=invoke=] |next callback| with the passed in
+                |value|.
+
+                If <a spec=webidl lt="an exception was thrown">an exception |E| was thrown</a>, then:
+
+                  1. [=AbortSignal/Remove=] |abort callback| from |subscriber|'s
+                     [=Subscriber/subscription controller=]'s [=AbortController/signal=].
+
+                     Note: This step is important, because the |abort callback| is only meant to be
+                     called for *consumer-initiated* unsubscriptions. When the producer terminates
+                     the subscription (via |subscriber|'s {{Subscriber/error()}} or
+                     {{Subscriber/complete()}} methods) like below, we have to ensure that
+                     |abort callback| is not run.
+
+                     Issue: This matches Chromium's implementation, but consider holding a reference
+                     to the originally-passed-in {{SubscribeOptions}}'s {{SubscribeOptions/signal}}
+                     and just invoking |abort callback| when *it* aborts. The result is likely the
+                     same, but needs investigation.
+
+                  1. Run |subscriber|'s {{Subscriber/error()}} method, given |E|, and return.
+
+             1. Run |subscriber|'s {{Subscriber/next()}} method with the passed in |value|.
+
+          : [=internal observer/error steps=]
+          :: [=AbortSignal/Remove=] |abort callback| from |subscriber|'s [=Subscriber/subscription
+             controller=]'s [=AbortController/signal=], and run |subscriber|'s
+             {{Subscriber/error()}} method, given the passed in <var ignore>error</var>.
+
+          : [=internal observer/complete steps=]
+          :: [=AbortSignal/Remove=] |abort callback| from |subscriber|'s [=Subscriber/subscription
+             controller=]'s [=AbortController/signal=], and run |subscriber|'s
+             {{Subscriber/complete()}} method.
+
+       1. Let |options| be a new {{SubscribeOptions}} whose {{SubscribeOptions/signal}} is
+          |subscriber|'s [=Subscriber/subscription controller=]'s [=AbortController/signal=].
+
+       1. <a for=Observable lt="subscribe to an Observable">Subscribe</a> to |sourceObservable|
+          given |sourceObserver| and |options|.
+
+    1. Return |observable|.
+
+  <wpt>
+    /dom/observable/tentative/observable-inspect.any.js
+  </wpt>
+</div>
+
 <div algorithm>
   The <dfn for=Observable method><code>finally(|callback|)</code></dfn> method steps are: