Re: [w3c/ServiceWorker] How to create registration objects (#1445)

jakearchibald commented on this pull request.



> @@ -477,7 +477,27 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
       };
     </pre>
 
-    A {{ServiceWorkerRegistration}} object represents a [=/service worker registration=]. Each {{ServiceWorkerRegistration}} object is associated with a <dfn for="ServiceWorkerRegistration">service worker registration</dfn> (a [=/service worker registration=]). Multiple separate objects implementing the {{ServiceWorkerRegistration}} interface across documents and workers can all be associated with the same [=/service worker registration=] simultaneously.
+    <section>
+      <h4 id="service-worker-registration-creation">Getting {{ServiceWorkerRegistration}} instances</h4>
+
+      An [=environment settings object=] has a <dfn for="environment settings object">service worker registration object map</dfn>, a [=/map=] where the [=map/keys=] are [=/service worker registrations=] and the [=map/values=] are {{ServiceWorkerRegistration}} objects.
+
+      <section algorithm="service-worker-registration-creation-algorithm">
+        To <dfn lt="get the service worker registration object|getting the service worker registration object">get the service worker registration object</dfn> representing |registration| (a [=/service worker registration=]) in |environment| (an [=environment settings object=]), run these steps:
+
+          1. Let |objectMap| be |environment|'s [=environment settings object/service worker registration object map=].
+          1. If |objectMap|[|registration|] does not [=map/exist=], then:
+            1. Let |registrationObject| be a new {{ServiceWorkerRegistration}} in |environment|'s [=environment settings object/Realm=].
+            1. Set |registrationObject|'s [=ServiceWorkerRegistration/service worker registration=] to |registration|.
+            1. Set |registrationObject|'s {{ServiceWorkerRegistration/installing}} attribute to null if |registration|'s [=service worker registration/installing worker=] is null, otherwise the result of [=getting the service worker object=] that represents |registration|'s [=service worker registration/installing worker=] in |environment|.

Ugh really? I kinda hate that form since you have to read on to realise it's an "otherwise". But I guess the real problem here is it's too long for a one-liner. I'll split it up.

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