Add an activity cache, to retrieve activities from their id.

src/known-activities.js

@@ -0,0 +1,58 @@
+const {Activity} = require('./activity.js')
+
+// A cache for activities, to be able to reference them by their id
+const KnownActivities = {
+  // Cache
+  activities: {},
+  // Methods
+  get: function(id) {
+    return KnownActivities.activities[id]
+  },
+  set: function(id, obj) {
+    KnownActivities.activities[id] = obj
+  },
+  // Retrieve an object
+  // The callback takes 3 arguments: load_ok, retrieved_activity, failure_message
+  retrieve: function(id, token, callback) {
+    if (KnownActivities.get(id)) {
+      callback(true, KnownActivities.get(id), undefined)
+    } else {
+      // Use ActivityPub protocol to get the activity
+      // The id is the link to the activity on the server
+      const request = new XMLHttpRequest()
+      request.onreadystatechange = function() {
+        if (request.readyState == 4 && request.status == 200) {
+          const answer = JSON.parse(request.responseText)
+          if (answer) {
+            const activity = new Activity(answer)
+            activity.loadActors(
+              function(load_ok, failure_message) {
+                if (load_ok) {
+                  KnownActivities.set(id, activity)
+                  callback(true, activity, undefined)
+                } else {
+                  callback(false, undefined, 'Unable to retrieve actors of activity.')
+                  console.log(answer)
+                }
+              })
+          } else {
+            callback(false, undefined, 'Unable to retrieve activity.')
+            console.log(answer)
+          }
+        } else if (request.readyState == 4) {
+          callback(false, undefined, 'Error during retrieval of activity.')
+        }
+      }
+      request.open('GET', id, true)
+      if (token) {
+        request.setRequestHeader('Authorization', 'Bearer ' + token)
+      }
+      request.setRequestHeader('Content-Type', 'application/activity+json')
+      request.setRequestHeader('Accept', 'application/activity+json')
+      request.send()
+    }
+  }
+}
+
+// Exported structures
+exports.KnownActivities = KnownActivities