fix(database): retrieve initial list content once (#820)

* fix(database): retrieve initial list content once

Closes #819

* test(database): add a child_added quirk test

Author: cartant

src/database/firebase_list_factory.spec.ts

@@ -391,6 +391,32 @@ describe('FirebaseListFactory', () => {
     });
 
 
+    it('should be resistant to non-asynchronous child_added quirks', (done: any) => {
+
+      // If push is called (or set or update, too, I guess) immediately after
+      // an on or once listener is added, it appears that the on or once
+      // child_added listeners are invoked immediately - i.e. not
+      // asynchronously - and the list implementation needs to support that.
+
+      questions.$ref.ref.push({ number: 1 })
+        .then(() => {
+          let calls = [];
+          questions.$ref.ref.once('child_added', (snap) => calls.push('child_added:' + snap.val().number));
+          skipAndTake(questions).subscribe(
+            (list) => {
+              expect(calls).toEqual(['child_added:2', 'pushed']);
+              expect(list.map(i => i.number)).toEqual([1, 2]);
+              done();
+            },
+            done.fail
+          );
+          questions.push({ number: 2 });
+          calls.push('pushed');
+        })
+        .catch(done.fail);
+    });
+
+
     it('should emit a new value when a child moves', (done: any) => {
        let question = skipAndTake(questions, 1, 2)
        subscription = _do.call(question, (data: any) => {

src/database/firebase_list_factory.ts

@@ -108,88 +108,71 @@ export function FirebaseListFactory (
  * is loaded, the observable starts emitting values.
  */
 function firebaseListObservable(ref: firebase.database.Reference | firebase.database.Query, {preserveSnapshot}: FirebaseListFactoryOpts = {}): FirebaseListObservable<any> {
+
   const toValue = preserveSnapshot ? (snapshot => snapshot) : utils.unwrapMapFn;
   const toKey = preserveSnapshot ? (value => value.key) : (value => value.$key);
-  // Keep track of callback handles for calling ref.off(event, handle)
-  const handles = [];
+
   const listObs = new FirebaseListObservable(ref, (obs: Observer<any[]>) => {
-    ref.once('value')
-      .then((snap) => {
-        let initialArray = [];
-        snap.forEach(child => {
-          initialArray.push(toValue(child))
-        });
-        return initialArray;
-      })
-      .then((initialArray) => {
-        const isInitiallyEmpty = initialArray.length === 0;
-        let hasInitialLoad = false;
-        let lastKey;
-
-        if (!isInitiallyEmpty) {
-          // The last key in the initial array tells us where
-          // to begin listening in realtime
-          lastKey = toKey(initialArray[initialArray.length - 1]);
-        }
 
-        const addFn = ref.on('child_added', (child: any, prevKey: string) => {
-          // If the initial load has not been set and the current key is
-          // the last key of the initialArray, we know we have hit the
-          // initial load
-          if (!isInitiallyEmpty && !hasInitialLoad) {
-            if (child.key === lastKey) {
-              hasInitialLoad = true;
-              obs.next(initialArray);
-              return;
-            }
-          }
-
-          if (hasInitialLoad) {
-            initialArray = onChildAdded(initialArray, toValue(child), toKey, prevKey);
-          }
-
-          // only emit the array after the initial load
-          if (hasInitialLoad) {
-            obs.next(initialArray);
-          }
-        }, err => {
-          if (err) { obs.error(err); obs.complete(); }
-        });
+    // Keep track of callback handles for calling ref.off(event, handle)
+    const handles = [];
+    let hasLoaded = false;
+    let lastLoadedKey: string = null;
+    let array = [];
 
-        handles.push({ event: 'child_added', handle: addFn });
+    // The list children are always added to, removed from and changed within
+    // the array using the child_added/removed/changed events. The value event
+    // is only used to determine when the initial load is complete.
 
-        let remFn = ref.on('child_removed', (child: any) => {
-          initialArray = onChildRemoved(initialArray, toValue(child), toKey);
-          if (hasInitialLoad) {
-            obs.next(initialArray);
-          }
-        }, err => {
-          if (err) { obs.error(err); obs.complete(); }
-        });
-        handles.push({ event: 'child_removed', handle: remFn });
-
-        let chgFn = ref.on('child_changed', (child: any, prevKey: string) => {
-          initialArray = onChildChanged(initialArray, toValue(child), toKey, prevKey)
-          if (hasInitialLoad) {
-            // This also manages when the only change is prevKey change
-            obs.next(initialArray);
-          }
-        }, err => {
-          if (err) { obs.error(err); obs.complete(); }
+    ref.once('value', (snap: any) => {
+      if (snap.exists()) {
+        snap.forEach((child: any) => {
+          lastLoadedKey = child.key;
         });
-        handles.push({ event: 'child_changed', handle: chgFn });
-
-        // If empty emit the array
-        if (isInitiallyEmpty) {
-          obs.next(initialArray);
-          hasInitialLoad = true;
+        if (array.find((child: any) => toKey(child) === lastLoadedKey)) {
+          hasLoaded = true;
+          obs.next(array);
         }
-      }, err => {
-        if (err) { 
-          obs.error(err); 
-          obs.complete(); 
-        }
-      });
+      } else {
+        hasLoaded = true;
+        obs.next(array);
+      }
+    }, err => {
+      if (err) { obs.error(err); obs.complete(); }
+    });
+
+    const addFn = ref.on('child_added', (child: any, prevKey: string) => {
+      array = onChildAdded(array, toValue(child), toKey, prevKey);
+      if (hasLoaded) {
+        obs.next(array);
+      } else if (child.key === lastLoadedKey) {
+        hasLoaded = true;
+        obs.next(array);
+      }
+    }, err => {
+      if (err) { obs.error(err); obs.complete(); }
+    });
+    handles.push({ event: 'child_added', handle: addFn });
+
+    let remFn = ref.on('child_removed', (child: any) => {
+      array = onChildRemoved(array, toValue(child), toKey);
+      if (hasLoaded) {
+        obs.next(array);
+      }
+    }, err => {
+      if (err) { obs.error(err); obs.complete(); }
+    });
+    handles.push({ event: 'child_removed', handle: remFn });
+
+    let chgFn = ref.on('child_changed', (child: any, prevKey: string) => {
+      array = onChildChanged(array, toValue(child), toKey, prevKey);
+      if (hasLoaded) {
+        obs.next(array);
+      }
+    }, err => {
+      if (err) { obs.error(err); obs.complete(); }
+    });
+    handles.push({ event: 'child_changed', handle: chgFn });
 
     return () => {
       // Loop through callback handles and dispose of each event with handle