[firestore] Support all onSnapshot parameter options

lib/modules/firestore/DocumentReference.js

@@ -5,12 +5,21 @@
 import CollectionReference from './CollectionReference';
 import DocumentSnapshot from './DocumentSnapshot';
 import Path from './Path';
-import { firestoreAutoId } from '../../utils';
+import { firestoreAutoId, isFunction, isObject } from '../../utils';
 
 export type WriteOptions = {
   merge?: boolean,
 }
 
+type DocumentListenOptions = {
+  includeMetadataChanges: boolean,
+}
+
+type Observer = {
+  next: (DocumentSnapshot) => void,
+  error?: (Object) => void,
+}
+
  /**
  * @class DocumentReference
  */
@@ -60,13 +69,64 @@ export default class DocumentReference {
       .then(result => new DocumentSnapshot(this._firestore, result));
   }
 
-  onSnapshot(onNext: Function, onError?: Function): () => void {
-    // TODO: Validation
+  onSnapshot(
+    optionsOrObserverOrOnNext: DocumentListenOptions | Observer | (DocumentSnapshot) => void,
+    observerOrOnNextOrOnError?: Observer | (DocumentSnapshot) => void | (Object) => void,
+    onError?: (Object) => void
+  ) {
+    let observer = {};
+    let docListenOptions = {};
+    // Called with: onNext, ?onError
+    if (isFunction(optionsOrObserverOrOnNext)) {
+      observer.next = optionsOrObserverOrOnNext;
+      if (observerOrOnNextOrOnError && !isFunction(observerOrOnNextOrOnError)) {
+        throw new Error('DocumentReference.onSnapshot failed: Second argument must be a valid function.');
+      }
+      observer.error = observerOrOnNextOrOnError;
+    } else if (optionsOrObserverOrOnNext && isObject(optionsOrObserverOrOnNext)) {
+      // Called with: Observer
+      if (optionsOrObserverOrOnNext.next) {
+        if (isFunction(optionsOrObserverOrOnNext.next)) {
+          if (optionsOrObserverOrOnNext.error && !isFunction(optionsOrObserverOrOnNext.error)) {
+            throw new Error('DocumentReference.onSnapshot failed: Observer.error must be a valid function.');
+          }
+          observer = optionsOrObserverOrOnNext;
+        } else {
+          throw new Error('DocumentReference.onSnapshot failed: Observer.next must be a valid function.');
+        }
+      } else if (optionsOrObserverOrOnNext.includeMetadataChanges) {
+        docListenOptions = optionsOrObserverOrOnNext;
+        // Called with: Options, onNext, ?onError
+        if (isFunction(observerOrOnNextOrOnError)) {
+          observer.next = observerOrOnNextOrOnError;
+          if (onError && !isFunction(onError)) {
+            throw new Error('DocumentReference.onSnapshot failed: Third argument must be a valid function.');
+          }
+          observer.error = onError;
+        // Called with Options, Observer
+        } else if (observerOrOnNextOrOnError && isObject(observerOrOnNextOrOnError) && observerOrOnNextOrOnError.next) {
+          if (isFunction(observerOrOnNextOrOnError.next)) {
+            if (observerOrOnNextOrOnError.error && !isFunction(observerOrOnNextOrOnError.error)) {
+              throw new Error('DocumentReference.onSnapshot failed: Observer.error must be a valid function.');
+            }
+            observer = observerOrOnNextOrOnError;
+          } else {
+            throw new Error('DocumentReference.onSnapshot failed: Observer.next must be a valid function.');
+          }
+        } else {
+          throw new Error('DocumentReference.onSnapshot failed: Second argument must be a function or observer.');
+        }
+      } else {
+        throw new Error('DocumentReference.onSnapshot failed: First argument must be a function, observer or options.');
+      }
+    } else {
+      throw new Error('DocumentReference.onSnapshot failed: Called with invalid arguments.');
+    }
     const listenerId = firestoreAutoId();
 
     const listener = (nativeDocumentSnapshot) => {
       const documentSnapshot = new DocumentSnapshot(this, nativeDocumentSnapshot);
-      onNext(documentSnapshot);
+      observer.next(documentSnapshot);
     };
 
     // Listen to snapshot events
@@ -76,16 +136,16 @@ export default class DocumentReference {
     );
 
     // Listen for snapshot error events
-    if (onError) {
+    if (observer.error) {
       this._firestore.on(
         this._firestore._getAppEventName(`onDocumentSnapshotError:${listenerId}`),
-        onError,
+        observer.error,
       );
     }
 
     // Add the native listener
     this._firestore._native
-      .documentOnSnapshot(this.path, listenerId);
+      .documentOnSnapshot(this.path, listenerId, docListenOptions);
 
     // Return an unsubscribe method
     return this._offDocumentSnapshot.bind(this, listenerId, listener);