Support FieldValue.increment (googlearchive#232)

Closes googlearchive#227 
Fixes googlearchive#225

AUTHORS

@@ -5,3 +5,4 @@
 
 Google Inc.
 Jana Moudrá <[email protected]>
+Andrew Babin <[email protected]>

CHANGELOG.md

@@ -6,6 +6,8 @@
 - **BREAKING** The `Promise` polyfil has been removed from the JS SDK. Users
   will have to include their own polyfil for `Promise`. 
 
+- Added `FieldValue.increment` static function.
+
 ## 5.0.4
 
 - Require at least Dart 2.1.0.

lib/src/firestore.dart

@@ -889,19 +889,17 @@ abstract class _Updatable {
 
 class _FieldValueDelete implements FieldValue {
   @override
-  dynamic _jsify() {
-    return firestore_interop.FieldValue.delete();
-  }
+  firestore_interop.FieldValue _jsify() =>
+      firestore_interop.FieldValue.delete();
 
   @override
   String toString() => 'FieldValue.delete()';
 }
 
 class _FieldValueServerTimestamp implements FieldValue {
   @override
-  dynamic _jsify() {
-    return firestore_interop.FieldValue.serverTimestamp();
-  }
+  firestore_interop.FieldValue _jsify() =>
+      firestore_interop.FieldValue.serverTimestamp();
 
   @override
   String toString() => 'FieldValue.serverTimestamp()';
@@ -941,23 +939,25 @@ class _FieldValueArrayRemove extends _FieldValueArray {
   String toString() => 'FieldValue.arrayRemove($elements)';
 }
 
+class _FieldValueIncrement implements FieldValue {
+  final num n;
+
+  _FieldValueIncrement(this.n);
+
+  @override
+  firestore_interop.FieldValue _jsify() =>
+      firestore_interop.FieldValue.increment(n);
+
+  @override
+  String toString() => 'FieldValue.increment($n)';
+}
+
 dynamic jsifyFieldValue(FieldValue fieldValue) => fieldValue._jsify();
 
 /// Sentinel values that can be used when writing document fields with set()
 /// or update().
 abstract class FieldValue {
-  factory FieldValue._fromJs(dynamic jsFieldValue) {
-    if (jsFieldValue == firestore_interop.FieldValue.delete()) {
-      return FieldValue.delete();
-    } else if (jsFieldValue == firestore_interop.FieldValue.serverTimestamp()) {
-      return FieldValue.serverTimestamp();
-    } else {
-      throw ArgumentError.value(jsFieldValue, 'jsFieldValue',
-          "Invalid value provided. We don't support dartfying object like arrayUnion or arrayRemove since not needed");
-    }
-  }
-
-  dynamic _jsify();
+  firestore_interop.FieldValue _jsify();
 
   /// Returns a sentinel used with set() or update() to include a
   /// server-generated timestamp in the written data.
@@ -989,6 +989,20 @@ abstract class FieldValue {
   static FieldValue arrayRemove(List elements) =>
       _FieldValueArrayRemove(elements);
 
+  /// Returns a special value that can be used with set() or update() that tells
+  /// the server to increment the field's current value by the given value.
+
+  /// If either the operand or the current field value uses floating point
+  /// precision, all arithmetic follows IEEE 754 semantics. If both values
+  /// are integers, values outside of JavaScript's safe number range
+  /// (Number.MIN_SAFE_INTEGER to Number.MAX_SAFE_INTEGER) are also subject
+  /// to precision loss. Furthermore, once processed by the Firestore backend,
+  /// all integer operations are capped between -2^63 and 2^63-1.
+
+  /// If the current field value is not of type number, or if the field does not
+  /// yet exist, the transformation sets the field to the given value.
+  static FieldValue increment(num n) => _FieldValueIncrement(n);
+
   FieldValue._();
 
   static final FieldValue _serverTimestamp = _FieldValueServerTimestamp();

lib/src/interop/firestore_interop.dart

@@ -194,6 +194,8 @@ abstract class FieldValue {
   /// server-generated timestamp in the written data.
   external static FieldValue serverTimestamp();
 
+  external static FieldValue increment(num n);
+
   /// Returns `true` if this [FieldValue] is equal to the provided [other].
   external bool isEqual(Object other);
 }

test/firestore_test.dart

@@ -438,6 +438,29 @@ void main() {
       expect(timeStamp, isA<DateTime>());
     });
 
+    test('increment', () async {
+      var docRef = ref.doc("increment");
+
+      await docRef.set({
+        'increment': {'int': 1, 'double': 3.141, 'nonnumber': 'firebase'}
+      });
+
+      await docRef.set({
+        'increment': {
+          'int': fs.FieldValue.increment(100),
+          'double': fs.FieldValue.increment(1.618),
+          'nonnumber': fs.FieldValue.increment(42)
+        }
+      }, fs.SetOptions(merge: true));
+
+      var snapshot = await docRef.get();
+      var snapshotData = snapshot.data();
+
+      expect(snapshotData, {
+        "increment": {'int': 101, 'double': 4.759, 'nonnumber': 42}
+      });
+    });
+
     test("update nested with dot notation", () async {
       var docRef = await ref.add({
         "greeting": {"text": "Good Morning"}