setting with compatible snapshot

lib/src/snapshot.dart

@@ -105,6 +105,15 @@ abstract class Snapshot implements DeepImmutable {
   ///
   /// Unmodified children and grandchildren are recycled. So, also their
   /// conversions are reused.
+  ///
+  /// [value] may either be a JSON-like object or a [Snapshot].
+  ///
+  /// When the new value equals the old value, this Snapshot will be returned.
+  /// In case the [value] argument was a compatible (i.e. with same decoder)
+  /// [Snapshot], the cache of the argument will be merged into this snapshot.
+  ///
+  /// When [value] is a compatible snapshot, value will be returned with the
+  /// cache of this snapshot merged.
   Snapshot set(dynamic value);
 
   /// Returns a snapshot with updated content at [path].
@@ -220,9 +229,45 @@ class _SnapshotImpl extends Snapshot {
 
   @override
   Snapshot set(newValue) {
-    if (newValue is Snapshot) {
-      // TODO recycle cache
+    if (newValue is _SnapshotImpl && _decoder == newValue._decoder) {
+      // the new value is a snapshot
+
+      if (DeepCollectionEquality().equals(value, newValue.value)) {
+        // content is identical: return this with cache from newValue
+
+        for (var k in newValue._childrenCache.keys) {
+          if (_childrenCache.containsKey(k)) {
+            _childrenCache[k] =
+                _childrenCache[k].set(newValue._childrenCache[k]);
+          } else {
+            _childrenCache[k] = newValue._childrenCache[k];
+          }
+        }
+
+        for (var t in newValue._decodingCache.keys) {
+          for (var f in newValue._decodingCache[t].keys) {
+            _decodingCache
+                .putIfAbsent(t, () => {})
+                .putIfAbsent(f, () => newValue._decodingCache[t][f]);
+          }
+        }
+        return this;
+      } else {
+        // we will return the new value with cache values from old value
+
+        for (var k in _childrenCache.keys) {
+          if (newValue._childrenCache.containsKey(k)) {
+            newValue._childrenCache[k] =
+                _childrenCache[k].set(newValue._childrenCache[k]);
+          } else {
+            newValue._childrenCache[k] = _childrenCache[k];
+          }
+        }
+
+        return newValue;
+      }
     }
+
     newValue = newValue is Snapshot ? newValue.as() : newValue;
     var isEqual = DeepCollectionEquality().equals(value, newValue);
     if (isEqual) return this;

test/snapshot_test.dart

@@ -201,6 +201,65 @@ void main() {
 
         expect(v.child('firstname').as(), 'John');
       });
+
+      group('Setting with compatible snapshot', () {
+        var decoder = SnapshotDecoder.from(SnapshotDecoder.defaultDecoder)
+          ..register<Snapshot, Address>((s) => Address(s))
+          ..seal();
+        var json = {
+          'firstname': 'Jane',
+          'lastname': 'Doe',
+          'address1': {'street': 'Mainstreet', 'number': '1', 'city': 'London'},
+          'address2': {'street': 'Mainstreet', 'number': '1', 'city': 'London'},
+          'address3': {'street': 'Mainstreet', 'number': '1', 'city': 'London'},
+        };
+        test('Should return this when content unchanged', () {
+          var person = Snapshot.fromJson(json, decoder: decoder);
+          var address1Snap = person.child('address1');
+          var address1 = address1Snap.as<Address>();
+          var address3Snap = person.child('address3');
+
+          var newValue = Snapshot.fromJson(json, decoder: decoder);
+          newValue.child('address1').as<Address>();
+          var address2Snap = newValue.child('address2');
+          var address2 = address2Snap.as<Address>();
+          var address3 = newValue.child('address3').as<Address>();
+
+          var v = person.set(newValue);
+
+          expect(v, same(person));
+          expect(v.child('address1'), same(address1Snap));
+          expect(v.child('address1').as<Address>(), same(address1));
+          expect(v.child('address2'), same(address2Snap));
+          expect(v.child('address2').as<Address>(), same(address2));
+          expect(v.child('address3'), same(address3Snap));
+          expect(v.child('address3').as<Address>(), same(address3));
+        });
+
+        test('Should return other when content changed', () {
+          var person = Snapshot.fromJson(json, decoder: decoder);
+          var address1Snap = person.child('address1');
+          var address1 = address1Snap.as<Address>();
+          var address3Snap = person.child('address3');
+
+          var newValue =
+              Snapshot.fromJson(json..['firstname'] = 'John', decoder: decoder);
+          newValue.child('address1').as<Address>();
+          var address2Snap = newValue.child('address2');
+          var address2 = address2Snap.as<Address>();
+          var address3 = newValue.child('address3').as<Address>();
+
+          var v = person.set(newValue);
+
+          expect(v, same(newValue));
+          expect(v.child('address1'), same(address1Snap));
+          expect(v.child('address1').as<Address>(), same(address1));
+          expect(v.child('address2'), same(address2Snap));
+          expect(v.child('address2').as<Address>(), same(address2));
+          expect(v.child('address3'), same(address3Snap));
+          expect(v.child('address3').as<Address>(), same(address3));
+        });
+      });
     });
 
     group('Snapshot.setPath()', () {