feat: add support for keepPreviousData & examples regarding this

packages/fl_query/example/lib/components/query_previous_data.dart

@@ -0,0 +1,70 @@
+import 'dart:convert';
+
+import 'package:fl_query/fl_query.dart';
+import 'package:flutter/material.dart';
+import 'package:http/http.dart' as http;
+
+final todoJob = QueryJob.withVariableKey<Map, void>(
+  preQueryKey: "todo",
+  task: (queryKey, _) async {
+    final res = await http.get(
+      Uri.parse(
+          "https://jsonplaceholder.typicode.com/todos/${getVariable(queryKey)}"),
+    );
+    return jsonDecode(res.body);
+  },
+  keepPreviousData: true,
+);
+
+class QueryPreviousDataExample extends StatefulWidget {
+  const QueryPreviousDataExample({Key? key}) : super(key: key);
+
+  @override
+  State<QueryPreviousDataExample> createState() =>
+      _QueryPreviousDataExampleState();
+}
+
+class _QueryPreviousDataExampleState extends State<QueryPreviousDataExample> {
+  int id = 1;
+
+  @override
+  Widget build(BuildContext context) {
+    return Column(
+      crossAxisAlignment: CrossAxisAlignment.start,
+      children: [
+        Text(
+          "# Query Variable Key with keepPreviousData",
+          style: Theme.of(context).textTheme.headline5,
+        ),
+        QueryBuilder(
+            job: todoJob(id.toString()),
+            externalData: null,
+            builder: (context, query) {
+              if (query.hasError) return Text(query.error.toString());
+              if (!query.hasData) return const CircularProgressIndicator();
+              return Text(jsonEncode(query.data ?? {}));
+            }),
+        Row(
+          children: [
+            IconButton(
+              icon: const Icon(Icons.remove),
+              onPressed: () {
+                setState(() {
+                  id -= 1;
+                });
+              },
+            ),
+            IconButton(
+              icon: const Icon(Icons.add),
+              onPressed: () {
+                setState(() {
+                  id += 1;
+                });
+              },
+            ),
+          ],
+        )
+      ],
+    );
+  }
+}

packages/fl_query/example/lib/main.dart

@@ -3,6 +3,7 @@ import 'package:fl_query_example/components/basic_query.dart';
 import 'package:fl_query_example/components/lazy_query.dart';
 import 'package:fl_query_example/components/mutation_variable_key.dart';
 import 'package:fl_query_example/components/query_external_data.dart';
+import 'package:fl_query_example/components/query_previous_data.dart';
 import 'package:fl_query_example/components/query_variable_key.dart';
 import 'package:fl_query/fl_query.dart';
 import 'package:flutter/material.dart';
@@ -53,6 +54,7 @@ class _MyHomePageState extends State<MyHomePage> with WidgetsBindingObserver {
             QueryExternalDataExample(),
             LazyQueryExample(),
             QueryVariableKeyExample(),
+            QueryPreviousDataExample(),
             Divider(),
             BasicMutationExample(),
             MutationVariableKeyExample(),