feat: add support for keepPreviousData & examples regarding this

packages/fl_query_hooks/example/lib/components/query_hook_previous_data.dart

@@ -0,0 +1,62 @@
+import 'dart:convert';
+
+import 'package:fl_query/fl_query.dart';
+import 'package:fl_query_hooks/fl_query_hooks.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_hooks/flutter_hooks.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 QueryHookPreviousDataExample extends HookWidget {
+  const QueryHookPreviousDataExample({Key? key}) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    final id = useState(1);
+    final query =
+        useQuery(job: todoJob(id.value.toString()), externalData: null);
+
+    return Column(
+      crossAxisAlignment: CrossAxisAlignment.start,
+      children: [
+        Text(
+          "# Query Variable Key with keepPreviousData",
+          style: Theme.of(context).textTheme.headline5,
+        ),
+        if (query.hasError)
+          Text(query.error.toString())
+        else if (!query.hasData)
+          const CircularProgressIndicator()
+        else
+          Text(jsonEncode(query.data ?? {})),
+        Row(
+          children: [
+            IconButton(
+              icon: const Icon(Icons.remove),
+              onPressed: () {
+                id.value -= 1;
+              },
+            ),
+            IconButton(
+              icon: const Icon(Icons.add),
+              onPressed: () {
+                id.value += 1;
+              },
+            ),
+          ],
+        )
+      ],
+    );
+  }
+}

packages/fl_query_hooks/example/lib/main.dart

@@ -3,6 +3,7 @@ import 'package:fl_query_hooks_example/components/basic_hook_query.dart';
 import 'package:fl_query_hooks_example/components/lazy_hook_query.dart';
 import 'package:fl_query_hooks_example/components/mutation_hook_variable_key.dart';
 import 'package:fl_query_hooks_example/components/query_hook_external_data.dart';
+import 'package:fl_query_hooks_example/components/query_hook_previous_data.dart';
 import 'package:fl_query_hooks_example/components/query_hook_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 {
             QueryHookExternalDataExample(),
             LazyHookQueryExample(),
             QueryHookVariableKeyExample(),
+            QueryHookPreviousDataExample(),
             Divider(),
             BasicHookMutationExample(),
             MutationHookVariableKeyExample(),

packages/fl_query_hooks/lib/src/use_query.dart

@@ -1,3 +1,5 @@
+// ignore_for_file: invalid_use_of_protected_member
+
 import 'package:fl_query/fl_query.dart';
 import 'package:fl_query_hooks/src/utils.dart';
 import 'package:flutter/widgets.dart';
@@ -31,10 +33,11 @@ Query<T, Outside> useQuery<T extends Object, Outside>({
   final oldOnData = usePrevious(onData);
   final oldOnError = usePrevious(onError);
 
-  final init = useCallback(() {
+  final init = useCallback(([T? previousData]) {
     query.value = queryBowl.addQuery<T, Outside>(
       job,
       externalData: externalData,
+      previousData: previousData,
       key: uKey,
       onData: onData,
       onError: onError,
@@ -66,7 +69,17 @@ Query<T, Outside> useQuery<T extends Object, Outside>({
     final hasOnDataChanged = oldOnData != onData && oldOnData != null;
     if (oldJob != null && oldJob.queryKey != job.queryKey) {
       disposeQuery();
-      init();
+
+      /// setting the new query's initial data as prev query's data
+      /// when [job.keepPreviousData] is true and both are dynamic
+      if (oldJob.isDynamic &&
+          job.isDynamic &&
+          oldJob.keepPreviousData == true &&
+          job.keepPreviousData == true) {
+        init(query.value.data);
+      } else {
+        init();
+      }
     } else if (oldExternalData != null &&
         externalData != null &&
         !isShallowEqual(oldExternalData, externalData)) {