add paginator

lib/helpers/paginator.dart

@@ -0,0 +1,70 @@
+import 'dart:async';
+
+import 'package:cloud_firestore/cloud_firestore.dart';
+import 'package:fcs/pages/widgets/callbacks.dart';
+import 'package:logging/logging.dart';
+
+typedef ToObj = Function(Map<String, dynamic> data, String id);
+
+/*
+ * Pagination load data in page 
+ */
+class Paginator<T> {
+  final log = Logger('Paginator');
+
+  final int rowPerLoad;
+  DocumentSnapshot prev;
+  bool ended = false;
+  bool isLoading = false;
+  List<T> values = [];
+  ToObj toObj;
+
+  Query pageQuery;
+
+  Paginator(this.pageQuery, {this.rowPerLoad = 20, this.toObj}) {
+    _clearState();
+  }
+
+  void _clearState() {
+    prev = null;
+    ended = false;
+    isLoading = false;
+    values = [];
+  }
+
+  void close() {
+    _clearState();
+  }
+
+  Future<void> refresh({CallBack onFinished}) async {
+    _clearState();
+    await load();
+    if (onFinished != null) onFinished();
+  }
+
+  Future<bool> load({CallBack onFinished}) async {
+    if (ended) return null;
+    isLoading = true;
+    Query _query =
+        prev != null ? pageQuery.startAfterDocument(prev) : pageQuery;
+    try {
+      await _query
+          .limit(rowPerLoad)
+          .getDocuments(source: Source.server)
+          .then((QuerySnapshot snapshot) {
+        int count = snapshot.documents.length;
+        ended = count < rowPerLoad;
+        prev = count > 0 ? snapshot.documents[count - 1] : prev;
+        snapshot.documents.forEach((e) {
+          values.add(toObj(e.data, e.documentID));
+        });
+      });
+    } catch (e) {
+      log.warning("Error!! $e");
+    } finally {
+      isLoading = false;
+      if (onFinished != null) onFinished();
+    }
+    return ended;
+  }
+}