add packages, receiving & processing

lib/helpers/pagination.dart

@@ -0,0 +1,70 @@
+import 'dart:async';
+
+import 'package:cloud_firestore/cloud_firestore.dart';
+import 'package:logging/logging.dart';
+
+/*
+ * Pagination load data in page 
+ */
+class Pagination {
+  final log = Logger('PaginationModel');
+
+  final int rowPerLoad;
+  DocumentSnapshot prev;
+  bool ended = false;
+
+  Query pageQuery;
+
+  Pagination(Query pageQuery, {this.rowPerLoad = 20}) {
+    this.pageQuery = pageQuery;
+    initData();
+  }
+
+  void initData() async {
+    _clearState();
+    load();
+  }
+
+  void _clearState() {
+    prev = null;
+    ended = false;
+    if (controller != null) controller.close();
+  }
+
+  StreamController<DocumentSnapshot> controller;
+  Stream<DocumentSnapshot> get stream {
+    if (controller != null) {
+      controller.close();
+    }
+    controller = StreamController<DocumentSnapshot>(onCancel: _clearState);
+    return controller.stream;
+  }
+
+  void close() {
+    _clearState();
+  }
+
+  Future<bool> load() async {
+    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) {
+          controller.add(e);
+        });
+        if (ended) {
+          controller.add(null);
+        }
+      });
+    } catch (e) {
+      log.warning("Error!! $e");
+    }
+    return ended;
+  }
+}