Add AnalyticsStorage class

A simple way to store the events. Since we are using protobufs, we can
efficiently store events in a binary form.

lib/analytics/storage.dart

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+import 'dart:typed_data';
+
+import 'package:buffer/buffer.dart';
+import 'package:path/path.dart' as p;
+import 'package:universal_io/io.dart';
+
+import 'generated/analytics.pb.dart' as pb;
+
+class AnalyticsStorage {
+  final String folderPath;
+
+  AnalyticsStorage(this.folderPath);
+
+  Future<void> appendEvent(pb.Event event) async {
+    var eventData = event.writeToBuffer();
+
+    var filePath = p.join(folderPath, 'analytics');
+    // print(filePath);
+
+    var intData = ByteData(4);
+    intData.setInt32(0, eventData.length);
+
+    var builder = BytesBuilder();
+    builder.add(intData.buffer.asUint8List());
+    builder.add(eventData);
+
+    await File(filePath).writeAsBytes(builder.toBytes(), mode: FileMode.append);
+  }
+
+  Future<List<pb.Event>> fetchAll() async {
+    var file = File(p.join(folderPath, 'analytics'));
+    var bytes = await file.readAsBytes();
+    var events = <pb.Event>[];
+
+    var reader = ByteDataReader(copy: false);
+    reader.add(bytes);
+    while (reader.remainingLength != 0) {
+      var len = reader.readUint32();
+      var bytes = reader.read(len);
+
+      var event = pb.Event.fromBuffer(bytes);
+      events.add(event);
+    }
+    return events;
+  }
+}

test/analytics/storage_test.dart

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+import 'package:fixnum/fixnum.dart';
+import 'package:test/test.dart';
+import 'package:universal_io/io.dart';
+
+import 'package:gitjournal/analytics/generated/analytics.pb.dart' as pb;
+import 'package:gitjournal/analytics/storage.dart';
+
+void main() {
+  test('Read and write', () async {
+    var dt = DateTime.now().add(const Duration(days: -1));
+    var ev = pb.Event(
+      name: 'test',
+      date: Int64(dt.millisecondsSinceEpoch ~/ 1000),
+      params: {'a': 'hello'},
+      pseudoId: 'id',
+      userProperties: {'b': 'c'},
+      sessionID: 'session',
+    );
+
+    var dir = await Directory.systemTemp.createTemp('_analytics_');
+    var storage = AnalyticsStorage(dir.path);
+    await storage.appendEvent(ev);
+    await storage.appendEvent(ev);
+
+    var events = await storage.fetchAll();
+    expect(events.length, 2);
+    expect(events[0].toDebugString(), ev.toDebugString());
+    expect(events[1].toDebugString(), ev.toDebugString());
+    // expect(events[0], ev);
+  });
+}