From b2217c72b2a8ef075e3a41395e18fc561aabd490 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Kl=C4=81vs=20Pried=C4=ABtis?= <klavs@prieditis.lv>
Date: Tue, 17 Sep 2019 15:10:50 +0300
Subject: [PATCH] dedupe DataLoader requests for the same item

---
 data_loader/lib/data_loader.dart | 19 ++++++++++++++-----
 data_loader/test/all_test.dart   | 25 +++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 5 deletions(-)

diff --git a/data_loader/lib/data_loader.dart b/data_loader/lib/data_loader.dart
index 4b013609..c806f908 100644
--- a/data_loader/lib/data_loader.dart
+++ b/data_loader/lib/data_loader.dart
@@ -25,13 +25,22 @@ class DataLoader<Id, Data> {
       var current = _queue.toList();
       _queue.clear();
 
-      var data = await loadMany(current.map((i) => i.id));
+      List<Id> loadIds =
+          current.map((i) => i.id).toSet().toList(growable: false);
 
-      for (int i = 0; i < current.length; i++) {
-        var item = current[i];
+      var data = await loadMany(
+        loadIds,
+      );
+
+      for (int i = 0; i < loadIds.length; i++) {
+        var id = loadIds[i];
         var value = data.elementAt(i);
-        if (cache) _cache[item.id] = value;
-        item.completer.complete(value);
+
+        if (cache) _cache[id] = value;
+
+        current
+            .where((item) => item.id == id)
+            .forEach((item) => item.completer.complete(value));
       }
     }
 
diff --git a/data_loader/test/all_test.dart b/data_loader/test/all_test.dart
index 5da29500..28f95eda 100644
--- a/data_loader/test/all_test.dart
+++ b/data_loader/test/all_test.dart
@@ -1,4 +1,5 @@
 import 'dart:async';
+
 import 'package:data_loader/data_loader.dart';
 import 'package:test/test.dart';
 
@@ -18,6 +19,30 @@ void main() {
     expect(batch, ['0.00', '1.00', '2.00']);
   });
 
+  test('dedupe', () async {
+    var loader = DataLoader<int, Map<int, List<int>>>((ids) {
+      return ids.map(
+        (i) => {i: ids.toList()},
+      );
+    });
+
+    var zero = loader.load(0);
+    var one = loader.load(1);
+    var two = loader.load(2);
+    var anotherZero = loader.load(0);
+    var batch = await Future.wait([zero, one, two, anotherZero]);
+
+    expect(
+      batch,
+      [
+        { 0: [0, 1, 2]},
+        { 1: [0, 1, 2]},
+        { 2: [0, 1, 2]},
+        { 0: [0, 1, 2]},
+      ],
+    );
+  });
+
   group('cache', () {
     DataLoader<int, _Unique> uniqueLoader, noCache;