Add in memory cache for thumbnails to improve hero experience

lib/ui/thumbnail_widget.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:photos/core/cache/image_cache.dart';
 import 'package:photos/core/cache/thumbnail_cache.dart';
 import 'package:photos/file_repository.dart';
 import 'package:photos/models/file.dart';
@@ -119,6 +120,12 @@ class _ThumbnailWidgetState extends State<ThumbnailWidget> {
         !_encounteredErrorLoadingThumbnail &&
         !_isLoadingThumbnail) {
       _isLoadingThumbnail = true;
+      final cachedThumbnail = ThumbnailFileLruCache.get(widget.file);
+      if (cachedThumbnail != null) {
+        _imageProvider = Image.file(cachedThumbnail).image;
+        _hasLoadedThumbnail = true;
+        return;
+      }
       getThumbnailFromServer(widget.file).then((file) {
         final imageProvider = Image.file(file).image;
         if (mounted) {

lib/utils/file_util.dart

@@ -101,14 +101,23 @@ Future<io.File> getFileFromServer(File file) async {
 
 Future<io.File> getThumbnailFromServer(File file) async {
   if (!file.isEncrypted) {
-    return ThumbnailCacheManager().getSingleFile(file.getThumbnailUrl());
+    return ThumbnailCacheManager()
+        .getSingleFile(file.getThumbnailUrl())
+        .then((data) {
+      ThumbnailFileLruCache.put(file, data);
+      return data;
+    });
   } else {
     return ThumbnailCacheManager()
         .getFileFromCache(file.getThumbnailUrl())
         .then((info) {
       if (info == null) {
-        return _downloadAndDecryptThumbnail(file);
+        return _downloadAndDecryptThumbnail(file).then((data) {
+          ThumbnailFileLruCache.put(file, data);
+          return data;
+        });
       } else {
+        ThumbnailFileLruCache.put(file, info.file);
         return info.file;
       }
     });