Reuse File objects in FileTransferController.

ui/file_manager/file_manager/foreground/js/file_transfer_controller.js

Index: ui/file_manager/file_manager/foreground/js/file_transfer_controller.js
diff --git a/ui/file_manager/file_manager/foreground/js/file_transfer_controller.js b/ui/file_manager/file_manager/foreground/js/file_transfer_controller.js
index 35ecd10b61ec694a1f9da2a66a4438a4b6fd9a87..90a744b390f65bc0983a8aea6aacee400dec3864 100644
--- a/ui/file_manager/file_manager/foreground/js/file_transfer_controller.js
+++ b/ui/file_manager/file_manager/foreground/js/file_transfer_controller.js
@@ -1261,7 +1261,6 @@ FileTransferController.prototype.simulateCommand_ = function(command, handler) {
  */
 FileTransferController.prototype.onFileSelectionChanged_ = function() {
   this.preloadedThumbnailImagePromise_ = null;
-  this.selectedAsyncData_ = {};

[fukino, 2017/04/26 03:11:33]

We should clear objects in selectedAsyncData_ if they are not selected.

[tetsui2, 2017/04/26 03:37:04]

Every time a file is copied, both onFileSelectionChanged_ and
onFileSelectionChangedThrottled are called. Therefore, setting {} here means
clearing and creating all the File objects for the selected files every time,
which leads to O(N^2) File object creation. File object creation requests
metadata internally, therefore it is heavy operation on MTP.

I'm not sure what would happen if a file is changed after the File object is
created and then copied. Besides that, I think not clearing selectedAsyncData_
here is correct.

[fukino, 2017/04/26 03:49:17]

I didn't mean that we should clear the selectedAsyncData_.
By referring this.selectionHandler_.selection.entries, metadata for entries
which are not in selection anymore should be removed.
Does this make sense?

[tetsui2, 2017/04/26 06:47:42]

Done.

 };
 
 /**
@@ -1274,7 +1273,9 @@ FileTransferController.prototype.onFileSelectionChangedThrottled_ = function() {
   for (var i = 0; i < entries.length; i++) {
     if (entries[i].isFile)
       fileEntries.push(entries[i]);
-    asyncData[entries[i].toURL()] = {externalFileUrl: '', file: null};
+    if (!(entries[i].toURL() in asyncData)) {
+      asyncData[entries[i].toURL()] = {externalFileUrl: '', file: null};
+    }
   }
   var containsDirectory = this.selectionHandler_.selection.directoryCount > 0;
 
@@ -1285,9 +1286,11 @@ FileTransferController.prototype.onFileSelectionChangedThrottled_ = function() {
   if (!containsDirectory) {
     for (var i = 0; i < fileEntries.length; i++) {
       (function(fileEntry) {
-        fileEntry.file(function(file) {
-          asyncData[fileEntry.toURL()].file = file;
-        });
+        if (!asyncData[fileEntry.toURL()]) {
+          fileEntry.file(function(file) {
+            asyncData[fileEntry.toURL()].file = file;
+          });
+        }
       })(fileEntries[i]);
     }
   }