chore(storage): Download to temp file

First downloads to a non-existent file, then copies the file to the location of the original in order to avoid iOS restrictions on overwriting data and potential failures which would result in deletion of the original file.

commit-id:7fbfb559

packages/amplify_core/lib/src/types/storage/download_file/download_file_request.dart

@@ -23,8 +23,37 @@ class DownloadFileRequest {
   File local;
   DownloadFileOptions? options;
 
-  DownloadFileRequest({required this.key, required this.local, this.options}) {
-    uuid = UUID.getUUID();
+  factory DownloadFileRequest({
+    required String key,
+    required File local,
+    DownloadFileOptions? options,
+  }) {
+    return DownloadFileRequest._(
+      uuid: UUID.getUUID(),
+      key: key,
+      local: local,
+      options: options,
+    );
+  }
+
+  DownloadFileRequest._({
+    required this.uuid,
+    required this.key,
+    required this.local,
+    this.options,
+  });
+
+  DownloadFileRequest copyWith({
+    String? key,
+    File? local,
+    DownloadFileOptions? options,
+  }) {
+    return DownloadFileRequest._(
+      uuid: uuid,
+      key: key ?? this.key,
+      local: local ?? this.local,
+      options: options ?? this.options,
+    );
   }
 
   Map<String, Object?> serializeAsMap() => {

packages/storage/amplify_storage_s3/lib/method_channel_storage_s3.dart

@@ -18,6 +18,8 @@ import 'dart:io';
 
 import 'package:amplify_core/amplify_core.dart';
 import 'package:flutter/services.dart';
+import 'package:path/path.dart' as path;
+import 'package:path_provider/path_provider.dart';
 
 import 'amplify_storage_s3.dart';
 
@@ -169,31 +171,36 @@ class AmplifyStorageS3MethodChannel extends AmplifyStorageS3 {
   Future<DownloadFileResult> downloadFile(
       {required DownloadFileRequest request,
       void Function(TransferProgress)? onProgress}) async {
+    // Download to a non-existent, empty file as required by iOS, then move
+    // the file into its correct position as determined by `request`.
+    final tempDir = await getTemporaryDirectory();
+    final tempFile = File(
+      path.join(tempDir.path, 'amplify_temp_${request.uuid}'),
+    );
     try {
       if (onProgress != null) {
         _transferProgressionCallbackMap[request.uuid] = onProgress;
       }
-      // Delete the file if it already exists since iOS will only write to
-      // a non-existent file; it will not overwrite the contents of an existing
-      // one.
-      if (Platform.isIOS && await request.local.exists()) {
-        await request.local.delete();
-      }
       final Map<String, dynamic>? data =
           (await _channel.invokeMapMethod<String, dynamic>(
         'downloadFile',
-        request.serializeAsMap(),
+        request.copyWith(local: tempFile).serializeAsMap(),
       ));
       if (data == null) {
         throw AmplifyException(
             AmplifyExceptionMessages.nullReturnedFromMethodChannel);
       }
-      DownloadFileResult result = _formatDownloadFileResult(data);
-      return result;
+      if (await request.local.exists()) {
+        await request.local.delete();
+      }
+      return DownloadFileResult(
+        file: await tempFile.copy(request.local.path),
+      );
     } on PlatformException catch (e) {
       throw _convertToStorageException(e);
     } finally {
       _transferProgressionCallbackMap.remove(request.uuid);
+      tempFile.delete().ignore();
     }
   }
 

packages/storage/amplify_storage_s3/pubspec.yaml

@@ -18,6 +18,8 @@ dependencies:
     sdk: flutter
   meta: ^1.7.0
   plugin_platform_interface: ^2.0.0
+  path_provider: ^2.0.0
+  path: any
 
 dev_dependencies:
   amplify_lints: ^1.0.0