Allow asynchronized method with savePath (#36)

dio/CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## 5.0.0-dev.1
 
+- Allow asynchronized method with `savePath`.
 - Allow `data` in all request methods.
 - A platform independent `HttpClientAdapter` can now be instantiated by doing
   `dio.httpClientAdapter = HttpClientAdapter();`.

dio/lib/src/dio/dio_for_native.dart

@@ -64,19 +64,19 @@ class DioForNative with DioMixin implements Dio {
       rethrow;
     }
     final File file;
-    if (savePath is String Function(Headers)) {
+    if (savePath is FutureOr<String> Function(Headers)) {
       // Add real Uri and redirect information to headers.
       response.headers
         ..add('redirects', response.redirects.length.toString())
         ..add('uri', response.realUri.toString());
-      file = File(savePath(response.headers));
+      file = File(await savePath(response.headers));
     } else if (savePath is String) {
       file = File(savePath);
     } else {
       throw ArgumentError.value(
         savePath.runtimeType,
         'savePath',
-        'The type must be `String` or `String Function(Headers)`.',
+        'The type must be `String` or `FutureOr<String> Function(Headers)`.',
       );
     }
 

dio/test/download_test.dart

@@ -8,6 +8,7 @@ import 'dart:io';
 import 'package:dio/dio.dart';
 import 'package:test/test.dart';
 
+import 'mock/adapters.dart';
 import 'utils.dart';
 
 void main() {
@@ -93,4 +94,19 @@ void main() {
     );
     //print(r);
   });
+
+  test('Test `savePath` types', () async {
+    Object? error;
+    final dio = Dio()
+      ..options.baseUrl = EchoAdapter.mockBase
+      ..httpClientAdapter = EchoAdapter();
+    try {
+      await dio.download('/test', 'testPath');
+      await dio.download('/test', (headers) => 'testPath');
+      await dio.download('/test', (headers) async => 'testPath');
+    } catch (e) {
+      error = e;
+    }
+    expect(error, null);
+  });
 }