List files with Cloud Storage on Flutter

Cloud Storage for Firebase allows you to list the contents of your Cloud Storage bucket. The SDKs return both the items and the prefixes of objects under the current Cloud Storage reference.

Projects that use the List API require Cloud Storage for Firebase Rules version 2. If you have an existing Firebase project, follow the steps in the Security Rules Guide.

list() uses the Google Cloud Storage List API. In Cloud Storage for Firebase, we use / as a delimiter, which allows us to emulate file system semantics. To allow for efficient traversal of large, hierarchical Cloud Storage buckets, the List API returns prefixes and items separately. For example, if you upload one file /images/uid/file1,

• root.child('images').listAll() will return /images/uid as a prefix.
• root.child('images/uid').listAll() will return the file as an item.

The Cloud Storage for Firebase SDK does not return object paths that contain two consecutive /s or end with a /. For example, consider a bucket that has the following objects:

• correctPrefix/happyItem
• wrongPrefix//sadItem
• lonelyItem/

The list operations on items in this bucket will give the following results:

• The list operation at the root returns the references to correctPrefix, wrongPrefix and lonelyItem as prefixes.
• The list operation at the correctPrefix/ returns the references to correctPrefix/happyItem as items.
• The list operation at the wrongPrefix/ doesn't return any references because wrongPrefix//sadItem contains two consecutive /s.
• The list operation at the lonelyItem/ doesn't return any references because the object lonelyItem/ ends with /.

List all files

You can use listAll to fetch all results for a directory. This is best used for small directories as all results are buffered in memory. The operation also may not return a consistent snapshot if objects are added or removed during the process.

For a large list, use the paginated list() method as listAll() buffers all results in memory.

The following example demonstrates listAll.

final storageRef = FirebaseStorage.instance.ref().child("files/uid");
final listResult = await storageRef.listAll();
for (var prefix in listResult.prefixes) {
  // The prefixes under storageRef.
  // You can call listAll() recursively on them.
}
for (var item in listResult.items) {
  // The items under storageRef.
}

Paginate list results

The list() API places a limit on the number of results it returns. list() provides a consistent pageview and exposes a pageToken that allows control over when to fetch additional results.

The pageToken encodes the path and version of the last item returned in the previous result. In a subsequent request using the pageToken, items that come after the pageToken are shown.

The following example demonstrates paginating a result:

Stream<ListResult> listAllPaginated(Reference storageRef) async* {
  String? pageToken;
  do {
    final listResult = await storageRef.list(ListOptions(
      maxResults: 100,
      pageToken: pageToken,
    ));
    yield listResult;
    pageToken = listResult.nextPageToken;
  } while (pageToken != null);
}

Handle errors

list() and listAll() fail if you haven't upgraded the Security Rules to version 2. Upgrade your Security Rules if you see this error:

Listing objects in a bucket is disallowed for rules_version = "1".
Please update storage security rules to rules_version = "2" to use list.

Other possible errors may indicate the user does not have the right permission. More information on errors can be found in the Handle Errors page.