Use file metadata with Cloud Storage on Flutter

After uploading a file to Cloud Storage reference, you can also get and update the file metadata, for example to view or update the content type. Files can also store custom key/value pairs with additional file metadata.

Note: By default, a Cloud Storage for Firebase bucket requires Firebase Authentication to perform any action on the bucket's data or files. You can change your Firebase Security Rules for Cloud Storage to allow unauthenticated access for specific situations. However, for most situations, we strongly recommend restricting access and setting up robust security rules (especially for production apps). Note that if you use Google App Engine and have a default Cloud Storage bucket with a name format of *.appspot.com, you may need to consider how your security rules impact access to App Engine files.

Get File Metadata

File metadata contains common properties such as name, size, and contentType (often referred to as MIME type) in addition to some less common ones like contentDisposition and timeCreated. This metadata can be retrieved from a Cloud Storage reference using the getMetadata() method.

// Create reference to the file whose metadata we want to retrieve
final forestRef = storageRef.child("images/forest.jpg");

// Get metadata properties
final metadata = await forestRef.getMetadata();

// Metadata now contains the metadata for 'images/forest.jpg'

Update File Metadata

You can update file metadata at any time after the file upload completes by using the updateMetadata() method. Refer to the full list for more information on what properties can be updated. Only the properties specified in the metadata are updated, all others are left unmodified.

// Create reference to the file whose metadata we want to change
final forestRef = storageRef.child("images/forest.jpg");

// Create file metadata to update
final newMetadata = SettableMetadata(
  cacheControl: "public,max-age=300",
  contentType: "image/jpeg",
);

// Update metadata properties
final metadata = await forestRef.updateMetadata(newMetadata);

// Updated metadata for 'images/forest.jpg' is returned

You can delete writable metadata properties by passing null:

// Delete the cacheControl property
final newMetadata = SettableMetadata(cacheControl: null);
final metadata = await forestRef.updateMetadata(newMetadata);

Handle Errors

There are a number of reasons why errors may occur on getting or updating metadata, including the file not existing, or the user not having permission to access the desired file. More information on errors can be found in the Handle Errors section of the docs.

Custom Metadata

You can specify custom metadata using the customMetadata parameter of the SettableMetadata constructor:

// Create reference to the file whose metadata we want to change
final forestRef = storageRef.child("images/forest.jpg");

// Create file metadata to update
final newCustomMetadata = SettableMetadata(
  customMetadata: {
    "location": "Yosemite, CA, USA",
    "activity": "Hiking",
  },
);

// Update metadata properties
final metadata = await forestRef.updateMetadata(newCustomMetadata);

// Updated metadata for 'images/forest.jpg' is returned

You can store app-specific data for each file in custom metadata, but we highly recommend using a database (such as the Firebase Realtime Database) to store and synchronize this type of data.

File Metadata Properties

A full list of metadata properties on a file is available below:

Property	Type	Settable?
bucket	String	No
generation	String	No
metageneration	String	No
metadataGeneration	String	No
fullPath	String	No
name	String	No
size	int	No
timeCreated	DateTime	No
updated	DateTime	No
md5Hash	String	No
cacheControl	String	Yes
contentDisposition	String	Yes
contentEncoding	String	Yes
contentLanguage	String	Yes
contentType	String	Yes
customMetadata	Map<String, String>	Yes

Uploading, downloading, and updating files is important, but so is being able to remove them. Let's learn how to delete files from Cloud Storage.