After uploading a file to Cloud Storage reference, you can also get
and update the file metadata, for example to update the content type. Files
can also store custom key/value pairs with additional file metadata.
File metadata contains common properties such as
name
,
size
, and
content_type
(often referred to as MIME type) in addition to some less common
ones like
content_disposition
and
time_created
. 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
StorageReference forest_ref = storage_ref.Child("images/forest.jpg");
// Get metadata properties
Future
future = forest_ref.GetMetadata();
// Wait for Future to complete...
if (future.Error() != firebase::storage::kErrorNone) {
// Uh-oh, an error occurred!
} else {
// We can now retrieve the metadata for 'images/forest.jpg'
Metadata* metadata = future.Result();
}
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
firebase::storage::StorageReference forest_ref = storage_ref.child("images/forest.jpg");
// Create file metadata to update
Metadata new_metadata;
newMetadata.set_cache_control("public,max-age=300");
newMetadata.set_content_type("image/jpeg");
// Update metadata properties
Future
future = forest_ref.UpdateMetadata(new_metadata);
// Wait for Future to complete...
if (future.Error() != firebase::storage::kErrorNone) {
// Uh-oh, an error occurred!
} else {
// We can now retrieve the updated metadata for 'images/forest.jpg'
Metadata* metadata = future.Result();
}
You can delete writable metadata properties by passing the empty string:
// Create file metadata with property to delete
StorageMetadata new_metadata;
new_metadata.set_content_type("");
// Delete the metadata property
Future
future = forest_ref.UpdateMetadata(new_metadata);
// Wait for Future to complete...
if (future.Error() != 0) {
// Uh-oh, an error occurred!
} else {
// metadata.content_type() should be an empty string
Metadata* metadata = future.Result();
}
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.
You can specify custom metadata as an
std::map
containing
std::string
properties.
std::map
* custom_metadata = metadata.custom_metadata();
custom_metadata->insert(std::make_pair("location", "Yosemite, CA, USA");
custom_metadata->insert(std::make_pair("activity", "Hiking");
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.
A full list of metadata properties on a file is available below:
Property
|
Type
|
Writable
|
bucket
|
const char*
|
NO
|
generation
|
const char*
|
NO
|
metageneration
|
const char*
|
NO
|
full_path
|
const char*
|
NO
|
name
|
const char*
|
NO
|
size
|
int64_t
|
NO
|
time_created
|
int64_t
|
NO
|
updated
|
int64_t
|
NO
|
cache_control
|
const char*
|
YES
|
content_disposition
|
const char*
|
YES
|
content_encoding
|
const char*
|
YES
|
content_language
|
const char*
|
YES
|
content_type
|
const char*
|
YES
|
download_urls
|
std::vector<std::string>
|
NO
|
custom_metadata
|
std::map<std::string, std::string>
|
YES
|
Next Steps
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.