Android 4.4 (API level 19) introduces the Storage Access Framework (SAF). The SAF makes it simple for users to browse and open documents, images, and other files across all of their their preferred document storage providers. A standard, easy-to-use UI lets users browse files and access recents in a consistent way across apps and providers.
Cloud or local storage services can participate in this ecosystem by implementing a {@link android.provider.DocumentsProvider} that encapsulates their services. Client apps that need access to a provider's documents can integrate with the SAF with just a few lines of code.
The SAF includes the following:
Some of the features offered by the SAF are as follows:
The SAF centers around a content provider that is a subclass of the {@link android.provider.DocumentsProvider} class. Within a document provider, data is structured as a traditional file hierarchy:
Figure 1. Document provider data model. A Root points to a single Document, which then starts the fan-out of the entire tree.
Note the following:
As stated above, the document provider data model is based on a traditional file hierarchy. However, you can physically store your data however you like, as long as it can be accessed through the {@link android.provider.DocumentsProvider} API. For example, you could use tag-based cloud storage for your data.
Figure 2 shows an example of how a photo app might use the SAF to access stored data:
Figure 2. Storage Access Framework Flow
Note the following:
Figure 3 shows a picker in which a user searching for images has selected a Google Drive account:
Figure 3. Picker
When the user selects Google Drive the images are displayed, as shown in figure 4. From that point on, the user can interact with them in whatever ways are supported by the provider and client app.
Figure 4. Images
On Android 4.3 and lower, if you want your app to retrieve a file from another app, it must invoke an intent such as {@link android.content.Intent#ACTION_PICK} or {@link android.content.Intent#ACTION_GET_CONTENT}. The user must then select a single app from which to pick a file and the selected app must provide a user interface for the user to browse and pick from the available files.
On Android 4.4 and higher, you have the additional option of using the {@link android.content.Intent#ACTION_OPEN_DOCUMENT} intent, which displays a picker UI controlled by the system that allows the user to browse all files that other apps have made available. From this single UI, the user can pick a file from any of the supported apps.
{@link android.content.Intent#ACTION_OPEN_DOCUMENT} is not intended to be a replacement for {@link android.content.Intent#ACTION_GET_CONTENT}. The one you should use depends on the needs of your app:
This section describes how to write client apps based on the {@link android.content.Intent#ACTION_OPEN_DOCUMENT} and {@link android.content.Intent#ACTION_CREATE_DOCUMENT} intents.
The following snippet uses {@link android.content.Intent#ACTION_OPEN_DOCUMENT} to search for document providers that contain image files:
private static final int READ_REQUEST_CODE = 42;
...
/**
* Fires an intent to spin up the "file chooser" UI and select an image.
*/
public void performFileSearch() {
// ACTION_OPEN_DOCUMENT is the intent to choose a file via the system's file
// browser.
Intent intent = new Intent(Intent.ACTION_OPEN_DOCUMENT);
// Filter to only show results that can be "opened", such as a
// file (as opposed to a list of contacts or timezones)
intent.addCategory(Intent.CATEGORY_OPENABLE);
// Filter to show only images, using the image MIME data type.
// If one wanted to search for ogg vorbis files, the type would be "audio/ogg".
// To search for all documents available via installed storage providers,
// it would be "*/*".
intent.setType("image/*");
startActivityForResult(intent, READ_REQUEST_CODE);
}
Note the following:
intent.setType("image/*") further filters to
display only documents that have the image MIME data type.Once the user selects a document in the picker, {@link android.app.Activity#onActivityResult onActivityResult()} gets called. The URI that points to the selected document is contained in the {@code resultData} parameter. Extract the URI using {@link android.content.Intent#getData getData()}. Once you have it, you can use it to retrieve the document the user wants. For example:
@Override
public void onActivityResult(int requestCode, int resultCode,
Intent resultData) {
// The ACTION_OPEN_DOCUMENT intent was sent with the request code
// READ_REQUEST_CODE. If the request code seen here doesn't match, it's the
// response to some other intent, and the code below shouldn't run at all.
if (requestCode == READ_REQUEST_CODE && resultCode == Activity.RESULT_OK) {
// The document selected by the user won't be returned in the intent.
// Instead, a URI to that document will be contained in the return intent
// provided to this method as a parameter.
// Pull that URI using resultData.getData().
Uri uri = null;
if (resultData != null) {
uri = resultData.getData();
Log.i(TAG, "Uri: " + uri.toString());
showImage(uri);
}
}
}
Once you have the URI for a document, you gain access to its metadata. This snippet grabs the metadata for a document specified by the URI, and logs it:
public void dumpImageMetaData(Uri uri) {
// The query, since it only applies to a single document, will only return
// one row. There's no need to filter, sort, or select fields, since we want
// all fields for one document.
Cursor cursor = getActivity().getContentResolver()
.query(uri, null, null, null, null, null);
try {
// moveToFirst() returns false if the cursor has 0 rows. Very handy for
// "if there's anything to look at, look at it" conditionals.
if (cursor != null && cursor.moveToFirst()) {
// Note it's called "Display Name". This is
// provider-specific, and might not necessarily be the file name.
String displayName = cursor.getString(
cursor.getColumnIndex(OpenableColumns.DISPLAY_NAME));
Log.i(TAG, "Display Name: " + displayName);
int sizeIndex = cursor.getColumnIndex(OpenableColumns.SIZE);
// If the size is unknown, the value stored is null. But since an
// int can't be null in Java, the behavior is implementation-specific,
// which is just a fancy term for "unpredictable". So as
// a rule, check if it's null before assigning to an int. This will
// happen often: The storage API allows for remote files, whose
// size might not be locally known.
String size = null;
if (!cursor.isNull(sizeIndex)) {
// Technically the column stores an int, but cursor.getString()
// will do the conversion automatically.
size = cursor.getString(sizeIndex);
} else {
size = "Unknown";
}
Log.i(TAG, "Size: " + size);
}
} finally {
cursor.close();
}
}
Once you have the URI for a document, you can open it or do whatever else you want to do with it.
Here is an example of how you might open a {@link android.graphics.Bitmap}:
private Bitmap getBitmapFromUri(Uri uri) throws IOException {
ParcelFileDescriptor parcelFileDescriptor =
getContentResolver().openFileDescriptor(uri, "r");
FileDescriptor fileDescriptor = parcelFileDescriptor.getFileDescriptor();
Bitmap image = BitmapFactory.decodeFileDescriptor(fileDescriptor);
parcelFileDescriptor.close();
return image;
}
Note that you should not do this operation on the UI thread. Do it in the background, using {@link android.os.AsyncTask}. Once you open the bitmap, you can display it in an {@link android.widget.ImageView}.
Here is an example of how you can get an {@link java.io.InputStream} from the URI. In this snippet, the lines of the file are being read into a string:
private String readTextFromUri(Uri uri) throws IOException {
InputStream inputStream = getContentResolver().openInputStream(uri);
BufferedReader reader = new BufferedReader(new InputStreamReader(
inputStream));
StringBuilder stringBuilder = new StringBuilder();
String line;
while ((line = reader.readLine()) != null) {
stringBuilder.append(line);
}
fileInputStream.close();
parcelFileDescriptor.close();
return stringBuilder.toString();
}
Your app can create a new document in a document provider using the {@link android.content.Intent#ACTION_CREATE_DOCUMENT} intent. To create a file you give your intent a MIME type and a file name, and launch it with a unique request code. The rest is taken care of for you:
// Here are some examples of how you might call this method.
// The first parameter is the MIME type, and the second parameter is the name
// of the file you are creating:
//
// createFile("text/plain", "foobar.txt");
// createFile("image/png", "mypicture.png");
// Unique request code.
private static final int WRITE_REQUEST_CODE = 43;
...
private void createFile(String mimeType, String fileName) {
Intent intent = new Intent(Intent.ACTION_CREATE_DOCUMENT);
// Filter to only show results that can be "opened", such as
// a file (as opposed to a list of contacts or timezones).
intent.addCategory(Intent.CATEGORY_OPENABLE);
// Create a file with the requested MIME type.
intent.setType(mimeType);
intent.putExtra(Intent.EXTRA_TITLE, fileName);
startActivityForResult(intent, WRITE_REQUEST_CODE);
}
Once you create a new document you can get its URI in {@link android.app.Activity#onActivityResult onActivityResult()}, so that you can continue to write to it.
If you have the URI for a document and the document's {@link android.provider.DocumentsContract.Document#COLUMN_FLAGS Document.COLUMN_FLAGS} contains {@link android.provider.DocumentsContract.Document#FLAG_SUPPORTS_DELETE SUPPORTS_DELETE}, you can delete the document. For example:
DocumentsContract.deleteDocument(getContentResolver(), uri);
You can use the SAF to edit a text document in place. This snippet fires the {@link android.content.Intent#ACTION_OPEN_DOCUMENT} intent and uses the category {@link android.content.Intent#CATEGORY_OPENABLE} to to display only documents that can be opened. It further filters to show only text files:
private static final int EDIT_REQUEST_CODE = 44;
/**
* Open a file for writing and append some text to it.
*/
private void editDocument() {
// ACTION_OPEN_DOCUMENT is the intent to choose a file via the system's
// file browser.
Intent intent = new Intent(Intent.ACTION_OPEN_DOCUMENT);
// Filter to only show results that can be "opened", such as a
// file (as opposed to a list of contacts or timezones).
intent.addCategory(Intent.CATEGORY_OPENABLE);
// Filter to show only text files.
intent.setType("text/plain");
startActivityForResult(intent, EDIT_REQUEST_CODE);
}
Next, from {@link android.app.Activity#onActivityResult onActivityResult()} (see Process results) you can call code to perform the edit. The following snippet gets a {@link java.io.FileOutputStream} from the {@link android.content.ContentResolver}. By default it uses “write” mode. It's best practice to ask for the least amount of access you need, so don’t ask for read/write if all you need is write:
private void alterDocument(Uri uri) {
try {
ParcelFileDescriptor pfd = getActivity().getContentResolver().
openFileDescriptor(uri, "w");
FileOutputStream fileOutputStream =
new FileOutputStream(pfd.getFileDescriptor());
fileOutputStream.write(("Overwritten by MyCloud at " +
System.currentTimeMillis() + "\n").getBytes());
// Let the document provider know you're done by closing the stream.
fileOutputStream.close();
pfd.close();
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
When your app opens a file for reading or writing, the system gives your app a URI permission grant for that file. It lasts until the user's device restarts. But suppose your app is an image-editing app, and you want users to be able to access the last 5 images they edited, directly from your app. If the user's device has restarted, you'd have to send the user back to the system picker to find the files, which is obviously not ideal.
To prevent this from happening, you can persist the permissions the system gives your app. Effectively, your app "takes" the persistable URI permission grant that the system is offering. This gives the user continued access to the files through your app, even if the device has been restarted:
final int takeFlags = intent.getFlags()
& (Intent.FLAG_GRANT_READ_URI_PERMISSION
| Intent.FLAG_GRANT_WRITE_URI_PERMISSION);
// Check for the freshest data.
getContentResolver().takePersistableUriPermission(uri, takeFlags);
There is one final step. You may have saved the most recent URIs your app accessed, but they may no longer be valid—another app may have deleted or modified a document. Thus, you should always call {@code getContentResolver().takePersistableUriPermission()} to check for the freshest data.
If you're developing an app that provides storage services for files (such as a cloud save service), you can make your files available through the SAF by writing a custom document provider. This section describes how to do this.
To implement a custom document provider, add the following to your application's manifest:
<provider> element that declares your custom storage
provider. com.example.android.storageprovider.MyCloudProvider.com.example.android.storageprovider) plus the type of content provider
(documents). For example, {@code com.example.android.storageprovider.documents}.android:exported set to "true".
You must export your provider so that other apps can see it.android:grantUriPermissions set to
"true". This setting allows the system to grant other apps access
to content in your provider. For a discussion of how to persist a grant for
a particular document, see Persist permissions.android:enabled="@bool/atLeastKitKat". In
addition to including this attribute in the manifest, you need to do the following:
<bool name="atLeastKitKat">false</bool>
<bool name="atLeastKitKat">true</bool>
Here are excerpts from a sample manifest that includes a provider:
<manifest... >
...
<uses-sdk
android:minSdkVersion="19"
android:targetSdkVersion="19" />
....
<provider
android:name="com.example.android.storageprovider.MyCloudProvider"
android:authorities="com.example.android.storageprovider.documents"
android:grantUriPermissions="true"
android:exported="true"
android:permission="android.permission.MANAGE_DOCUMENTS"
android:enabled="@bool/atLeastKitKat">
<intent-filter>
<action android:name="android.content.action.DOCUMENTS_PROVIDER" />
</intent-filter>
</provider>
</application>
</manifest>
The {@link android.content.Intent#ACTION_OPEN_DOCUMENT} intent is only available on devices running Android 4.4 and higher. If you want your application to support {@link android.content.Intent#ACTION_GET_CONTENT} to accommodate devices that are running Android 4.3 and lower, you should disable the {@link android.content.Intent#ACTION_GET_CONTENT} intent filter in your manifest for devices running Android 4.4 or higher. A document provider and {@link android.content.Intent#ACTION_GET_CONTENT} should be considered mutually exclusive. If you support both of them simultaneously, your app will appear twice in the system picker UI, offering two different ways of accessing your stored data. This would be confusing for users.
Here is the recommended way of disabling the {@link android.content.Intent#ACTION_GET_CONTENT} intent filter for devices running Android version 4.4 or higher:
<bool name="atMostJellyBeanMR2">true</bool>
<bool name="atMostJellyBeanMR2">false</bool>
<!-- This activity alias is added so that GET_CONTENT intent-filter
can be disabled for builds on API level 19 and higher. -->
<activity-alias android:name="com.android.example.app.MyPicker"
android:targetActivity="com.android.example.app.MyActivity"
...
android:enabled="@bool/atMostJellyBeanMR2">
<intent-filter>
<action android:name="android.intent.action.GET_CONTENT" />
<category android:name="android.intent.category.OPENABLE" />
<category android:name="android.intent.category.DEFAULT" />
<data android:mimeType="image/*" />
<data android:mimeType="video/*" />
</intent-filter>
</activity-alias>
Usually when you write a custom content provider, one of the tasks is implementing contract classes, as described in the Content Providers developers guide. A contract class is a {@code public final} class that contains constant definitions for the URIs, column names, MIME types, and other metadata that pertain to the provider. The SAF provides these contract classes for you, so you don't need to write your own:
For example, here are the columns you might return in a cursor when your document provider is queried for documents or the root:
private static final String[] DEFAULT_ROOT_PROJECTION =
new String[]{Root.COLUMN_ROOT_ID, Root.COLUMN_MIME_TYPES,
Root.COLUMN_FLAGS, Root.COLUMN_ICON, Root.COLUMN_TITLE,
Root.COLUMN_SUMMARY, Root.COLUMN_DOCUMENT_ID,
Root.COLUMN_AVAILABLE_BYTES,};
private static final String[] DEFAULT_DOCUMENT_PROJECTION = new
String[]{Document.COLUMN_DOCUMENT_ID, Document.COLUMN_MIME_TYPE,
Document.COLUMN_DISPLAY_NAME, Document.COLUMN_LAST_MODIFIED,
Document.COLUMN_FLAGS, Document.COLUMN_SIZE,};
The next step in writing a custom document provider is to subclass the abstract class {@link android.provider.DocumentsProvider}. At minimum, you need to implement the following methods:
These are the only methods you are strictly required to implement, but there are many more you might want to. See {@link android.provider.DocumentsProvider} for details.
Your implementation of {@link android.provider.DocumentsProvider#queryRoots queryRoots()} must return a {@link android.database.Cursor} pointing to all the root directories of your document providers, using columns defined in {@link android.provider.DocumentsContract.Root}.
In the following snippet, the {@code projection} parameter represents the specific fields the caller wants to get back. The snippet creates a new cursor and adds one row to it—one root, a top level directory, like Downloads or Images. Most providers only have one root. You might have more than one, for example, in the case of multiple user accounts. In that case, just add a second row to the cursor.
@Override
public Cursor queryRoots(String[] projection) throws FileNotFoundException {
// Create a cursor with either the requested fields, or the default
// projection if "projection" is null.
final MatrixCursor result =
new MatrixCursor(resolveRootProjection(projection));
// If user is not logged in, return an empty root cursor. This removes our
// provider from the list entirely.
if (!isUserLoggedIn()) {
return result;
}
// It's possible to have multiple roots (e.g. for multiple accounts in the
// same app) -- just add multiple cursor rows.
// Construct one row for a root called "MyCloud".
final MatrixCursor.RowBuilder row = result.newRow();
row.add(Root.COLUMN_ROOT_ID, ROOT);
row.add(Root.COLUMN_SUMMARY, getContext().getString(R.string.root_summary));
// FLAG_SUPPORTS_CREATE means at least one directory under the root supports
// creating documents. FLAG_SUPPORTS_RECENTS means your application's most
// recently used documents will show up in the "Recents" category.
// FLAG_SUPPORTS_SEARCH allows users to search all documents the application
// shares.
row.add(Root.COLUMN_FLAGS, Root.FLAG_SUPPORTS_CREATE |
Root.FLAG_SUPPORTS_RECENTS |
Root.FLAG_SUPPORTS_SEARCH);
// COLUMN_TITLE is the root title (e.g. Gallery, Drive).
row.add(Root.COLUMN_TITLE, getContext().getString(R.string.title));
// This document id cannot change once it's shared.
row.add(Root.COLUMN_DOCUMENT_ID, getDocIdForFile(mBaseDir));
// The child MIME types are used to filter the roots and only present to the
// user roots that contain the desired type somewhere in their file hierarchy.
row.add(Root.COLUMN_MIME_TYPES, getChildMimeTypes(mBaseDir));
row.add(Root.COLUMN_AVAILABLE_BYTES, mBaseDir.getFreeSpace());
row.add(Root.COLUMN_ICON, R.drawable.ic_launcher);
return result;
}
Your implementation of {@link android.provider.DocumentsProvider#queryChildDocuments queryChildDocuments()} must return a {@link android.database.Cursor} that points to all the files in the specified directory, using columns defined in {@link android.provider.DocumentsContract.Document}.
This method gets called when you choose an application root in the picker UI. It gets the child documents of a directory under the root. It can be called at any level in the file hierarchy, not just the root. This snippet makes a new cursor with the requested columns, then adds information about every immediate child in the parent directory to the cursor. A child can be an image, another directory—any file:
@Override
public Cursor queryChildDocuments(String parentDocumentId, String[] projection,
String sortOrder) throws FileNotFoundException {
final MatrixCursor result = new
MatrixCursor(resolveDocumentProjection(projection));
final File parent = getFileForDocId(parentDocumentId);
for (File file : parent.listFiles()) {
// Adds the file's display name, MIME type, size, and so on.
includeFile(result, null, file);
}
return result;
}
Your implementation of {@link android.provider.DocumentsProvider#queryDocument queryDocument()} must return a {@link android.database.Cursor} that points to the specified file, using columns defined in {@link android.provider.DocumentsContract.Document}.
The {@link android.provider.DocumentsProvider#queryDocument queryDocument()} method returns the same information that was passed in {@link android.provider.DocumentsProvider#queryChildDocuments queryChildDocuments()}, but for a specific file:
@Override
public Cursor queryDocument(String documentId, String[] projection) throws
FileNotFoundException {
// Create a cursor with the requested projection, or the default projection.
final MatrixCursor result = new
MatrixCursor(resolveDocumentProjection(projection));
includeFile(result, documentId, null);
return result;
}
You must implement {@link android.provider.DocumentsProvider#openDocument openDocument()} to return a {@link android.os.ParcelFileDescriptor} representing the specified file. Other apps can use the returned {@link android.os.ParcelFileDescriptor} to stream data. The system calls this method once the user selects a file and the client app requests access to it by calling {@link android.content.ContentResolver#openFileDescriptor openFileDescriptor()}. For example:
@Override
public ParcelFileDescriptor openDocument(final String documentId,
final String mode,
CancellationSignal signal) throws
FileNotFoundException {
Log.v(TAG, "openDocument, mode: " + mode);
// It's OK to do network operations in this method to download the document,
// as long as you periodically check the CancellationSignal. If you have an
// extremely large file to transfer from the network, a better solution may
// be pipes or sockets (see ParcelFileDescriptor for helper methods).
final File file = getFileForDocId(documentId);
final boolean isWrite = (mode.indexOf('w') != -1);
if(isWrite) {
// Attach a close listener if the document is opened in write mode.
try {
Handler handler = new Handler(getContext().getMainLooper());
return ParcelFileDescriptor.open(file, accessMode, handler,
new ParcelFileDescriptor.OnCloseListener() {
@Override
public void onClose(IOException e) {
// Update the file with the cloud server. The client is done
// writing.
Log.i(TAG, "A file with id " +
documentId + " has been closed!
Time to " +
"update the server.");
}
});
} catch (IOException e) {
throw new FileNotFoundException("Failed to open document with id "
+ documentId + " and mode " + mode);
}
} else {
return ParcelFileDescriptor.open(file, accessMode);
}
}
Suppose your document provider is a password-protected cloud storage service and you want to make sure that users are logged in before you start sharing their files. What should your app do if the user is not logged in? The solution is to return zero roots in your implementation of {@link android.provider.DocumentsProvider#queryRoots queryRoots()}. That is, an empty root cursor:
public Cursor queryRoots(String[] projection) throws FileNotFoundException {
...
// If user is not logged in, return an empty root cursor. This removes our
// provider from the list entirely.
if (!isUserLoggedIn()) {
return result;
}
The other step is to call {@code getContentResolver().notifyChange()}. Remember the {@link android.provider.DocumentsContract}? We’re using it to make this URI. The following snippet tells the system to query the roots of your document provider whenever the user's login status changes. If the user is not logged in, a call to {@link android.provider.DocumentsProvider#queryRoots queryRoots()} returns an empty cursor, as shown above. This ensures that a provider's documents are only available if the user is logged into the provider.
private void onLoginButtonClick() {
loginOrLogout();
getContentResolver().notifyChange(DocumentsContract
.buildRootsUri(AUTHORITY), null);
}