Sharing a File

Once you have set up your app to share files using content URIs, you can respond to other apps' requests for those files. One way to respond to these requests is to provide a file selection interface from the server app that other applications can invoke. This approach allows a client application to let users select a file from the server app and then receive the selected file's content URI.

This lesson shows you how to create a file selection Activity in your app that responds to requests for files.

Receive File Requests

To receive requests for files from client apps and respond with a content URI, your app should provide a file selection Activity. Client apps start this Activity by calling startActivityForResult() with an Intent containing the action ACTION_PICK. When the client app calls startActivityForResult(), your app can return a result to the client app, in the form of a content URI for the file the user selected.

To learn how to implement a request for a file in a client app, see the lesson Requesting a Shared File.

Create a File Selection Activity

To set up the file selection Activity, start by specifying the Activity in your manifest, along with an intent filter that matches the action ACTION_PICK and the categories CATEGORY_DEFAULT and CATEGORY_OPENABLE. Also add MIME type filters for the files your app serves to other apps. The following snippet shows you how to specify the new Activity and intent filter:

<manifest xmlns:android="http://schemas.android.com/apk/res/android">
    ...
        <application>
        ...
            <activity
                android:name=".FileSelectActivity"
                android:label="@File Selector" >
                <intent-filter>
                    <action
                        android:name="android.intent.action.PICK"/>
                    <category
                        android:name="android.intent.category.DEFAULT"/>
                    <category
                        android:name="android.intent.category.OPENABLE"/>
                    <data android:mimeType="text/plain"/>
                    <data android:mimeType="image/*"/>
                </intent-filter>
            </activity>

Define the file selection Activity in code

Next, define an Activity subclass that displays the files available from your app's files/images/ directory in internal storage and allows the user to pick the desired file. The following snippet demonstrates how to define this Activity and respond to the user's selection:

public class MainActivity extends Activity {
    // The path to the root of this app's internal storage
    private File mPrivateRootDir;
    // The path to the "images" subdirectory
    private File mImagesDir;
    // Array of files in the images subdirectory
    File[] mImageFiles;
    // Array of filenames corresponding to mImageFiles
    String[] mImageFilenames;
    // Initialize the Activity
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        ...
        // Set up an Intent to send back to apps that request a file
        mResultIntent =
                new Intent("com.example.myapp.ACTION_RETURN_FILE");
        // Get the files/ subdirectory of internal storage
        mPrivateRootDir = getFilesDir();
        // Get the files/images subdirectory;
        mImagesDir = new File(mPrivateRootDir, "images");
        // Get the files in the images subdirectory
        mImageFiles = mImagesDir.listFiles();
        // Set the Activity's result to null to begin with
        setResult(Activity.RESULT_CANCELED, null);
        /*
         * Display the file names in the ListView mFileListView.
         * Back the ListView with the array mImageFilenames, which
         * you can create by iterating through mImageFiles and
         * calling File.getAbsolutePath() for each File
         */
         ...
    }
    ...
}

Respond to a File Selection

Once a user selects a shared file, your application must determine what file was selected and then generate a content URI for the file. Since the Activity displays the list of available files in a ListView, when the user clicks a file name the system calls the method onItemClick(), in which you can get the selected file.

When using an intent to send a file's URI from one app to another, you must be careful to get an URI that other apps can read. Doing so on devices running Android 6.0 (API level 23) and later requires special care because of changes to the permissions model in that version of Android, particularly READ_EXTERNAL_STORAGE's becoming a dangerous permission, which the receiving app might lack.

With these considerations in mind, we recommend that you avoid using Uri.fromFile(), which presents several drawbacks. This method:

• Does not allow file sharing across profiles.
• Requires that your app have WRITE_EXTERNAL_STORAGE permission on devices running Android 4.4 (API level 19) or lower.
• Requires that receiving apps have the READ_EXTERNAL_STORAGE permission, which will fail on important share targets, like Gmail, that don't have that permission.

Instead of using Uri.fromFile(), you can use URI permissions to grant other apps access to specific URIs. While URI permissions don’t work on file:// URIs generated by Uri.fromFile(), they do work on URIs associated with Content Providers. The FileProvider API can help you create such URIs. This approach also works with files that are not in external storage, but in the local storage of the app sending the intent.

In onItemClick(), get a File object for the file name of the selected file and pass it as an argument to getUriForFile(), along with the authority that you specified in the <provider> element for the FileProvider. The resulting content URI contains the authority, a path segment corresponding to the file's directory (as specified in the XML meta-data), and the name of the file including its extension. How FileProvider maps directories to path segments based on XML meta-data is described in the section Specify Sharable Directories.

The following snippet shows you how to detect the selected file and get a content URI for it:

    protected void onCreate(Bundle savedInstanceState) {
        ...
        // Define a listener that responds to clicks on a file in the ListView
        mFileListView.setOnItemClickListener(
                new AdapterView.OnItemClickListener() {
            @Override
            /*
             * When a filename in the ListView is clicked, get its
             * content URI and send it to the requesting app
             */
            public void onItemClick(AdapterView<?> adapterView,
                    View view,
                    int position,
                    long rowId) {
                /*
                 * Get a File for the selected file name.
                 * Assume that the file names are in the
                 * mImageFilename array.
                 */
                File requestFile = new File(mImageFilename[position]);
                /*
                 * Most file-related method calls need to be in
                 * try-catch blocks.
                 */
                // Use the FileProvider to get a content URI
                try {
                    fileUri = FileProvider.getUriForFile(
                            MainActivity.this,
                            "com.example.myapp.fileprovider",
                            requestFile);
                } catch (IllegalArgumentException e) {
                    Log.e("File Selector",
                          "The selected file can't be shared: " +
                          clickedFilename);
                }
                ...
            }
        });
        ...
    }

Remember that you can only generate content URIs for files that reside in a directory you've specified in the meta-data file that contains the <paths> element, as described in the section Specify Sharable Directories. If you call getUriForFile() for a File in a path that you haven't specified, you receive an IllegalArgumentException.

Grant Permissions for the File

Now that you have a content URI for the file you want to share with another app, you need to allow the client app to access the file. To allow access, grant permissions to the client app by adding the content URI to an Intent and then setting permission flags on the Intent. The permissions you grant are temporary and expire automatically when the receiving app's task stack is finished.

The following code snippet shows you how to set read permission for the file:

    protected void onCreate(Bundle savedInstanceState) {
        ...
        // Define a listener that responds to clicks in the ListView
        mFileListView.setOnItemClickListener(
                new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView,
                    View view,
                    int position,
                    long rowId) {
                ...
                if (fileUri != null) {
                    // Grant temporary read permission to the content URI
                    mResultIntent.addFlags(
                        Intent.FLAG_GRANT_READ_URI_PERMISSION);
                }
                ...
             }
             ...
        });
    ...
    }

Caution: Calling setFlags() is the only way to securely grant access to your files using temporary access permissions. Avoid calling Context.grantUriPermission() method for a file's content URI, since this method grants access that you can only revoke by calling Context.revokeUriPermission().

Don’t use Uri.fromFile(). It forces receiving apps to have the READ_EXTERNAL_STORAGE permission, won’t work at all if you are trying to share across users, and in versions of Android lower than 4.4 (API level 19), would require your app to have WRITE_EXTERNAL_STORAGE. And really important share targets, such as the Gmail app, don't have the READ_EXTERNAL_STORAGE, causing this call to fail. Instead, you can use URI permissions to grant other apps access to specific URIs. While URI permissions don’t work on file:// URIs as is generated by Uri.fromFile(), they do work on Uris associated with Content Providers. Rather than implement your own just for this, you can and should use FileProvider as explained in File Sharing.

Share the File with the Requesting App

To share the file with the app that requested it, pass the Intent containing the content URI and permissions to setResult(). When the Activity you have just defined is finished, the system sends the Intent containing the content URI to the client app. The following code snippet shows you how to do this:

    protected void onCreate(Bundle savedInstanceState) {
        ...
        // Define a listener that responds to clicks on a file in the ListView
        mFileListView.setOnItemClickListener(
                new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView,
                    View view,
                    int position,
                    long rowId) {
                ...
                if (fileUri != null) {
                    ...
                    // Put the Uri and MIME type in the result Intent
                    mResultIntent.setDataAndType(
                            fileUri,
                            getContentResolver().getType(fileUri));
                    // Set the result
                    MainActivity.this.setResult(Activity.RESULT_OK,
                            mResultIntent);
                    } else {
                        mResultIntent.setDataAndType(null, "");
                        MainActivity.this.setResult(RESULT_CANCELED,
                                mResultIntent);
                    }
                }
        });

Provide users with an way to return immediately to the client app once they have chosen a file. One way to do this is to provide a checkmark or Done button. Associate a method with the button using the button's android:onClick attribute. In the method, call finish(). For example:

    public void onDoneClick(View v) {
        // Associate a method with the Done button
        finish();
    }