Android file sharing application

Sharing a File

This lesson teaches you to

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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:

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:

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 , android.view.View, int, long)»>onItemClick() , in which you can get the selected file.

In , android.view.View, int, long)»>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

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:

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

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:

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() .

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:

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:

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Sharing Content between Android apps

Sharing is caring, as they say, but sharing on Android means something perhaps slightly different. ‘Sharing’ is really shorthand for sending content such as text, formatted text, files, or images between apps.

So if ‘sharing’ == sending content, it makes slightly more sense that it is implemented using ACTION_SEND (or ACTION_SEND_MULTIPLE) Intents and its dozen extras.

While that approach is perfectly valid, I prefer to use ShareCompat, a set of classes in the v4 Support Library designed to make it easy to build intents for sharing content.

Sharing text

Sharing plain text is, as you might imagine, a good place to start. In fact, there’s not a whole lot to it:

ShareCompat.IntentBuilder uses a fluent API where you can chain together multiple method calls, using only the ones you need. For sharing, one of the most important parts is picking the right mime type — this is how apps filter what type of content they can receive. By using text/plain, we signify that our Intent will only contain plain text. Then, of course, setText() is how we actually add the CharSequence to the Intent to send. And while you can certainly send styled text using setText(), there’s no guarantee that the receiving app will honor that styling, so you should ensure that the text is legible with or without styling.

You’ll note we then use resolveActivity() before calling startActivity(). As mentioned in Protecting Implicit Intents with Runtime Checks, this is critical to prevent an ActivityNotFoundException when there is no Activity available to handle the mime type you have selected. While probably not as much of a concern with text/plain, it may be much more common with other types.

Note: when you use startActivity(shareIntent), that respects any default apps the user has set (i.e., if they’ve previously selected sharing all “text/plain” items to a certain app). If you’d like to instead always show a disambiguation chooser, use the intent generated from IntentBuilder.createChooserIntent() as explained in the ACTION_CHOOSER documentation.

Sharing HTML text

Some apps, most notably email clients, also support formatting with HTML. The changes, compared to plain text, are fairly minor:

The differences here are that we use of setHtmlText() in place of setText() and a mime type of text/html replacing text/plain. Here ShareCompat actually does a little bit extra: setHtmlText() also uses Html.fromHtml() to create a fallback formatted text to pass along to the receiving app if you haven’t previously called setText() yourself.

Given that many of the apps that can receive HTML text are email clients, there’s a number of helper methods to set the subject, to:, cc:, and bcc: email addresses as well — consider adding at least a subject to any share intent for best compatibility with email apps.

Of course, you’ll still want to call resolveActivity() just as before — nothing changes there.

Receiving text

While the focus so far has been on the sending side, it is helpful to know exactly what is happening on the other side (if not just to build a simple receiving app to install on your emulator for testing purposes). Receiving Activities add an intent filter to the Activity:

The action is obviously the more critical part — without that there’s nothing that would denote this as an ACTION_SEND (the action behind sharing). The mime type, same as with our sending code, is also present here. What isn’t as obvious are the two categories. From the element documentation:

Note: In order to receive implicit intents, you must include the CATEGORY_DEFAULT category in the intent filter. The methods startActivity() and startActivityForResult() treat all intents as if they declared the CATEGORY_DEFAULT category. If you do not declare it in your intent filter, no implicit intents will resolve to your activity.

So CATEGORY_DEFAULT is required for our use case. Then, CATEGORY_BROWSABLE allows web pages to natively share into apps without any extra effort required on the receiving side.

And to actually extract the information from the Intent, the useful ShareCompat.IntentReader can be used:

Similar to IntentBuilder, IntentReader is just a simple wrapper that make it easy to extract information.

Sharing files and images

While sending and receiving text is straightforward enough (create text, include it in Intent), sending files (and particularly images — the most common type by far) has an additional wrinkle: file permissions.

The simplest code you might try might look like

And that almost works — the tricky part is in getting a Uri to the File that other apps can actually read, particularly when it comes to Android 6.0 Marshmallow devices and runtime permissions (which include the now dangerous READ_EXTERNAL_STORAGE and WRITE_EXTERNAL_STORAGE permissions).

My plea: 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 prior to KitKat, would require your app to have WRITE_EXTERNAL_STORAGE. And really important share targets, like Gmail, won’t have the READ_EXTERNAL_STORAGE permission — so it’ll just 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 the File Sharing Training.

Once you have it set up, our code becomes:

Using FileProvider.getUriForFile(), you’ll get a Uri actually suitable for sending to another app — they’ll be able to read it without any storage permissions — instead, you are specifically granting them read permission with FLAG_GRANT_READ_URI_PERMISSION.

Note: we don’t call setType() anywhere when building our ShareCompat (even though in the video I did set it). As explained in the setDataAndType() Javadoc, the type is automatically inferred from the data URI using getContentResolver().getType(uriToImage). Since FileProvider returns the correct mime type automatically, we don’t need to manually specify a mime type at all.

If you’re interested in learning more about avoiding the storage permission, consider watching my Forget the Storage Permission talk or at least go through the slides, which covers this topic in depth at 14:55 (slide 11).

Receiving files

Receiving files isn’t too different from text because you’re still going to use ShareCompat.IntentReader. For example, to make a Bitmap out of an incoming file, it would look like:

Of course, you’re free to do whatever you want with the InputStream — watch out for images that are so large you hit an OutOfMemoryException. All of the things you know about loading Bitmaps still apply.

The Support Library is your friend

With both ShareCompat (and its IntentBuilder and IntentReader) and FileProvider in the v4 Support Library, you’ll be able to include sharing text, HTML text, and files in your app with the best practices by default.

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Sharing a File

This lesson teaches you to

You should also read

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:

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:

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 , android.view.View, int, long)»>onItemClick() , in which you can get the selected file.

In , android.view.View, int, long)»>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

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:

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

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:

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() .

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:

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:

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