Device Storage API

The Device Storage API is used to access the file system within a Web app. Accessing the file system can be highly sensitive, and for that reason this API is available for privileged apps only.

Accessing device storage

This section explains what is needed to access device storage.

Entry point

It's possible to access storage areas by using the navigator.getDeviceStorage() and navigator.getDeviceStorages() methods:

• navigator.getDeviceStorage() accepts a string parameter representing the name of the default storage area to be accessed. The method returns a DeviceStorage object, which is used to access the related storage area. It returns the storage area whose .default attribute is true. This is controlled by the user via Settings App > Media Storage > Default media location.
• navigator.getDeviceStorages() on the other hand accepts a string representing the name of the storage area to be accessed, and returns an Array of DeviceStorage objects, one per physical storage area.

Firefox OS provides the following storage names:

• apps: This storage area is used to store the user data needed by apps. As it is critical data, accessing this storage area requires some extra privileges (see below) and is available to certified applications only.
• music: This is the storage area where music and sounds are stored.
• pictures: This is the storage area where pictures are stored.
• sdcard: This is the storage area that grants access to the device's SDCard. sdcard is also used as the name of the device's default internal storage, which is a bit unfortunate and potentially confusing; be aware.
• videos: This is the storage area where videos are stored.

var pics = navigator.getDeviceStorage('pictures');

Accessing different storage areas with navigator.getDeviceStorages()

When using navigator.getDeviceStorages(), if there is more than one storage area then the internal one will be named for example sdcard and the physical storage area will be called something else (sometimes it's extsdcard, sometimes it's sdcard1). This varies by device manufacturer. The names of files on the sdcard storage area will be /sdcard/path/filename, and the names of files on the sdcard1 storage area will be /sdcard1/path/filename, or whatever.

Note that the /sdcard and /sdcard1 are storage names. Their actual mount points on the system are determined via vold and/or /system/etc/volume.cfg file.) DeviceStorage transparently maps the storageName into the actual mountPoint (so you don't need the mount point if you're just accessing the files through device storage).

If you want to determine the mount point to examine the filesystem from an adb shell, then you can determine the vold mount points by using the command adb shell vdc volume list  (this requires a root shell).

On the Flame, you'll see something like this:

110 0 sdcard /storage/sdcard 4
110 0 sdcard1 /storage/sdcard1 4
200 0 Volumes listed.

For volumes that aren't managed by vold (for example, the sdcard volume on a Nexus 4/5), the mount point is found in /system/etc/volume.cfg.

Device storage permissions

To be able to use these storage areas, the application must declare them in its application manifest. For example, if the application wants to access the sdcard storage area, it must have the "device-storage:sdcard" permission in its manifest.

"permissions": {
  "device-storage:videos":{ "access": "readonly" },
  "device-storage:pictures":{ "access": "readwrite" }
}

As mentioned above, using device-storage:apps also needs some extra permissions, in the form of the webapps-manage permission, which allows access to the navigator.mozApps.mgmt API for managing installed open web apps.

"permissions": {
  "device-storage:apps":{ "access": "readwrite" },
  "webapps-manage":{ }
}

All of the device-storage name permissions are privileged level, except for apps, which is certified. webapps-manage is certified level.

Using a storage

Once an application gets access to a storage area, it's possible to add, get and remove files inside the area.

Add a file

Adding a file is done using the addNamed or add methods. The former allows to set an explicit name when storing a file while the latter creates a name automatically when the file is stored. Both methods are asynchronous and return a DOMRequest object to handle the success or error of the operation. This is very important as writing and reading files on a physical support is a slow process.

Those two methods expect a Blob as their first parameter. This object will be turned into a file under the hood and stored. When creating a Blob object, it's mandatory to give it a type. This type, which is a MIME type, is important because some storage areas have restrictions based on the type:

• music only accepts Blob with a valid audio MIME type
• pictures only accepts Blob with a valid image MIME type
• videos only accepts Blob with a valid video MIME type

var sdcard = navigator.getDeviceStorage("sdcard");
var file   = new Blob(["This is a text file."], {type: "text/plain"});

var request = sdcard.addNamed(file, "my-file.txt");

request.onsuccess = function () {
  var name = this.result;
  console.log('File "' + name + '" successfully wrote on the sdcard storage area');
}

// An error typically occur if a file with the same name already exist
request.onerror = function () {
  console.warn('Unable to write the file: ' + this.error);
}

Get a file

Retrieving a file can be done in two ways: by using its name or by iterating the whole list of files.

The easiest way is to retrieve a file by its name using the get and getEditable methods. The former provides a File object (which act like a read only file) when the latter provides a FileHandle object (which allows updating the underlaying file). Both methods are asynchronous and return a DOMRequest object to handle the success or error of the operation.

var sdcard = navigator.getDeviceStorage('sdcard');

var request = sdcard.get("my-file.txt");

request.onsuccess = function () {
  var file = this.result;
  console.log("Get the file: " + file.name);
}

request.onerror = function () {
  console.warn("Unable to get the file: " + this.error);
}

The other way to retrieve files is by browsing the content of the storage area. This is possible using the enumerate and enumerateEditable methods. The former provides File objects when the latter provides FileHandle objects. Both methods are asynchronous and return a DOMCursor object to iterate along the list of files. A DOMCursor is nothing less than a DOMRequest with extra power to iterate asynchronously along a list of things (files in that case).

var pics = navigator.getDeviceStorage('pictures');

// Let's browse all the images available
var cursor = pics.enumerate();

cursor.onsuccess = function () {
  if(cursor.result.name !== null) {
    var file = cursor.result;
    console.log("File found: " + file.name);
 
    // Once we found a file we check if there is other results
    // Then we move to the next result, which call the cursor
    // success with the next file as result.
      this.continue();
    }
}

cursor.onerror = function () { 
    console.warn("No file found: " + this.error); 
}

It's possible to limit the number of results by passing two optional parameters to the enumerate and enumerateEditable methods.

• The first parameter can be a string representing a sub folder to search inside.
• The second parameter can be an object with a since property, which allows you to limit the search to a given time period.

var pics = navigator.getDeviceStorage('pictures');

// Lets retrieve picture from the last week.
var param = {
  since: new Date((+new Date()) - 7*24*60*60*1000)
}

var cursor = pics.enumerate(param);

cursor.onsuccess = function () {
  var file = this.result;
  console.log("Picture taken on: " + file.lastModifiedDate);

  if (!this.done) {
    this.continue();
  }
}

Delete a file

A file can be removed from the storage area by simply using the delete method. This method just needs the name of the file to delete. As all the other methods from the DeviceStorage interface, this one is also asynchronous and returns a DOMRequest object to handle the success or error of the operation.

var sdcard = navigator.getDeviceStorage('sdcard');

var request = sdcard.delete("my-file.txt");

request.onsuccess = function () {
  console.log("File deleted");
}

request.onerror = function () {
  console.log("Unable to delete the file: " + this.error);
}

Storage information

Beyond accessing files, a storage area provides a few methods to easily reach some important information

Available space

One of the most important things to know when storing files on a device is the amount of space available. The DeviceStorage interface provides two useful functions dedicated to space:

• freeSpace() to get the amount of free space available to store new files;
• usedSpace() to get the amount of space used to store the files;

As those methods are asynchronous, they return a DOMRequest object to handle the success or error of the operation.

var videos = navigator.getDeviceStorage('videos');

var request = videos.usedSpace();

request.onsuccess = function () {
  // The result is express in bytes, lets turn it into megabytes
  var size = this.result / 1048576;
  console.log("The videos on your device use a total of " + size.toFixed(2) + "Mo of space.");
}

request.onerror = function () {
  console.warn("Unable to get the space used by videos: " + this.error);
}

Listening for change

As many applications can use a same storage area at the same time, it's sometimes useful for an application to be aware of a change in that storage area. It's also useful for an application performing an asynchronous action because it doesn't have to relay on all the DOMRequest objects returned by each method of the DeviceStorage interface.

To that end, a change event is triggered each time a file is created, modified or deleted. This event can be captured using the onchange property or the addEventListener() method. The event handler gets a DeviceStorageChangeEvent object which is a regular Event object with two extra properties:

• DeviceStorageChangeEvent.reason which gives the reason of the change (created, modified or deleted).
• DeviceStorageChangeEvent.path which gives the full path to the file affected by the change.

var sdcard = navigator.getDeviceStorage('sdcard');

sdcard.onchange = function (change) {
  var reason = change.reason;
  var path   = change.path;

  console.log('The file "' + path + '" has been ' + reason);
}

Specification

Not part of any specification.

Browser compatibility

See also

• navigator.getDeviceStorage()
• navigator.getDeviceStorages()
• DeviceStorage
• DeviceStorageChangeEvent
• Firefox OS Contact Exported Sample App