- Content providers are a means for IPC (Interprocess communication) on Android.
- The system has several content providers for built-in apps. App authors can also provide their own custom content providers as an API.
- In reality, custom content providers are not used frequently because it is often easier and more reliable to share data through a cloud service (like AWS AppSync) etc. Using custom content providers produces runtime dependencies that most users down't want to deal with.
The MediaStore is one example of a built-in ContentProvider on Android. You can think of it as a SQL database that provides access to the music, video, and pictures on the device. The MediaStore is not without its problems though. It is unable to access many formats of audio such as WMA and APE. The following steps show how to query the MediaStore:
Note: The following code won't work on Android Q as the READ/WRITE external storage permissions are going away in favor of scoped storage access.
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Set the proper permission in the AndroidManifest.xml and request the permission using
ContextCompat.checkSelfPermissionsee: https://developer.android.com/reference/android/Manifest.permission and https://developer.android.com/training/permissions/requesting<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>if (ContextCompat.checkSelfPermission(this, PERMISSION) != PackageManager.PERMISSION_GRANTED) { // Permission is not granted // Should we show an explanation? if (ActivityCompat.shouldShowRequestPermissionRationale(this, PERMISSION)) { // Show an explanation to the user *asynchronously* -- don't block // this thread waiting for the user's response! After the user // sees the explanation, try again to request the permission. } else { // No explanation needed; request the permission ActivityCompat.requestPermissions( this, new String[]{PERMISSION}, MY_PERMISSIONS_REQUEST_ACCESS_MEDIA); // MY_PERMISSIONS_REQUEST_ACCESS_MEDIA is an // app-defined int constant. The callback method gets the // result of the request. } } else { // Permission has already been granted readMedia(); }
@Override public void onRequestPermissionsResult( int requestCode, String permissions[], int[] grantResults) { switch (requestCode) { case MY_PERMISSIONS_REQUEST_ACCESS_MEDIA: { // If request is cancelled, the result arrays are empty. if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) { // permission was granted, yay! Do the // media-related task you need to do. readMedia(); } else { // permission denied, boo! Disable the // functionality that depends on this permission. } return; } } }
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Define a Projection (similar to specifying columns in a SQL SELECT statement)`
private static String[] PROJECTION = new String[] { MediaStore.Audio.AudioColumns.ALBUM, MediaStore.Audio.AudioColumns.TITLE };
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Get a
Cursorobject usinggetContentResolver().querywith the proper URL and Projection. -
Iterate over the cursor using
moveToNext()etc. or use aCursorAdapterin a RecyclerView. To get data from an individual column in aCursor, you need the column's index within the Cursor. You can either create a constant as the index will be the same order as your projection, or you can query theCursordirectly.private void readMedia() { Cursor cursor = getContentResolver(). query(MediaStore.Audio.Media.EXTERNAL_CONTENT_URI, PROJECTION, null, null, null); int idxAlbum = cursor.getColumnIndexOrThrow( MediaStore.Audio.AudioColumns.ALBUM); int idxTitle = cursor.getColumnIndexOrThrow( MediaStore.Audio.AudioColumns.TITLE); Log.i(TAG, "Media Rows: " + cursor.getCount()) ; while (cursor.moveToNext()) { String album = cursor.getString(idxAlbum); String title = cursor.getString(idxTitle); Log.i(TAG, "Album: " + album + " Title: " + title); } }
public class ProviderDemo extends ContentProvider {
static final String TAG = "ProviderDemo";
static final String AUTHORITY = "content://com.example.android.lifecycle.providerdemo";
public static final Uri CONTENT_URI = Uri.parse(AUTHORITY);
static final String SINGLE_RECORD_MIME_TYPE = "vnd.android.cursor.item/vnd.example.android.lifecycle.status";
static final String MULTIPLE_RECORDS_MIME_TYPE = "vnd.android.cursor.dir/vnd.example.android.lifecycle.status";
@Override
public boolean onCreate() {
// Used to initialize this content provider. This method runs on UI
// thread, so should be quick. Good place to instantiate database
// helper, if using database.
Log.d(TAG, "onCreate");
return true;
}
@Override
public String getType(Uri uri) {
// Returns the mime time for the given uri. Typically, this MIME
// type will either be something like
// vnd.android.cursor.item/vnd.example.android.lifecycle.status for
// a single item or
// vnd.android.cursor.dir/vnd.example.android.lifecycle.status for
// multiple items.
String ret = getContext().getContentResolver().getType(System.CONTENT_URI);
Log.d(TAG, "getType returning: " + ret);
return ret;
}
@Override
public Uri insert(Uri uri, ContentValues values) {
// Inserts the values into the provider returning uri that points to
// the newly inserted record.
Log.d(TAG, "insert uri: " + uri.toString());
return null;
}
@Override
public int update(Uri uri, ContentValues values, String selection,
String[] selectionArgs) {
// Updates records(s) specified by either the uri or selection/
// selectionArgs combo. Returns number of records affected.
Log.d(TAG, "update uri: " + uri.toString());
return 0;
}
@Override
public int delete(Uri uri, String selection, String[] selectionArgs) {
// Deletes records(s) specified by either the uri or selection/
// selectionArgs combo. Returns number of records affected.
Log.d(TAG, "delete uri: " + uri.toString());
return 0;
}
@Override
public Cursor query(Uri uri, String[] projection, String selection,
String[] selectionArgs, String sortOrder) {
// Queries the provider for the record(s) specified by either uri or
// selection/selectionArgs/grouping/having combo.
Log.d(TAG, "query with uri: " + uri.toString());
return null;
}
}<provider
android:name=".ProviderDemo"
android:authorities="com.example.android.lifecycle.providerdemo" />A content URI is a URI that identifies data in a provider. It consists of:
- The scheme, which is always content:// for a content URI
- The authority, which is a unique string identifying the specific content provider
- The path, which identifies a particular record or collection of records managed by the provider
In the preceding example, the full URI for the "words" table is:
content://user_dictionary/words
- content:// — the scheme identifying this as a content URI
- user_dictionary — the authority of the system user dictionary provider
- words — the path corresponding to the “words” table
The ContentResolver uses the authority to identify the ContentProvider to contact.
- An application implementing a ContentProvider specifies the provider’s authority in the application manifest.
The ContentProvider uses the path to choose the table to access.
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A provider usually has a path for each table it exposes.
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Many providers allow you to access a single row in a table by appending an ID value to the end of the URI.
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For example, to retrieve a row whose _ID is 4 from user dictionary, you can use this content URI:
Uri singleUri = ContentUri.withAppendedId(UserDictionary.Words.CONTENT_URI,4);
Note: The Uri and Uri.Builder classes contain convenience methods for constructing well-formed Uri objects from strings. The ContentUris class contains convenience methods for appending id values to a URI. The previous snippet uses withAppendedId() to append an id to the UserDictionary content URI.