Content Providers are again one of the fundamental building blocks of the Android Platform. So far, we have looked at Activities, Services, which are basic building blocks. We have also looked at intents, notifications and variations of the same. None of these have dealt with stored data. Content Provider brings us to the concept of how to deal with data stored in the Android mobile esp. if the data has to be shared across applications.
Before we move on to shared data, let us understand the basics of data storage support provided by Android.
There are 4 ways we can store data:
- 1. Preferences
- 2. Files
- 3. RDBMS (SQLite)
- 4. Network
Preferences – Preferences are used to store user preferences for a single application or across applications in a mobile. They are typically name-value pairs accessible to the context.
Files – Files can be stored directly on to the mobile or to extended storage mediums. They are by default not accessible to applications other than the one that created it.
Database (RDBMS) – Android support creation of databases based on SQLite. These are again private to the application that created it.
Network – Android provides API support to store data over the network on to a server, probably
Note that all these are various forms of storing data and most often for a single application. Irrespective of how data is stored, if it has to be shared across applications, Here comes the concept of Content Providers.
Content Providers are the only way to share data across Android applications. They store and retrieve data thus making it accessible to all. Android platform provides default implementations of content providers for data types like audio, video, images, contact information etc.
In the sample I would like to first show how to work with existing content providers like the contact information. We will first view the existing contacts on the phone. We will insert a new contact (hardcoded name and phone number), update the same contact and delete the same contact, in this example. Note that the update and delete will not work till we create a contact through this example.
Introduction to Content Providers:
Irrespective of how the data is stored, Content Providers give a uniform interface to access the data. Data is exposed as a simple table with rows and columns where row is a record and column is a particular data type with a specific meaning. Like a row could be about a single person and the columns could be the person’s first name, number, address, email id etc.
Each record is identified by a unique _ID field which is the key to the record. Each content provider exposes a unique URI that identifies its data set uniquely. This URI is equivalent to a table name in a database. The URI consists of various parts: eg: content://com.colllabera.labs.sai/tasks/123 is a unique URI. content:// is a standard prefix. com.collabera.labs.sai is the authority, tasks is the table name, 123 is the unique _ID.
For the native content providers, these unique URIs are declared as constants in an interface. So, in our program we will be using constants like People.CONTENT_URI which internally translates to content://contacts/people
Let us now look at the code to view all the existing contacts:
//Here is the button to click for viewing the contacts
Button view = (Button)findViewById(R.id.viewButton);
…
//The method / class that gets invoked when the View button is clicked
view.setOnClickListener(new OnClickListener() {
public void onClick(View v){
displayContacts();
Log.i("NativeContentProvider", "Completed Displaying Contact list");
}
});
…
//Here is the displayContacts() method
private void displayContacts() {
String[] columns = new String[] {People.NAME,People.NUMBER};
Uri mContacts = People.CONTENT_URI;
Cursor mCur = managedQuery(mContacts, // Contact URI
columns, // Which columns to return
null, // Which rows to return
null, // Where clause parameters
null // Order by clause
);
if (mCur.moveToFirst()) {
String name = null;
String phoneNo = null;
do {
name = mCur.getString(mCur.getColumnIndex(People.NAME));
phoneNo = mCur.getString(mCur.getColumnIndex(People.NUMBER));
Toast.makeText(NativeContentProvider.this, name + " " + phoneNo, Toast.LENGTH_SHORT).show();
} while (mCur.moveToNext());
}
}
Here we are using the Activity.managedQuery(..) to create and execute a query against the provided URI. The comments against the parameters in the code is self-explanatory. This returns a cursor object that can be iterated using the two methods moveToFirst() and moveToNext(). For simplicity sake, I have just toasted the contact name and phone number retrieved. An advanced tutorial can start a new activity that can display this in a ListView.
Now, we can move on to creating a new contact. While the button related code will be very similar to the above, let us look at the actual createContact() method.
private void createContact(String name, String phone) {
ContentValues contact = new ContentValues();
contact.put(People.NAME, name);
insertUri = getContentResolver().insert(People.CONTENT_URI, contact);
Log.d(getClass().getSimpleName(),insertUri.toString());
Uri phoneUri = Uri.withAppendedPath(insertUri, People.Phones.CONTENT_DIRECTORY);
contact.clear();
contact.put(People.Phones.TYPE, People.TYPE_MOBILE);
contact.put(People.NUMBER, phone);
updateUri = getContentResolver().insert(phoneUri, contact);
Toast.makeText(NativeContentProvider.this, "Created a new contact: " + name + " " + phone, Toast.LENGTH_SHORT).show();
Log.d(getClass().getSimpleName(),updateUri.toString());
}
Here we need to understand 2 new classes: ContentResolver and ContentValues. A ContentResolver provides applications access to the content data / model. We can get a handle to a ContentResolver by calling the getContentResolver() method within the Activity. This provides methods to insert, update and delete data. In order to insert data, we need to provide it through a ContentValues object. A ContentValues Object is nothing but a name, value pair where the name of the column is to be mentioned. So, we pass the URI and the ContentValues to insert() method which returns a unique URI with the new ID created. Once we get the ID of the new person/contact inserted, we insert his/her mobile phone details into the related Phones table by using the returned insertUri. The insertUri which is unique to the new record is stored as a class variable to use it in the delete method later. The phoneUri is also stored for updating the same in the updateContact() method later.
Note that People is a class that has implemented various interfaces like android.priovider.BaseColumns, android.provider.Contacts.Phones, android.provider.Contact.PeopleColumns etc. These constants come from the interfaces.
With the above understanding let us see the update and delete methods:
private void updateContact(String phone) {
if (updateUri == null) {
Toast.makeText(NativeContentProvider.this, "There is nothing to update, Please create a contact and then click update", Toast.LENGTH_LONG).show();
} else {
ContentValues newPhone = new ContentValues();
newPhone.put(People.Phones.TYPE, People.TYPE_MOBILE);
newPhone.put(People.NUMBER, phone);
getContentResolver().update(updateUri, newPhone, null,null);
Toast.makeText(NativeContentProvider.this, "Updated the phone number to: " + phone, Toast.LENGTH_SHORT).show();
Log.i(getClass().getSimpleName(), "Updated the phone number");
}
}
private void deleteContact() {
if (updateUri == null) {
Toast.makeText(NativeContentProvider.this, "Please create a contact by clicking create button, then I can delete the same", Toast.LENGTH_LONG).show();
} else {
getContentResolver().delete(insertUri, null, null);
Toast.makeText(NativeContentProvider.this, "Deleted contact at: " + insertUri.toString(), Toast.LENGTH_SHORT).show();
updateUri = null;
insertUri = null;
Log.i(getClass().getSimpleName(),"Deleted the contact inserted by this program");
}
}
These methods only manipulate the freshly created record, for simplicity sake. They call upon the update() and delete() method on the ContentResolver.
The complete code for this example is available here.
Please note that you must add the following permissions to the AndroidManifest.xml file to be able to access the contacts.
<uses-permission android:name="android.permission.READ_CONTACTS"></uses-permission>
<uses-permission android:name="android.permission.WRITE_CONTACTS"></uses-permission>
Otherwise you get a SecurityException.
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