Android - Content Providers

2021-12-07 14:05 Android开发

<blockquote > A content provider component supplies data from one application to others on request. Such requests are handled by the methods of the ContentResolver class. A content provider can use different ways to store its data and the data can be stored in a database, in files, or even over a network. </blockquote> <img alt="content provider" src="http://www.tutorialspoint.com/android/images/content.jpg" /> <h4 align="center" >CONTENTPROVIDER</h4> <blockquote >sometimes it is required to share data across applications. This is where content providers become very useful.</blockquote> Content providers let you centralize content in one place and have many different applications access it as needed. A content provider behaves very much like a database where you can query it, edit its content, as well as add or delete content using insert(), update(), delete(), and query() methods. In most cases this data is stored in anSQlitedatabase. A content provider is implemented as a subclass ofContentProviderclass and must implement a standard set of APIs that enable other applications to perform transactions. <pre class="prettyprint notranslate prettyprinted" > public class My Application extends ContentProvider { }</pre> <h2 >Content URIs</h2> To query a content provider, you specify the query string in the form of a URI which has following format: <pre class="prettyprint notranslate prettyprinted" > <prefix>://<authority>/<data_type>/<id></pre> Here is the detail of various parts of the URI − <table class="table table-bordered" > <tbody > <tr > <th >Part</th> <th >Description</th> </tr> <tr > <td >prefix</td> <td >This is always set to content://</td> </tr> <tr > <td >authority</td> <td >This specifies the name of the content provider, for examplecontacts,browseretc. For third-party content providers, this could be the fully qualified name, such ascom.tutorialspoint.statusprovider</td> </tr> <tr > <td >data_type</td> <td >This indicates the type of data that this particular provider provides. For example, if you are getting all the contacts from theContactscontent provider, then the data path would bepeopleand URI would look like thiscontent://contacts/people</td> </tr> <tr > <td >id</td> <td >This specifies the specific record requested. For example, if you are looking for contact number 5 in the Contacts content provider then URI would look like thiscontent://contacts/people/5.</td> </tr> </tbody> </table> <h2 >Create Content Provider</h2> This involves number of simple steps to create your own content provider. <ul class="list" > <li > First of all you need to create a Content Provider class that extends theContentProviderbaseclass. </li> <li > Second, you need to define your content provider URI address which will be used to access the content. </li> <li > Next you will need to create your own database to keep the content. Usually, Android uses SQLite database and framework needs to overrideonCreate()method which will use SQLite Open Helper method to create or open the provider's database. When your application is launched, theonCreate()handler of each of its Content Providers is called on the main application thread. </li> <li > Next you will have to implement Content Provider queries to perform different database specific operations. </li> <li > Finally register your Content Provider in your activity file using <provider> tag. </li> </ul> Here is the list of methods which you need to override in Content Provider class to have your Content Provider working: <img alt="content provider" src="http://www.tutorialspoint.com/android/images/content1.jpg" /> <h4 align="center" >CONTENTPROVIDER</h4> <ul class="list" > <li > onCreate()This method is called when the provider is started. </li> <li > query()This method receives a request from a client. The result is returned as a Cursor object. </li> <li > insert()This method inserts a new record into the content provider. </li> <li > delete()This method deletes an existing record from the content provider. </li> <li > update()This method updates an existing record from the content provider. </li> <li > getType()This method returns the MIME type of the data at the given URI. </li> </ul> <h2 >Example</h2> This example will explain you how to create your ownContentProvider. So let's follow the following steps to similar to what we followed while creatingHello World Example− <table class="table table-bordered" > <tbody > <tr > <th class="fivepct" >Step</th> <th >Description</th> </tr> <tr > <td >1</td> <td >You will use Android StudioIDE to create an Android application and name it asMy Applicationunder a packagecom.example.My Application, with blank Activity.</td> </tr> <tr > <td >2</td> <td >Modify main activity fileMainActivity.javato add two new methodsonClickAddName()andonClickRetrieveStudents().</td> </tr> <tr > <td >3</td> <td >Create a new java file calledStudentsProvider.javaunder the packagecom.example.My Applicationto define your actual provider and associated methods.</td> </tr> <tr > <td >4</td> <td >Register your content provider in yourAndroidManifest.xmlfile using <provider.../> tag</td> </tr> <tr > <td >5</td> <td >Modify the default content ofres/layout/activity_main.xmlfile to include a small GUI to add students records.</td> </tr> <tr > <td >6</td> <td >No need to change string.xml.Android studio take care of string.xml file.</td> </tr> <tr > <td >7</td> <td >Run the application to launch Android emulator and verify the result of the changes done in the application.</td> </tr> </tbody> </table> Following is the content of the modified main activity filesrc/com.example.My Application/MainActivity.java. This file can include each of the fundamental life cycle methods. We have added two new methodsonClickAddName()andonClickRetrieveStudents()to handle user interaction with the application. <pre class="prettyprint notranslate prettyprinted" > package com.example.My Application; import android.net.Uri; import android.os.Bundle; import android.app.Activity; import android.content.ContentValues; import android.content.CursorLoader; import android.database.Cursor; import android.view.Menu; import android.view.View; import android.widget.EditText; import android.widget.Toast; public class MainActivity extends Activity { @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); } @Override public boolean onCreateOptionsMenu(Menu menu) { getMenuInflater().inflate(R.menu.main, menu); return true; } public void onClickAddName(View view) { // Add a new student record ContentValues values = new ContentValues(); values.put(StudentsProvider.NAME, ((EditText)findViewById(R.id.editText2)).getText().toString()); values.put(StudentsProvider.GRADE, ((EditText)findViewById(R.id.editText3)).getText().toString()); Uri uri = getContentResolver().insert( StudentsProvider.CONTENT_URI, values); Toast.makeText(getBaseContext(), uri.toString(), Toast.LENGTH_LONG).show(); } public void onClickRetrieveStudents(View view) { // Retrieve student records String URL = "content://com.example.provider.College/students"; Uri students = Uri.parse(URL); Cursor c = managedQuery(students, null, null, null, "name"); if (c.moveToFirst()) { do{ Toast.makeText(this, c.getString(c.getColumnIndex(StudentsProvider._ID)) + ", " + c.getString(c.getColumnIndex( StudentsProvider.NAME)) + ", " + c.getString(c.getColumnIndex( StudentsProvider.GRADE)), Toast.LENGTH_SHORT).show(); } while (c.moveToNext()); } } }</pre> Create new file StudentsProvider.java undercom.example.My Applicationpackage and following is the content ofsrc/com.example.My Application/StudentsProvider.java− <pre class="prettyprint notranslate prettyprinted" > package com.example.My Application; import java.util.HashMap; import android.content.ContentProvider; import android.content.ContentUris; import android.content.ContentValues; import android.content.Context; import android.content.UriMatcher; import android.database.Cursor; import android.database.SQLException; import android.database.sqlite.SQLiteDatabase; import android.database.sqlite.SQLiteOpenHelper; import android.database.sqlite.SQLiteQueryBuilder; import android.net.Uri; import android.text.TextUtils; public class StudentsProvider extends ContentProvider { static final String PROVIDER_NAME = "com.example.provider.College"; static final String URL = "content://" + PROVIDER_NAME + "/students"; static final Uri CONTENT_URI = Uri.parse(URL); static final String _ID = "_id"; static final String NAME = "name"; static final String GRADE = "grade"; private static HashMap<String, String> STUDENTS_PROJECTION_MAP; static final int STUDENTS = 1; static final int STUDENT_ID = 2; static final UriMatcher uriMatcher; static{ uriMatcher = new UriMatcher(UriMatcher.NO_MATCH); uriMatcher.addURI(PROVIDER_NAME, "students", STUDENTS); uriMatcher.addURI(PROVIDER_NAME, "students/#", STUDENT_ID); } /** * Database specific constant declarations */ private SQLiteDatabase db; static final String DATABASE_NAME = "College"; static final String STUDENTS_TABLE_NAME = "students"; static final int DATABASE_VERSION = 1; static final String CREATE_DB_TABLE = " CREATE TABLE " + STUDENTS_TABLE_NAME + " (_id INTEGER PRIMARY KEY AUTOINCREMENT, " + " name TEXT NOT NULL, " + " grade TEXT NOT NULL);"; /** * Helper class that actually creates and manages * the provider's underlying data repository. */ private static class DatabaseHelper extends SQLiteOpenHelper { DatabaseHelper(Context context){ super(context, DATABASE_NAME, null, DATABASE_VERSION); } @Override public void onCreate(SQLiteDatabase db) { db.execSQL(CREATE_DB_TABLE); } @Override public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { db.execSQL("DROP TABLE IF EXISTS " + STUDENTS_TABLE_NAME); onCreate(db); } } @Override public boolean onCreate() { Context context = getContext(); DatabaseHelper dbHelper = new DatabaseHelper(context); /** * Create a write able database which will trigger its * creation if it doesn't already exist. */ db = dbHelper.getWritableDatabase(); return (db == null)? false:true; } @Override public Uri insert(Uri uri, ContentValues values) { /** * Add a new student record */ long rowID = db.insert( STUDENTS_TABLE_NAME, "", values); /** * If record is added successfully */ if (rowID > 0) { Uri _uri = ContentUris.withAppendedId(CONTENT_URI, rowID); getContext().getContentResolver().notifyChange(_uri, null); return _uri; } throw new SQLException("Failed to add a record into " + uri); } @Override public Cursor query(Uri uri, String[] projection, String selection,String[] selectionArgs, String sortOrder) { SQLiteQueryBuilder qb = new SQLiteQueryBuilder(); qb.setTables(STUDENTS_TABLE_NAME); switch (uriMatcher.match(uri)) { case STUDENTS: qb.setProjectionMap(STUDENTS_PROJECTION_MAP); break; case STUDENT_ID: qb.appendWhere( _ID + "=" + uri.getPathSegments().get(1)); break; default: throw new IllegalArgumentException("Unknown URI " + uri); } if (sortOrder == null || sortOrder == ""){ /** * By default sort on student names */ sortOrder = NAME; } Cursor c = qb.query(db, projection, selection, selectionArgs,null, null, sortOrder); /** * register to watch a content URI for changes */ c.setNotificationUri(getContext().getContentResolver(), uri); return c; } @Override public int delete(Uri uri, String selection, String[] selectionArgs) { int count = 0; switch (uriMatcher.match(uri)){ case STUDENTS: count = db.delete(STUDENTS_TABLE_NAME, selection, selectionArgs); break; case STUDENT_ID: String id = uri.getPathSegments().get(1); count = db.delete( STUDENTS_TABLE_NAME, _ID + " = " + id + (!TextUtils.isEmpty(selection) ? " AND (" + selection + ')' : ""), selectionArgs); break; default: throw new IllegalArgumentException("Unknown URI " + uri); } getContext().getContentResolver().notifyChange(uri, null); return count; } @Override public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) { int count = 0; switch (uriMatcher.match(uri)){ case STUDENTS: count = db.update(STUDENTS_TABLE_NAME, values, selection, selectionArgs); break; case STUDENT_ID: count = db.update(STUDENTS_TABLE_NAME, values, _ID + " = " + uri.getPathSegments().get(1) + (!TextUtils.isEmpty(selection) ? " AND (" +selection + ')' : ""), selectionArgs); break; default: throw new IllegalArgumentException("Unknown URI " + uri ); } getContext().getContentResolver().notifyChange(uri, null); return count; } @Override public String getType(Uri uri) { switch (uriMatcher.match(uri)){ /** * Get all student records */ case STUDENTS: return "vnd.android.cursor.dir/vnd.example.students"; /** * Get a particular student */ case STUDENT_ID: return "vnd.android.cursor.item/vnd.example.students"; default: throw new IllegalArgumentException("Unsupported URI: " + uri); } } }</pre> Following will the modified content ofAndroidManifest.xmlfile. Here we have added <provider.../> tag to include our content provider: <pre class="prettyprint notranslate prettyprinted" > <?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.example.My Application" android:versionCode="1" android:versionName="1.0" > <uses-sdk android:minSdkVersion="8" android:targetSdkVersion="22" /> <application android:allowBackup="true" android:icon="@drawable/ic_launcher" android:label="@string/app_name" android:theme="@style/AppTheme" > <activity android:name="com.example.My Application.MainActivity" android:label="@string/app_name" > <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity> <provider android:name="StudentsProvider" <android:authorities="com.example.provider.College"> </provider> </application> </manifest></pre> Following will be the content ofres/layout/activity_main.xmlfile to include a button to broadcast your custom intent − <pre class="prettyprint notranslate prettyprinted" > <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent" android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin" android:paddingRight="@dimen/activity_horizontal_margin" android:paddingTop="@dimen/activity_vertical_margin" android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity"/"> <TextView android:id="@+id/textView1" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Content provider" android:layout_alignParentTop="true" android:layout_centerHorizontal="true" android:textSize="30dp" /> <TextView android:id="@+id/textView2" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Tutorials point " android:textColor="#ff87ff09" android:textSize="30dp" android:layout_below="@+id/textView1" android:layout_centerHorizontal="true" /> <ImageButton android:layout_width="wrap_content" android:layout_height="wrap_content" android:id="@+id/imageButton" android:src="@drawable/abc" android:layout_below="@+id/textView2" android:layout_centerHorizontal="true" /> <Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:id="@+id/button2" android:text="Add Name" android:layout_below="@+id/editText3" android:layout_alignRight="@+id/textView2" android:layout_alignEnd="@+id/textView2" android:layout_alignLeft="@+id/textView2" android:layout_alignStart="@+id/textView2" android:onClick="onClickAddName"/> <EditText android:layout_width="wrap_content" android:layout_height="wrap_content" android:id="@+id/editText" android:layout_below="@+id/imageButton" android:layout_alignRight="@+id/imageButton" android:layout_alignEnd="@+id/imageButton" /> <EditText android:layout_width="wrap_content" android:layout_height="wrap_content" android:id="@+id/editText2" android:layout_alignTop="@+id/editText" android:layout_alignLeft="@+id/textView1" android:layout_alignStart="@+id/textView1" android:layout_alignRight="@+id/textView1" android:layout_alignEnd="@+id/textView1" android:hint="Name" android:textColorHint="@android:color/holo_blue_light" /> <EditText android:layout_width="wrap_content" android:layout_height="wrap_content" android:id="@+id/editText3" android:layout_below="@+id/editText" android:layout_alignLeft="@+id/editText2" android:layout_alignStart="@+id/editText2" android:layout_alignRight="@+id/editText2" android:layout_alignEnd="@+id/editText2" android:hint="Grade" android:textColorHint="@android:color/holo_blue_bright" /> <Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Retrive student" android:id="@+id/button" android:layout_below="@+id/button2" android:layout_alignRight="@+id/editText3" android:layout_alignEnd="@+id/editText3" android:layout_alignLeft="@+id/button2" android:layout_alignStart="@+id/button2" android:onClick="onClickRetrieveStudents"/> </RelativeLayout></pre> Make sure you have following content ofres/values/strings.xmlfile: <pre class="prettyprint notranslate prettyprinted" > <?xml version="1.0" encoding="utf-8"?> <resources> <string name="app_name">My Application</string> <string name="action_settings">Settings</string> </resources>;</pre> Let's try to run our modifiedMy Applicationapplication we just created. I assume you had created yourAVDwhile doing environment set-up. To run the app from Android Studio IDE, open one of your project's activity files and click Run<img alt="Android StudioRun Icon" class="inline" src="http://www.tutorialspoint.com/android/images/eclipse_run.jpg" />icon from the tool bar. Android Studio installs the app on your AVD and starts it and if everything is fine with your set-up and application, it will display following Emulator window, be patience because it may take sometime based on your computer speed: <img alt="Android Content Provider Demo" src="http://www.tutorialspoint.com/android/images/content2.jpg" /> Now let's enter studentNameandGradeand finally click onAdd Namebutton, this will add student record in the database and will flash a message at the bottom showing ContentProvider URI along with record number added in the database. This operation makes use of ourinsert()method. Let's repeat this process to add few more students in the database of our content provider. <img alt="Add Records using ContentProvider" src="http://www.tutorialspoint.com/android/images/content3.jpg" /> Once you are done with adding records in the database, now its time to ask ContentProvider to give us those records back, so let's clickRetrieve Studentsbutton which will fetch and display all the records one by one which is as per our the implementation of ourquery()method. You can write activities against update and delete operations by providing callback functions inMainActivity.javafile and then modify user interface to have buttons for update and deleted operations in the same way as we have done for add and read operations. This way you can use existing Content Provider like Address Book or you can use Content Provider concept in developing nice database oriented applications where you can perform all sort of database operations like read, write, update and delete as explained above in the example.