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C # tutorial
Content provider by requesting component provides data from one application to another application. These requests by the class ContentResolver way to deal with. Content providers can use different ways to store data. Data can be stored in a database, file, or even a network.
Sometimes the need to share data between applications. In this case the content provider becomes very useful.
Content providers can make content focus, you can have a number of different applications to access, if necessary. Content providers and acts like a database. You can query, edit its content, use insert (), update (), delete () and query () to add or delete content. In most cases the data is stored in a SQLite database.
The content provider is implemented as a subclass of the class ContentProvider class. We need to implement a series of standard API, so other applications to execute the transaction.
public class MyApplication extends ContentProvider { }
To query the content provider, you need to be in the form of the following format to specify the URI query string:
<prefix>://<authority>/<data_type>/<id>
The following is a specific description of each part of the URI:
section | Explanation |
---|
prefix | Prefix: has been set as content: // |
authority | Authorization: Specifies the name of the content provider, such as contacts, browser, etc. Third party content providers may be a full name, such as: cn.programmer.statusprovider |
data_type | Data Type: This indicates that this particular type of content provider of the data. For example: You want to provide by the content provider to get all of the Contacts address book, the data path is the people, then the URI will look as follows: content: // contacts / people |
id | This request specified a particular record. For example: You provide the content provider Contacts Find contact ID number is 5, then the URI looks like this: content: // contacts / people / 5 |
Simple steps described here providers to create their own content.
The following is the content provider to get your work, you need some method in the class ContentProvider rewritten:
This example explains how to create your own content provider. Let's follow the steps below:
step | description |
---|
1 | Create Android applications using Android Studio named Content Provider, in the package cn.w3resource.contentprovider, and the establishment of space activities. |
2 | Modify the main activities MainActivity.java file to add two new methods onClickAddName () and onClickRetrieveStudents (). |
3 | Created under the new Java package cn.w3resource.contentprovider StudentsProvider.java file to define the actual provider, and associated methods. |
4 | Use <provider /en. /> tag in AndroidManifest.xml registered content provider. |
5 | Modify the default content res / layout / activity_main.xml file to add the student record contains a simple interface. |
6 | Without modifying strings.xml, Android Studio will pay attention strings.xml file. |
7 | Launch Android emulator to run the application and verify the results of changes made to the application. |
Here is the content of the modified main activity file src / cn.w3resource.contentprovider / MainActivity.java of. This file contains each of the basic life-cycle approach. We've added two new methods, onClickAddName () and onClickRetrieveStudents () to let the application handle user interaction.
package cn.w3resource.contentprovider; 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; import cn.w3resource.contentprovider.R; 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.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()); } } }
Create a new file in the package StudentsProvider.java cn.w3resource.contentprovider. The following is the content of src / cn.w3resource.contentprovider / StudentsProvider.java of.
package cn.w3resource.contentprovider; 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); } /** * 数据库特定常量声明 */ 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);"; /** * 创建和管理提供者内部数据源的帮助类. */ 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); /** * 如果不存在,则创建一个可写的数据库。 */ db = dbHelper.getWritableDatabase(); return (db == null)? false:true; } @Override public Uri insert(Uri uri, ContentValues values) { /** * 添加新学生记录 */ long rowID = db.insert( STUDENTS_TABLE_NAME, "", values); /** * 如果记录添加成功 */ 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 == ""){ /** * 默认按照学生姓名排序 */ sortOrder = NAME; } Cursor c = qb.query(db, projection, selection, selectionArgs,null, null, sortOrder); /** * 注册内容URI变化的监听器 */ 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)){ /** * 获取所有学生记录 */ case STUDENTS: return "vnd.android.cursor.dir/vnd.example.students"; /** * 获取一个特定的学生 */ case STUDENT_ID: return "vnd.android.cursor.item/vnd.example.students"; default: throw new IllegalArgumentException("Unsupported URI: " + uri); } } }
The following is the AndroidManifest.xml file modifications. Here Added <provider /en. /> tag to include our content providers:
<?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="https://www.android.com/" package="cn.w3resource.contentprovider" 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="cn.w3resource.contentprovider.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>
Here is the content of res / layout / activity_main.xml file:
<RelativeLayout xmlns:android="https://www.android.com/" xmlns:tools="https://www.android.com/" 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="内容提供者实例" 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="www.w3resource.cn" 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/ic_launcher" 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="添加" 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="姓名" 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="年级" android:textColorHint="@android:color/holo_blue_bright" /> <Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="查询" 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>
Ensure res / values / strings.xml file has the following contents:
<?xml version="1.0" encoding="utf-8"?> <resources> <string name="app_name">Content Provider</string> <string name="action_settings">Settings</string> </resources>
Let's just run a modified Content Provider application. I assume you've created AVD during the installation environment. Open your project in the active file, click on the toolbar Icon to run the application In Android Studio. Android Studio install the application on the AVD and starts it. If all goes well, it will be displayed on the emulator window as follows:
Enter the name and year, and click on the "Add" button, which will add a student record in the data, and at the bottom to delete a message. Information content displays a number of records added to the database content provider URI. This operation uses the insert () method. Repeat this process to add more students in our content provider's database.
Once you complete the database record is added, it is time to give back to the content provider requires these records. Click the "Search" button, which will come to retrieve and display all of the data recorded by implementing the query () method.
You can provide MainActivity.java callback method to prepare update and delete operations, and modify the user interface to add update and delete operations.
You can use existing content provided in this way by such contacts. You can also in this way to develop a good database-oriented application, you can be performed as described above is known as an instance of database operations, such as read, write, update and delete.