Android中的内容提供者(带有示例)

简介

Android中的内容提供者( Content Provider)是一种能够向外部提供访问自己私有数据的机制。它可以将数据安全地共享给其他应用，也可以和其他应用共享其他应用的数据。通过内容提供者，应用可以在不知道具体 数据存储位置的情况下获取数据。这种机制也能在应用之间共享数据，从而帮助开发者编写更加高效、更加健壮的代码。

创建ContentProvider

public class MyContentProvider extends ContentProvider {
    //定义一个静态变量，这个变量用来描述ContentProvider对外提供的数据。
    public static final String AUTHORITY = "com.example.my.contentprovider";
    public static final Uri CONTENT_URI = Uri.parse("content://"+ AUTHORITY + "/person");

    private static final int PERSON_COLLECTION = 1;
    private static final int PERSON_SINGLE = 2;
    //定义一个UriMatcher，用来匹配Uri，不匹配则返回-1
    private static final UriMatcher uriMatcher;
    static {
        uriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
        uriMatcher.addURI(AUTHORITY,"person",PERSON_COLLECTION);
        uriMatcher.addURI(AUTHORITY,"person/#",PERSON_SINGLE);
    }
    //声明一些私有变量和方法，用来对Provider进行一些操作，如增、删、改、查等。
    private SQLiteDatabase database;

    @Override
    public boolean onCreate() {
        MyDatabaseHelper helper = new MyDatabaseHelper(getContext());
        database = helper.getWritableDatabase();
        return (database == null) ? false : true;
    }

    @Override
    public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
        switch (uriMatcher.match(uri)){
            case PERSON_COLLECTION:
                return database.query("person",projection,selection,selectionArgs,null,null,sortOrder);
            case PERSON_SINGLE:
                long id = ContentUris.parseId(uri);
                String where = "id=" + id;
                if (TextUtils.isEmpty(selection)){
                    selection = where;
                }
                else {
                    selection = where + " and " + selection;
                }
                return database.query("person",projection,selection,selectionArgs,null,null,sortOrder);
            default:
                throw new IllegalArgumentException("URI 不支持。");
        }
    }

    @Override
    public Uri insert(Uri uri, ContentValues values) {
        long rowId = database.insert("person",null,values);
        if (rowId > 0){
            Uri insertedUri = ContentUris.withAppendedId(CONTENT_URI,rowId);
            getContext().getContentResolver().notifyChange(insertedUri,null);
            return insertedUri;
        }else {
            throw new SQLException("无法保存" + uri);
        }
    }

    @Override
    public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) {
        int count = 0;
        switch (uriMatcher.match(uri)){
            case PERSON_COLLECTION:
                count = database.update("person",values,selection,selectionArgs);
                break;
            case PERSON_SINGLE:
                long id = ContentUris.parseId(uri);
                String where = "id =" + id;
                if (TextUtils.isEmpty(selection)){
                    selection = where;
                }
                else {
                    selection = where + " and " + selection;
                }
                count = database.update("person",values,selection,selectionArgs);
                break;
            default:
                throw new IllegalArgumentException("URI不支持。");
        }
        getContext().getContentResolver().notifyChange(uri,null);
        return count;
    }

    @Override
    public int delete(Uri uri, String selection, String[] selectionArgs) {
        int count = 0;
        switch (uriMatcher.match(uri)){
            case PERSON_COLLECTION:
                count = database.delete("person",selection,selectionArgs);
                break;
            case PERSON_SINGLE:
                long id = ContentUris.parseId(uri);
                String where = "id =" + id;
                if (TextUtils.isEmpty(selection)){
                    selection = where;
                }
                else {
                    selection = where + " and " + selection;
                }
                count = database.delete("person",selection,selectionArgs);
                break;
            default:
                throw new IllegalArgumentException("URI 不支持。");
        }
        getContext().getContentResolver().notifyChange(uri,null);
        return count;
    }

    @Override
    public String getType(Uri uri) {
        switch (uriMatcher.match(uri)){
            case PERSON_COLLECTION:
                return "vnd.android.cursor.dir/person";
            case PERSON_SINGLE:
                return "vnd.android.cursor.item/person";
            default:
                throw new IllegalArgumentException("URI 不支持。");
        }
    }
}

注册ContentProvider

<!--在AndroidManifest.xml中注册ContentProvider-->
<provider
    android:name=".MyContentProvider"
    android:authorities="com.example.my.contentprovider"
    android:enabled="true"
    android:exported="true"
    android:grantUriPermissions="true"
    android:multiprocess="true"
    android:readPermission="android.permission.READ_CONTACTS"
    android:writePermission="android.permission.WRITE_CONTACTS">
</provider>

ContentProvider使用

public class MainActivity extends AppCompatActivity {
    private ListView mListView;
    private SimpleCursorAdapter mAdapter;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mListView = findViewById(R.id.listview);
        mAdapter = new SimpleCursorAdapter(this,
                android.R.layout.simple_list_item_2,
                null,
                new String[] {"name","age"},
                new int[] {android.R.id.text1,android.R.id.text2},
                CursorAdapter.FLAG_REGISTER_CONTENT_OBSERVER);
        mListView.setAdapter(mAdapter);

        //访问内容提供者
        Uri uri = Uri.parse("content://com.example.my.contentprovider/person");
        String[] projection = {"name","age"};
        String selection = null;
        String[] selectionArgs = null;
        String sortOrder = null;
        Cursor cursor = getContentResolver().query(uri,projection,selection,selectionArgs,sortOrder);
        mAdapter.swapCursor(cursor);
    }
}

以上就是关于Android中的内容提供者的介绍和示例。使用ContentProvider能够实现应用之间的数据共享，为开发提供了更加多样化的操作方式。