Android--ContentProvider

一个

个人理解：ContentProvider对应的还有一个ContentResolver，它们的作用就是，应用之间数据的交互，举个例子就是：一个工程是ContentProvider，一个工程是ContentResolver。ContentProvider的数据库可以给ContentResolver访问。

步骤：
我们就先写ContentProvider吧。创一个工程（不是activity，而是工程）。然后类继承ContentProvider。其中要重写的方法就有CRUD了。这里的CRUD的作用是当另一个工程访问该工程数据库时，该工程时如何返回数据给另一个工程的。
好吧，上代码：

/*
    操作student的provider类
 */
public class StudentProvider extends ContentProvider {


    private DBHelper dbHelper;

    //里面参数代表的是，当查询不到时，然会这个参数，这个参数的值为-1
    private static UriMatcher matcher=new UriMatcher(UriMatcher.NO_MATCH);

    //com.example.mycotentprovider.studentprovider/student  不根据id查询
    //com.example.mycotentprovider.studentprovider/student/3  根据id查询
    //保存一些合法uri
    static {
        //里面的code并不是要查询的id，而是标识它是根据id查询还是不是根据id查询，也就是1是根据id查询，2是不跟据id查询
        matcher.addURI("com.example.mycotentprovider.studentprovider","/student",1);
        matcher.addURI("com.example.mycotentprovider.studentprovider","/student/#",2);// #匹配任意数字

    }

    public StudentProvider(){
        Log.e("TAG","StudentProvider()");
    }
	
    @Override
    public boolean onCreate() {
        dbHelper=new DBHelper(getContext());
        Log.e("TAG","StudentProvider()  onCreate()");
        return false;
    }


    //com.example.mycotentprovider.studentprovider/student  不根据id查询
    //com.example.mycotentprovider.studentprovider/student/3  根据id查询
    @Nullable
    @Override
    public Cursor query(@NonNull Uri uri, @Nullable String[] projection, @Nullable String selection, @Nullable String[] selectionArgs, @Nullable String sortOrder) {
        Log.e("TAG","StudentProvider() query");

        //得到连接对象
        SQLiteDatabase database = dbHelper.getReadableDatabase();

        //匹配uri，返回code
        int code = matcher.match(uri);
        if(code==1){//不根据id查询
            Cursor cursor = database.query("student", projection, selection, selectionArgs, null, null, null);
            return cursor;
        }else if(code==2){//根据id查询
            //得到id
            long id = ContentUris.parseId(uri);
            Cursor cursor = database.query("student", projection, "_id=?", new String[]{id+""}, null, null, null);
            return cursor;
        }else{
            throw new RuntimeException("查询Uri不合法");
        }
    }

	//不知道干什么的....（不理它）
    @Nullable
    @Override
    public String getType(@NonNull Uri uri) {
        Log.e("TAG","StudentProvider() getType");
        return null;
    }

	//添加数据
    @Nullable
    @Override
    public Uri insert(@NonNull Uri uri, @Nullable ContentValues values) {
        Log.e("TAG","StudentProvider() insert");
        SQLiteDatabase database = dbHelper.getReadableDatabase();
        int code=matcher.match(uri);
        if(code==1){
            long id = database.insert("student", null, values);
            uri=ContentUris.withAppendedId(uri,id);
            database.close();
            return uri;
        }else{
            database.close();
            throw new RuntimeException("添加Uri不合法");
        }
    }

    /*
        根据id和不跟据id删除
     */
    @Override
    public int delete(@NonNull Uri uri, @Nullable String selection, @Nullable String[] selectionArgs) {
        Log.e("TAG","StudentProvider()  delete");
        SQLiteDatabase database=dbHelper.getReadableDatabase();
        int code = matcher.match(uri);
        int deleteCount = -1;
        if(code==1){
            deleteCount = database.delete("student", selection, selectionArgs);
        }else if(code==2){
            long id =ContentUris.parseId(uri);
            deleteCount = database.delete("student", "_id=" + id, null);
        }else{
            database.close();
            throw new RuntimeException("删除Uri不合法");
        }
        database.close();
        return deleteCount;
    }

	//根据id或不根据id更新
    @Override
    public int update(@NonNull Uri uri, @Nullable ContentValues values, @Nullable String selection, @Nullable String[] selectionArgs) {
        Log.e("TAG","StudentProvider() update");

        SQLiteDatabase database=dbHelper.getReadableDatabase();
        int code = matcher.match(uri);
        int count=-1;
        if(code==1){
            count=database.update("student",values,selection,selectionArgs);
        }else if(code==2){
            long id = ContentUris.parseId(uri);
            count=database.update("student",values,"_id="+id,null);
        }else{
            database.close();
            throw new RuntimeException("更新Uri不合法");
        }
        return count;
    }
}

上面每个CRUD的方法里面都有code，code是标识，是去判断传过来的Uri是根据id还是不根据id。因为另一个工程是来取数据。所以这个工程下也得要有个sql数据库。这样为什么是sql数据库呢，个人理解是。如果是mysql之类的数据库…是联网请求数据的。那另一个工程直接联网请求不香吗…

差点忘了。由于要将数据库数据提供给另一个工程。所以要记得配置文件

<provider
        android:authorities="com.example.mycotentprovider.studentprovider"
        android:name="com.example.mycotentprovider.StudentProvider"
        android:exported="true"/>

其中authorities是唯一标识。然后exported的默认值是false。true代表运行别的应用程序访问。authorities一般都设为全类名，然后全部小写。

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ContentResolver代码：

public class MyContentResolver extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_my_content_resolver);
    }

    public void query(View view){
        //1，得到ContentResolver对象
        ContentResolver resolver=this.getContentResolver();
        //2，调用其query，得到cursor
        Uri uri=Uri.parse("content://com.example.mycotentprovider.studentprovider/student/1");
        Cursor cursor = resolver.query(uri, null, null, null, null);
        //3，取出cursor中的数据，并显示
        if(cursor.moveToNext()){
            int id=cursor.getInt(0);
            String name=cursor.getString(1);
            Toast.makeText(this,id+"-----"+name,Toast.LENGTH_SHORT).show();

        }
    }
    public void insert(View view){
        ContentResolver resolver=getContentResolver();
        Uri uri=Uri.parse("content://com.example.mycotentprovider.studentprovider/student/");
        ContentValues values=new ContentValues();
        values.put("name","Sam");
        Uri insert = resolver.insert(uri, values);
        Toast.makeText(this,uri.toString()+"",Toast.LENGTH_SHORT).show();
    }

    public void update(View view){
        ContentResolver resolver=getContentResolver();
        Uri uri=Uri.parse("content://com.example.mycotentprovider.studentprovider/student/2");
        ContentValues values=new ContentValues();
        values.put("name","jack");
        int updateCount = resolver.update(uri, values, null, null);
        Toast.makeText(this,""+updateCount,Toast.LENGTH_SHORT).show();
    }

    public void delete(View view){
        ContentResolver resolver=getContentResolver();
        Uri uri=Uri.parse("content://com.example.mycotentprovider.studentprovider/student/2");
        int deleteCount = resolver.delete(uri, null, null);

        Toast.makeText(this,""+deleteCount,Toast.LENGTH_SHORT).show();
    }

}

上面的就不讲太多了吧。就是通过合法的Uri，然后通过ContentResolver的对象调用相对应的方法。

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下面一个Uri的组成