关于RecyclerViewCursorAdapter

RecyclerView是谷歌推荐使用的也是现在比较常用的控件，用来代替ListView。CursorAdapter经常会配合数据库使用，然而在RecyclerView中使用CursorAdapter时会出错！查了一下，CursorAdapter只和ListView适配，RecyclerView并不兼容，看来需要一个改造过的CursorAdapter。还好，Github上有大神实现了RecyclerViewCursorAdapter：传送门。在里面找到主要用到了两个类，代码这里就直接贴出来了：

RecyclerViewCursorAdapter.java

public abstract class RecyclerViewCursorAdapter<VH extends RecyclerView.ViewHolder> extends RecyclerView.Adapter<VH> implements Filterable,
        CursorFilter.CursorFilterClient {

    /**
     * Call when bind view with the cursor
     *
     * @param holder
     * @param cursor
     */
    public abstract void onBindViewHolder(VH holder, Cursor cursor);

    /**
     * This field should be made private, so it is hidden from the SDK.
     * {@hide}
     */
    protected boolean mDataValid;

    /**
     * The current cursor
     */
    protected Cursor mCursor;

    /**
     * This field should be made private, so it is hidden from the SDK.
     * {@hide}
     */
    protected Context mContext;

    /**
     * The row id column
     */
    protected int mRowIDColumn;

    /**
     * This field should be made private, so it is hidden from the SDK.
     * {@hide}
     */
    protected ChangeObserver mChangeObserver;
    /**
     * This field should be made private, so it is hidden from the SDK.
     * {@hide}
     */
    protected DataSetObserver mDataSetObserver;

    /**
     * This field should be made private, so it is hidden from the SDK.
     * {@hide}
     */
    protected CursorFilter mCursorFilter;

    /**
     * This field should be made private, so it is hidden from the SDK.
     * {@hide}
     */
    protected FilterQueryProvider mFilterQueryProvider;

    /**
     * If set the adapter will register a content observer on the cursor and will call
     * {@link #onContentChanged()} when a notification comes in.  Be careful when
     * using this flag: you will need to unset the current Cursor from the adapter
     * to avoid leaks due to its registered observers.  This flag is not needed
     * when using a CursorAdapter with a
     * {@link android.content.CursorLoader}.
     */
    public static final int FLAG_REGISTER_CONTENT_OBSERVER = 0x02;

    /**
     * Recommended constructor.
     *
     * @param c       The cursor from which to get the data.
     * @param context The context
     * @param flags   Flags used to determine the behavior of the adapter;
     *                Currently it accept {@link #FLAG_REGISTER_CONTENT_OBSERVER}.
     */
    public RecyclerViewCursorAdapter(Context context, Cursor c, int flags) {
        init(context, c, flags);
    }

    void init(Context context, Cursor c, int flags) {

        boolean cursorPresent = c != null;
        mCursor = c;
        mDataValid = cursorPresent;
        mContext = context;
        mRowIDColumn = cursorPresent ? c.getColumnIndexOrThrow("_id") : -1;
        if ((flags & FLAG_REGISTER_CONTENT_OBSERVER) == FLAG_REGISTER_CONTENT_OBSERVER) {
            mChangeObserver = new ChangeObserver();
            mDataSetObserver = new MyDataSetObserver();
        } else {
            mChangeObserver = null;
            mDataSetObserver = null;
        }

        if (cursorPresent) {
            if (mChangeObserver != null) c.registerContentObserver(mChangeObserver);
            if (mDataSetObserver != null) c.registerDataSetObserver(mDataSetObserver);
        }
        setHasStableIds(true);
    }

    /**
     * Returns the cursor.
     *
     * @return the cursor.
     */
    @Override
    public Cursor getCursor() {
        return mCursor;
    }

    /**
     * @see RecyclerView.Adapter#getItemCount()
     */
    @Override
    public int getItemCount() {
        if (mDataValid && mCursor != null) {
            return mCursor.getCount();
        } else {
            return 0;
        }
    }

    /**
     * @param position Adapter position to query
     * @return
     * @see RecyclerView.Adapter#getItemId(int)
     */
    @Override
    public long getItemId(int position) {
        if (mDataValid && mCursor != null) {
            if (mCursor.moveToPosition(position)) {
                return mCursor.getLong(mRowIDColumn);
            } else {
                return 0;
            }
        } else {
            return 0;
        }
    }

    @Override
    public void onBindViewHolder(VH holder, int position) {
        if (!mDataValid) {
            throw new IllegalStateException("this should only be called when the cursor is valid");
        }
        if (!mCursor.moveToPosition(position)) {
            throw new IllegalStateException("couldn't move cursor to position " + position);
        }
        onBindViewHolder(holder, mCursor);
    }

    /**
     * Change the underlying cursor to a new cursor. If there is an existing cursor it will be
     * closed.
     *
     * @param cursor The new cursor to be used
     */
    public void changeCursor(Cursor cursor) {
        Cursor old = swapCursor(cursor);
        if (old != null) {
            old.close();
        }
    }

    /**
     * Swap in a new Cursor, returning the old Cursor.  Unlike
     * {@link #changeCursor(Cursor)}, the returned old Cursor is <em>not</em>
     * closed.
     *
     * @param newCursor The new cursor to be used.
     * @return Returns the previously set Cursor, or null if there wasa not one.
     * If the given new Cursor is the same instance is the previously set
     * Cursor, null is also returned.
     */
    public Cursor swapCursor(Cursor newCursor) {
        if (newCursor == mCursor) {
            return null;
        }
        Cursor oldCursor = mCursor;
        if (oldCursor != null) {
            if (mChangeObserver != null) oldCursor.unregisterContentObserver(mChangeObserver);
            if (mDataSetObserver != null) oldCursor.unregisterDataSetObserver(mDataSetObserver);
        }
        mCursor = newCursor;
        if (newCursor != null) {
            if (mChangeObserver != null) newCursor.registerContentObserver(mChangeObserver);
            if (mDataSetObserver != null) newCursor.registerDataSetObserver(mDataSetObserver);
            mRowIDColumn = newCursor.getColumnIndexOrThrow("_id");
            mDataValid = true;
            // notify the observers about the new cursor
            notifyDataSetChanged();
        } else {
            mRowIDColumn = -1;
            mDataValid = false;
            // notify the observers about the lack of a data set
            notifyDataSetChanged();
//            notifyDataSetInvalidated();
        }
        return oldCursor;
    }

    /**
     * <p>Converts the cursor into a CharSequence. Subclasses should override this
     * method to convert their results. The default implementation returns an
     * empty String for null values or the default String representation of
     * the value.</p>
     *
     * @param cursor the cursor to convert to a CharSequence
     * @return a CharSequence representing the value
     */
    public CharSequence convertToString(Cursor cursor) {
        return cursor == null ? "" : cursor.toString();
    }

    /**
     * Runs a query with the specified constraint. This query is requested
     * by the filter attached to this adapter.
     * <p/>
     * The query is provided by a
     * {@link FilterQueryProvider}.
     * If no provider is specified, the current cursor is not filtered and returned.
     * <p/>
     * After this method returns the resulting cursor is passed to {@link #changeCursor(Cursor)}
     * and the previous cursor is closed.
     * <p/>
     * This method is always executed on a background thread, not on the
     * application's main thread (or UI thread.)
     * <p/>
     * Contract: when constraint is null or empty, the original results,
     * prior to any filtering, must be returned.
     *
     * @param constraint the constraint with which the query must be filtered
     * @return a Cursor representing the results of the new query
     * @see #getFilter()
     * @see #getFilterQueryProvider()
     * @see #setFilterQueryProvider(FilterQueryProvider)
     */
    public Cursor runQueryOnBackgroundThread(CharSequence constraint) {
        if (mFilterQueryProvider != null) {
            return mFilterQueryProvider.runQuery(constraint);
        }

        return mCursor;
    }

    public Filter getFilter() {
        if (mCursorFilter == null) {
            mCursorFilter = new CursorFilter(this);
        }
        return mCursorFilter;
    }

    /**
     * Returns the query filter provider used for filtering. When the
     * provider is null, no filtering occurs.
     *
     * @return the current filter query provider or null if it does not exist
     * @see #setFilterQueryProvider(FilterQueryProvider)
     * @see #runQueryOnBackgroundThread(CharSequence)
     */
    public FilterQueryProvider getFilterQueryProvider() {
        return mFilterQueryProvider;
    }

    /**
     * Sets the query filter provider used to filter the current Cursor.
     * The provider's
     * {@link FilterQueryProvider#runQuery(CharSequence)}
     * method is invoked when filtering is requested by a client of
     * this adapter.
     *
     * @param filterQueryProvider the filter query provider or null to remove it
     * @see #getFilterQueryProvider()
     * @see #runQueryOnBackgroundThread(CharSequence)
     */
    public void setFilterQueryProvider(FilterQueryProvider filterQueryProvider) {
        mFilterQueryProvider = filterQueryProvider;
    }

    /**
     * Called when the {@link ContentObserver} on the cursor receives a change notification.
     * The default implementation provides the auto-requery logic, but may be overridden by
     * sub classes.
     *
     * @see ContentObserver#onChange(boolean)
     */
    protected abstract void onContentChanged();

    private class ChangeObserver extends ContentObserver {
        public ChangeObserver() {
            super(new Handler());
        }

        @Override
        public boolean deliverSelfNotifications() {
            return true;
        }

        @Override
        public void onChange(boolean selfChange) {
            onContentChanged();
        }
    }

    private class MyDataSetObserver extends DataSetObserver {
        @Override
        public void onChanged() {
            mDataValid = true;
            notifyDataSetChanged();
        }

        @Override
        public void onInvalidated() {
            mDataValid = false;
            notifyDataSetChanged();
//            notifyDataSetInvalidated();
        }
    }
}

CursorFilter.java

class CursorFilter extends Filter {

    CursorFilterClient mClient;

    interface CursorFilterClient {
        CharSequence convertToString(Cursor cursor);

        Cursor runQueryOnBackgroundThread(CharSequence constraint);

        Cursor getCursor();

        void changeCursor(Cursor cursor);
    }

    CursorFilter(CursorFilterClient client) {
        mClient = client;
    }

    @Override
    public CharSequence convertResultToString(Object resultValue) {
        return mClient.convertToString((Cursor) resultValue);
    }

    @Override
    protected FilterResults performFiltering(CharSequence constraint) {
        Cursor cursor = mClient.runQueryOnBackgroundThread(constraint);

        FilterResults results = new FilterResults();
        if (cursor != null) {
            results.count = cursor.getCount();
            results.values = cursor;
        } else {
            results.count = 0;
            results.values = null;
        }
        return results;
    }

    @Override
    protected void publishResults(CharSequence constraint, FilterResults results) {
        Cursor oldCursor = mClient.getCursor();

        if (results.values != null && results.values != oldCursor) {
            mClient.changeCursor((Cursor) results.values);
        }
    }
}

把上面这两个类直接放到工程中，让RecyclerView的Adapter继承RecyclerViewCursorAdapter，然后就可以像ListView的CursorAdapter那样使用它了。

Loader机制

Loader，顾名思义，就是一个加载器。是Android 3.0后推出的一个用于异步加载数据的机制。可用与Activity和Fragment中。它能检测数据源，当数据源内容改变时它们能够传递新的结果，当配置改变后需要重新创建时，它们会重新连接到最后一个Loader的游标。这样，它们不需要重新查询它们的数据。常用于ContentProvider、CursorAdapter和数据库配合使用。Google提供了一个标准的Loader，另外还有一个抽象类AsyncTaskLoader。本文只是简单的谈谈这个Loader的使用，即CursorLoader，它是Loader的标准实现，如果你的数据能够用Cursor表示，比如来自SQLiteDatabase的数据就是标准的Cursor，那么这个类对你而言就够用了。

我们就来配合前面提到的RecyclerViewCursorAdapter，让Loader机制结合RecyclerViewCursorAdapter写一个demo，上图看看效果：

LoaderDemo.gif

简单使用的过程

建一个bean类，Book.java：

package com.sonnyzoom.loaderdemo.bean;

/**
 * Created by zoom on 2016/3/30.
 */
public class Book {

    private String id;
    private String name;
    private int price;

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }
    
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getPrice() {
        return price;
    }

    public void setPrice(int price) {
        this.price = price;
    }
}

数据库：BookDBHelper.java：

package com.sonnyzoom.loaderdemo.db;

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

/**
 * Created by zoom on 2016/3/30.
 */
public class BookDBHelper extends SQLiteOpenHelper {

    private static final String DATABASE_NAME = "Book.db";
    private static final int DATABASE_VERSION = 1;

    public static final String TABLE_NAME = "BookInfo";
    public static final String ID = "_id";
    public static final String NAME = "name";
    public static final String PRICE = "price";

    private static final String CREATE_DATABASE="CREATE TABLE "+TABLE_NAME+" ( "
            +ID+" INTEGER PRIMARY KEY AUTOINCREMENT ,"
            +NAME+" TEXT,"
            +PRICE+" INTEGER)";

    private volatile static BookDBHelper helper;

    private BookDBHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    public static BookDBHelper getInstance(Context context) {

        if (helper == null) {
            synchronized (BookDBHelper.class) {
                if (helper == null) {
                    helper = new BookDBHelper(context);
                }
            }
        }

        return helper;
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(CREATE_DATABASE);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }
}

一个基本的ContentProvider类，关于ContentProvider就不多说了，不要忘记在AndroidManifest里面注册。

package com.sonnyzoom.loaderdemo.provider;

import android.content.ContentProvider;
import android.content.ContentUris;
import android.content.ContentValues;
import android.content.UriMatcher;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.net.Uri;
import android.support.annotation.Nullable;

import com.sonnyzoom.loaderdemo.db.BookDBHelper;

/**
 * Created by zoom on 2016/3/30.
 */
public class BookProvider extends ContentProvider {

    private static final String AUTHORITY="com.sonnyzoom.loaderdemo.provider.bookprovider";
    public static final Uri URI_BOOK_ALL=Uri.parse("content://"+AUTHORITY+"/book");

    private static UriMatcher matcher;
    private BookDBHelper helper;
    private SQLiteDatabase db;

    private static final int BOOK_ALL=0;
    private static final int BOOK_ONE=1;

    static {
        matcher=new UriMatcher(UriMatcher.NO_MATCH);
        matcher.addURI(AUTHORITY,"book/",BOOK_ALL);
        matcher.addURI(AUTHORITY,"book/#",BOOK_ONE);
    }

    @Override
    public boolean onCreate() {
        helper=BookDBHelper.getInstance(getContext());
        return true;
    }

    @Nullable
    @Override
    public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {

        switch (matcher.match(uri)){

            case BOOK_ALL:

                break;
            case BOOK_ONE:
                long id= ContentUris.parseId(uri);
                selection="_id=?";
                selectionArgs=new String[]{String.valueOf(id)};
                break;
            default:
                throw new IllegalArgumentException("Wrong Uri:"+uri);
        }

        db=helper.getReadableDatabase();
        Cursor cursor=db.query(BookDBHelper.TABLE_NAME,projection,selection,selectionArgs,null,null,sortOrder);
        cursor.setNotificationUri(getContext().getContentResolver(),URI_BOOK_ALL);
        return cursor;
    }

    @Nullable
    @Override
    public String getType(Uri uri) {
        return null;
    }

    @Nullable
    @Override
    public Uri insert(Uri uri, ContentValues values) {

        if (matcher.match(uri)!=BOOK_ALL){
            throw new IllegalArgumentException("Wrong Uri:"+uri);
        }
        db=helper.getReadableDatabase();
        long rowId = db.insert(BookDBHelper.TABLE_NAME, null, values);
        if (rowId>0){
            notifyDataSetChanged();
            return ContentUris.withAppendedId(uri,rowId);
        }

        return null;
    }

    @Override
    public int delete(Uri uri, String selection, String[] selectionArgs) {
        return 0;
    }

    @Override
    public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) {
        return 0;
    }

    private void notifyDataSetChanged() {
        getContext().getContentResolver().notifyChange(URI_BOOK_ALL,null);
    }
}

然后是RecyclerView的适配器，BookAdapter.java，继承上面我们开头提过的RecyclerViewCursorAdapter：

package com.sonnyzoom.loaderdemo.adapter;

import android.content.Context;
import android.database.Cursor;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import com.sonnyzoom.loaderdemo.R;
import com.sonnyzoom.loaderdemo.bean.Book;
import com.sonnyzoom.loaderdemo.db.BookDBHelper;

/**
 * Created by zoom on 2016/3/30.
 */
public class BookAdapter extends RecyclerViewCursorAdapter<BookAdapter.BookViewHolder> {

    private LayoutInflater inflater;

    public BookAdapter(Context context, Cursor c, int flags) {
        super(context, c, flags);
        inflater=LayoutInflater.from(context);
    }

    @Override
    public void onBindViewHolder(BookViewHolder holder, Cursor cursor) {

        Book book=new Book();
        book.setId(cursor.getString(cursor.getColumnIndex(BookDBHelper.ID)));
        book.setName(cursor.getString(cursor.getColumnIndex(BookDBHelper.NAME)));
        book.setPrice(cursor.getInt(cursor.getColumnIndex(BookDBHelper.PRICE)));

        holder.name.setText(book.getName());
        holder.price.setText(Integer.toString(book.getPrice()));

    }

    @Override
    protected void onContentChanged() {}

    @Override
    public BookViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {

        View v=inflater.inflate(R.layout.book_item,parent,false);

        return new BookViewHolder(v);
    }

    class BookViewHolder extends RecyclerView.ViewHolder{

        public TextView name;
        public TextView price;

        public BookViewHolder(View itemView) {
            super(itemView);
            name= (TextView) itemView.findViewById(R.id.book_name);
            price= (TextView) itemView.findViewById(R.id.book_price);

        }
    }
}

MainActivity里面主要的是CursorLoader ：

    private void initLoader() {

        getLoaderManager().initLoader(1, null, new LoaderManager.LoaderCallbacks<Cursor>() {
            @Override
            public Loader<Cursor> onCreateLoader(int id, Bundle args) {

                CursorLoader loader = new CursorLoader(MainActivity.this, BookProvider.URI_BOOK_ALL, null, null, null, null);

                return loader;
            }

            @Override
            public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
                adapter.swapCursor(data);
            }

            @Override
            public void onLoaderReset(Loader<Cursor> loader) {
                adapter.swapCursor(null);
            }
        });
    }

MainActivity.java全部代码：

package com.sonnyzoom.loaderdemo;

import android.app.LoaderManager;
import android.content.ContentValues;
import android.content.CursorLoader;
import android.content.Loader;
import android.database.Cursor;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.DefaultItemAnimator;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

import com.sonnyzoom.loaderdemo.adapter.BookAdapter;
import com.sonnyzoom.loaderdemo.db.BookDBHelper;
import com.sonnyzoom.loaderdemo.provider.BookProvider;

public class MainActivity extends AppCompatActivity {

    private RecyclerView recyclerView;
    private FloatingActionButton fab;

    private BookAdapter adapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        initViews();
        initLoader();

        if (fab != null) {
            fab.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view) {
                    showDialog();
                }
            });
        }

    }

    private void showDialog() {

        final AlertDialog dialog = new AlertDialog.Builder(this).create();
        View v = getLayoutInflater().inflate(R.layout.dialog, null);
        dialog.setView(v);
        dialog.setCancelable(false);
        dialog.show();

        final EditText name = (EditText) v.findViewById(R.id.edit_name);
        final EditText price = (EditText) v.findViewById(R.id.edit_price);

        Button cancel = (Button) v.findViewById(R.id.btn_cancel);
        Button save = (Button) v.findViewById(R.id.btn_save);

        save.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                String mName = name.getText().toString();
                String mPrice = price.getText().toString();

                ContentValues values = new ContentValues();
                values.put(BookDBHelper.NAME, mName);
                values.put(BookDBHelper.PRICE, Integer.valueOf(mPrice));

                getContentResolver().insert(BookProvider.URI_BOOK_ALL, values);
                dialog.dismiss();
            }
        });

        cancel.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                dialog.dismiss();
            }
        });

    }

    private void initLoader() {

        getLoaderManager().initLoader(1, null, new LoaderManager.LoaderCallbacks<Cursor>() {
            @Override
            public Loader<Cursor> onCreateLoader(int id, Bundle args) {

                CursorLoader loader = new CursorLoader(MainActivity.this, BookProvider.URI_BOOK_ALL, null, null, null, null);

                return loader;
            }

            @Override
            public void onLoadFinished(Loader<Cursor> loader, Cursor data) {
                adapter.swapCursor(data);
            }

            @Override
            public void onLoaderReset(Loader<Cursor> loader) {
                adapter.swapCursor(null);
            }
        });

    }

    private void initViews() {

        fab = (FloatingActionButton) findViewById(R.id.fab);
        recyclerView = (RecyclerView) findViewById(R.id.recyclerView);

        if (adapter == null) {

            Cursor c = getContentResolver().query(BookProvider.URI_BOOK_ALL, null, null, null, null);
            adapter = new BookAdapter(this, c, 1);
        }

        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        recyclerView.setItemAnimator(new DefaultItemAnimator());
        recyclerView.setHasFixedSize(true);
        recyclerView.setAdapter(adapter);

    }

}

Loader机制的用法当然不只有这一种，还有更多的功能等你去发掘。这里只是浅谈，文章如有什么错误，欢迎在下方评论、交流！
文章源码地址：Github