Android IPC机制（四）用ContentProvider进行进程间通信

本文首发于微信公众号「后厂技术官」

前言

ContentProvider为存储和获取数据提供统一的接口，它可以在不同的应用程序之间共享数据，本身就是适合进程间通信的。ContentProvider底层实现也是Binder，但是使用起来比AIDL要容易许多。系统也预制了很多的ContentProvider，例如通讯录，音视频等，这些操作本身就是跨进程进行通信。这篇文章主要是我们来自己实现用ContentProvider来进行进程间通信，而非介绍ContentProvider怎么使用。

1. 建立数据库，方便ContentProvider使用

我们创建数据库,并创建表”game_provider.db”，里面有两个字段分别存储游戏的名字和游戏的描述。(DbOpenHelper.java)

package com.example.liuwangshu.mooncontentprovider;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class DbOpenHelper extends SQLiteOpenHelper {
    private static final String DB_NAME="game_provider.db";
     static final String GAME_TABLE_NAME="game";
    private static final int DB_VERSION=1;
    private String CREATE_GAME_TABLE="create table if not exists " + GAME_TABLE_NAME +"(_id integer primary key," + "name TEXT, "+"describe TEXT)";
    public DbOpenHelper(Context context) {
        super(context, DB_NAME, null, DB_VERSION);
    }
    @Override
    public void onCreate(SQLiteDatabase db) {
       db.execSQL(CREATE_GAME_TABLE);
    }
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }
}

2. 使用ContentProvider对数据库进行操作

在initProvoder方法中，我们开启线程来对数据库进行操作，删除表的所有数据，再添加数据，并实现了query和insert方法。(GameProvider.java)

package com.example.liuwangshu.mooncontentprovider;
import android.content.ContentProvider;
import android.content.ContentValues;
import android.content.Context;
import android.content.UriMatcher;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.net.Uri;
public class GameProvider extends ContentProvider {
    public static final String AUTHORITY = "com.example.liuwangshu.mooncontentprovide.GameProvider";
    public static final Uri GAME_CONTENT_URI = Uri.parse("content://" + AUTHORITY + "/game");
    private static final UriMatcher mUriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
    private SQLiteDatabase mDb;
    private Context mContext;
    private String table;
    static {
        mUriMatcher.addURI(AUTHORITY, "game", 0);
    }
    @Override
    public boolean onCreate() {
        table = DbOpenHelper.GAME_TABLE_NAME;
        mContext = getContext();
        initProvoder();
        return false;
    }
    private void initProvoder() {
        mDb = new DbOpenHelper(mContext).getWritableDatabase();
        new Thread(new Runnable() {
            @Override
            public void run() {
                mDb.execSQL("delete from " + DbOpenHelper.GAME_TABLE_NAME);
                mDb.execSQL("insert into game values(1,'九阴真经ol','最好玩的武侠网游');");
            }
        }).start();
    }
    @Override
    public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
        String table = DbOpenHelper.GAME_TABLE_NAME;
        Cursor mCursor = mDb.query(table, projection, selection, selectionArgs, null, sortOrder, null);
        return mCursor;
    }
    @Override
    public String getType(Uri uri) {
        return null;
    }
    @Override
    public Uri insert(Uri uri, ContentValues values) {
        mDb.insert(table, null, values);
        mContext.getContentResolver().notifyChange(uri, null);
        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;
    }
}

在manifest文件中，我们要让ContentProvider运行在另一个进程，如果不大了解如何开启进程，可以查看本系列的第一篇文章Android IPC机制（一）开启多进程

<provider android:authorities="com.example.liuwangshu.mooncontentprovide. GameProvider"
            android:name=".GameProvider"
            android:process=":provider"
        ></provider>

3. 在Activity中调用另一个进程的GameProvider的方法

在Activity中我们在GameProvider再插入一条数据（此前GameProvider初始化时已经插入了一条数据），然后调用GameProvider的query方法来查询数据库中有几条数据并打印出来。

package com.example.liuwangshu.mooncontentprovider;

import android.content.ContentValues;
import android.database.Cursor;
import android.net.Uri;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
public class ContentProviderActivity extends AppCompatActivity {
    private final static String TAG = "ContentProviderActivity";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_content_provider);
        Uri uri = Uri.parse("content://com.example.liuwangshu.mooncontentprovide.GameProvider");
        ContentValues mContentValues = new ContentValues();
        mContentValues.put("_id", 2);
        mContentValues.put("name", "大航海时代ol");
        mContentValues.put("describe", "最好玩的航海网游");
        getContentResolver().insert(uri, mContentValues);
        Cursor gameCursor = getContentResolver().query(uri, new String[]{"name", "describe"}, null, null, null);
        while (gameCursor.moveToNext()) {
            Game mGame = new Game(gameCursor.getString(0), gameCursor.getString(1));
            Log.i(TAG, mGame.gameName + "---" + mGame.gameDescribe);
        }
    }
}

Bean文件 Game.java在Android IPC机制（三）在Android Studio中使用AIDL实现跨进程方法调用这篇文章中用过，直接拿过来用：

package com.example.liuwangshu.mooncontentprovider;
import android.os.Parcel;
import android.os.Parcelable;
public class Game implements Parcelable {
    public String gameName;
    public String gameDescribe;

    public Game(String gameName, String gameDescribe) {
        this.gameName = gameName;
        this.gameDescribe = gameDescribe;
    }
    protected Game(Parcel in) {
        gameName = in.readString();
        gameDescribe = in.readString();
    }
    public static final Creator<Game> CREATOR = new Creator<Game>() {
        @Override
        public Game createFromParcel(Parcel in) {
            return new Game(in);
        }
        @Override
        public Game[] newArray(int size) {
            return new Game[size];
        }
    };
    @Override
    public int describeContents() {
        return 0;
    }
    @Override
    public void writeToParcel(Parcel dest, int flags) {
        dest.writeString(gameName);
        dest.writeString(gameDescribe);
    }
}