Android IPC機制（四）用ContentProvider進行進程間通信

ContentProvider為存儲和獲取數據提供統一的接口，它可以在不同的應用程序之間共享數據，本身就是適合進程間通信的。ContentProvider底層實現也是Binder，但是使用起來比AIDL要容易許多。系統也預制了很多的ContentProvider，例如通訊錄，音視頻等，這些操作本身就是跨進程進行通信。這篇文章主要是我們來自己實現用ContentProvider來進行進程間通信，而非介紹ContentProvider怎麼使用。

1. 建立數據庫，方便ContentProvider使用

我們創建數據庫,並創建表”game_provider.db”，裡面有兩個字段分別存儲游戲的名字和游戲的描述。(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對數據庫進行操作
在initProvoder方法中，我們開啟線程來對數據庫進行操作，刪除表的所有數據，再添加數據，並實現了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,'九陰真經ol','最好玩的武俠網游');");
            }
        }).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，我們要讓ContentProvider運行在另一個進程，如果不大了解如何開啟進程，可以查看本系列的第一篇文章Android IPC機制（一）開啟多進程

3. 在Activity中調用另一個進程的GameProvider的方法
在Activity中我們在GameProvider再插入一條數據（此前GameProvider初始化時已經插入了一條數據），然後調用GameProvider的query方法來查詢數據庫中有幾條數據並打印出來。

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", "大航海時代ol");
        mContentValues.put("describe", "最好玩的航海網游");
        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機制（三）在Android Studio中使用AIDL實現跨進程方法調用這篇文章中用過，直接拿過來用：

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 CREATOR = new Creator() {
        @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);
    }
}

我們運行程序，發現GameProvider運行在另一個線程

log中也打印出了我們想要的結果，打出了兩條游戲信息：