**Short version**: what is the best practice way to access maintain a history for certain messages from both an activity and from a service?

**Long version**:
I have an activity and a service, which both may be running or not. I want to keep a message log (history) in an object an persist it in a file and be able to e.g. delete entries.

When I have such history in the service and one in the activity I run into sync problems. So, any advice, what the best solution would be?

* ideally I could use the methods from the history class in both the service and activity. Probably not possible.
* I could write and read the file in each action. Probably not very efficient in the long run.
* do I really need to setup a service for the history and handle all actions with it via intents?

It is a bit similiar to "[proper way to access DB from both Activity and a started Service?](https://stackoverflow.com/questions/10820138/proper-way-to-access-db-from-both-activity-and-a-started-service)", but with just an own class instead of a SQLite DB.

Any advice?

---

**Conclusion**: Use a ContentProvider with a SQLite-DB. Short version of the code:

```
package com.example.history;

import android.content.ContentProvider;
import android.content.ContentUris;
import android.content.ContentValues;
import android.content.Context;
import android.content.UriMatcher;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.database.sqlite.SQLiteQueryBuilder;
import android.net.Uri;

public class HistoryContentProvider extends ContentProvider {

    static final String PROVIDER_NAME = "com.example.HistoryContentProvider";
    static final String URL = "content://" + PROVIDER_NAME + "/history";
    static final Uri CONTENT_URI = Uri.parse(URL);

    static final String id = "id";
    static final String normalized_number = "normalized_number";
    static final String display_name = "display_name";

    static final int uriCode = 1;
    static final UriMatcher uriMatcher;
    static {
        uriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
        uriMatcher.addURI(PROVIDER_NAME, "history", uriCode);
    }

    @Override
    public boolean onCreate() {
        Context context = getContext();
        DatabaseHelper dbHelper = new DatabaseHelper(context);
        db = dbHelper.getWritableDatabase();
        if (db != null) {
            return true;
        }
        return false;
    }

    @Override
    public Cursor query(Uri uri, String[] projection, String selection,
            String[] selectionArgs, String sortOrder) {
        SQLiteQueryBuilder qb = new SQLiteQueryBuilder();
        qb.setTables(TABLE_NAME);

        Cursor c = qb.query(db, projection, selection, selectionArgs, null, null, sortOrder);
        c.setNotificationUri(getContext().getContentResolver(), uri);
        return c;
    }

    @Override
    public String getType(Uri uri) {
        switch (uriMatcher.match(uri)) {
        case uriCode:
            return "vnd.android.cursor.dir/history";
        default:
            throw new IllegalArgumentException("Unsupported URI: " + uri);
        }   }

    @Override
    public Uri insert(Uri uri, ContentValues values) {
        long rowID = db.insert(TABLE_NAME, "", values);
        if (rowID > 0) {
            Uri _uri = ContentUris.withAppendedId(CONTENT_URI, rowID);
            getContext().getContentResolver().notifyChange(_uri, null);
            return _uri;
        }
        throw new SQLException("Failed to add a record into " + uri);
    }

    @Override
    public int delete(Uri uri, String selection, String[] selectionArgs) {
        int count = 0;
        switch (uriMatcher.match(uri)) {
        case uriCode:
            count = db.delete(TABLE_NAME, selection, selectionArgs);
            getContext().getContentResolver().notifyChange(uri, null);
            break;
        default:
            throw new IllegalArgumentException("Unknown URI " + uri);
        }
        return count;
    }

    @Override
    public int update(Uri uri, ContentValues values, String selection,
            String[] selectionArgs) {
        int count = 0;
        switch (uriMatcher.match(uri)) {
        case uriCode:
            count = db.update(TABLE_NAME, values, selection, selectionArgs);
            getContext().getContentResolver().notifyChange(uri, null);
            break;
        default:
            throw new IllegalArgumentException("Unknown URI " + uri);
        }
        return count;
    }

    private SQLiteDatabase db;
    static final String DATABASE_NAME = "historyDb";
    static final String TABLE_NAME = "history";
    static final int DATABASE_VERSION = 3;
    static final String CREATE_DB_TABLE = " CREATE TABLE " + TABLE_NAME
            + " (id INTEGER PRIMARY KEY AUTOINCREMENT, "
            + normalized_number + " TEXT NOT NULL, "
            + display_name + " TEXT NOT NULL, ";

    private static class DatabaseHelper extends SQLiteOpenHelper {
        DatabaseHelper(Context context) {
            super(context, DATABASE_NAME, null, DATABASE_VERSION);
        }

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

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
            db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);
            onCreate(db);
        }
    }

}

```