remove(key: string) : Promise<?string> {
        return new Promise(resolve => {
            this.databaseInitialized.then(() => {
                this.get(key).then((val: ?string) => {
                    this.db.transaction(tx => {
                        tx.executeSql('DELETE FROM preference WHERE key = ?', [key]);
                    });
                    resolve(val);
                });
            }, () => {
                debugger;
            });
        });
    }

    set(key: string, value: string) : Promise<string> {
        return new Promise(resolve => {
            this.databaseInitialized.then(() => {
                this.db.transaction(tx => {
                    tx.executeSql('INSERT OR IGNORE INTO preference (key,value) VALUES(?,?)', [key,value]);
                    tx.executeSql('UPDATE preference SET value = ? WHERE key = ?', [key,value]);
                });
                resolve(value);
            }, () => {
                debugger;
            });
        });
    }

    get(key: string): Promise<?string> {
        return new Promise(resolve => {
            this.databaseInitialized.then(() => {
                this.db.transaction(tx => {
                    tx.executeSql('SELECT value FROM preference WHERE key = ?', [key], (tx, results) => {
                        const data: ?Object = results.rows.item(0);
                        resolve(data ? data.value : null);
                    }, (er) => {
                        debugger;
                        resolve(null)
                    });
                });
            }, () => {
                debugger;
            });
        });
    }