public void ChangesTvsStateToOnState()
        {
            // Arrange
            IState  on = new OnState();
            Product tv = new Tv(on);

            // Act
            var theStateOfTv = tv.ChangeState();

            // Assert
            Assert.Equal("Current device's state is ON now.", theStateOfTv);
        }
        public void TurnsOnAndThenTurnsOffTheTv()
        {
            // Arrange
            IState  on = new OnState();
            Product tv = new Tv(on);

            // Act
            var theStateOfTv = tv.ChangeState();

            Assert.Equal("Current device's state is ON now.", theStateOfTv);

            // Assert
            var theStateOfTvIsOff = tv.ChangeState(new OffState());

            Assert.Equal("Current device's state is OFF now.", theStateOfTvIsOff);
        }