버튼을 클릭 한 후 캔버스에 요소를 하나 더 추가하는 방법은 무엇입니까?

빨간색 선이 그려지는 View를 확장하는 DrawView 클래스가 있습니다. 버튼을 클릭하면 화면에 한 줄 더 표시됩니다 (canvas.drawLine (200, 500, 500, 500, paint)). 올바르게하는 방법?

DrawView 클래스 :

public class DrawView extends View {
    Paint paint = new Paint();

    public DrawView(Context context) {
        super(context);
        paint.setColor(Color.RED);
    }

    @Override
    public void onDraw(Canvas canvas) {
          canvas.drawLine(200, 500, 200, 800, paint);  
    }
}

MainActivity 클래스 :

public class MainActivity extends AppCompatActivity {

    DrawView drawView;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);
        drawView = new DrawView(this);
        drawView.setBackgroundColor(Color.WHITE);
        LinearLayout container = (LinearLayout) findViewById(R.id.conteiner);
        container.addView(drawView);

         Button important = (Button) findViewById(R.id.important);

         important.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {
                Toast.makeText(getApplicationContext(),
                        "Button 1 clicked",
                        Toast.LENGTH_LONG).show();
            }
        });
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.attracti.redline.MainActivity">

    <LinearLayout
        android:id="@+id/conteiner"
        android:orientation="horizontal"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_above="@+id/important">
    </LinearLayout>

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Draw"
        android:id="@+id/important"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true" />
</RelativeLayout>