load a layout xml file in a View class android

I have this class:

public class MainActivity extends Activity{
    public void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState); 
        setContentView(R.layout.main);//LOAD MY LAYOUT
    }
}

And more code (obviously). And I have the next UI:

And I want to draw a line to simulate the union between images.

I tried with:

Canvas canvas = new Canvas();
Paint paint = new Paint();
paint.setColor(Color.BLACK);
canvas.drawLine(0, 0, 300, 700, paint);

(The above are just a test to know if I can draw a line.) But I can't draw anything because when I create a canvas like that in my activity it's not attached to anything, I'm just drawing on a surface that is never part of the screen.

Therefore, I have the following classes.

public class MainActivity extends Activity  {
/** Called when the activity is first created. */
Paint mPaint;
float Mx1,My1;
float x,y;
@Override

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    MyView view1 =new MyView(this);
    setContentView(view1);

    mPaint = new Paint();
    mPaint.setAntiAlias(true);
    mPaint.setDither(true);
    mPaint.setColor(0xFFFF0000);
    mPaint.setStyle(Paint.Style.STROKE);
    mPaint.setStrokeJoin(Paint.Join.ROUND);
    mPaint.setStrokeWidth(10);
}

public class MyView extends View {
    private Bitmap  mBitmap;
    private Canvas  mCanvas;
    private Path    mPath;
    private Paint   mBitmapPaint;

    public MyView(Context c) {
        super(c);
        mPath = new Path();
        mBitmapPaint = new Paint(Paint.DITHER_FLAG);
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
        mCanvas = new Canvas(mBitmap);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawColor(0xFFAAAAAA);
        canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);
        canvas.drawPath(mPath, mPaint);
    }

    private float mX, mY;
    private static final float TOUCH_TOLERANCE = 4;

    private void touch_start(float x, float y) {
        mPath.reset();
        mPath.moveTo(x, y);
        mX = x;
        mY = y;
    }
    private void touch_move(float x, float y) {
        float dx = Math.abs(x - mX);
        float dy = Math.abs(y - mY);
        if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
            mX = x;
            mY = y;
        }
    }
    private void touch_up() {
        mPath.lineTo(mX, mY);
        // commit the path to our offscreen
        mCanvas.drawPath(mPath, mPaint);
        // kill this so we don't double draw
        mPath.reset();
    }

    @Override
    public boolean onTouchEvent(MotionEvent event){
        float x = event.getX();
        float y = event.getY();
        switch (event.getAction()){
            case MotionEvent.ACTION_DOWN:
                touch_start(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_MOVE:
                touch_move(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_UP:
                touch_up();
               invalidate();
                break;
        }
        return true;
    }
}
}

This is a example I found. In a view class the method:

setContentView(R.layout.main);

doesn't work.

My question is: How I can load my UI (which I have set as an image) in the extended View class? because in the example I just load a empty screen. I don't know how to load my layout file.xml

Thanks for your help!!!