Question

I would like to know when will the method

protected void onDraw(final Canvas canvas) {}

will be called. I am asking about the control flow.constructor of this class is called from other class.When control comes to the constructor will it simply call all methods in this class??

Also i want to do some drawing when the draw image is touched and moved. for that i used onTouchEvent(MotionEvent event).But i dont know how to invoke onDraw after i do some coding in onTouch.That is i do change some coordinate values how will call onDraw to redraw image?

Can anyone help?

public class DrawView extends View {
Paint paint = new Paint();
public DrawView(Context context) {
    // TODO Auto-generated constructor stub
    super(context);
}

@Override
protected void onDraw(final Canvas canvas) {
    // TODO Auto-generated method stub

        paint.setColor(Color.BLACK);
        paint.setStrokeWidth(3); 
        canvas.drawRect(30, 350, 50, 400, paint);
        super.onDraw(canvas);
// some other drawings
}


@Override
public boolean onTouchEvent(MotionEvent event) {
    // TODO Auto-generated method stub

    switch (event.getAction()) {
    case MotionEvent.ACTION_DOWN://some code
        break;

case MotionEvent.ACTION_MOVE://some code

            break;
case MotionEvent.ACTION_UP://some code  
    break;
        default:            break;
        }
        return super.onTouchEvent(event);
    }
}

Answer 1

/**
* @author rajeshcp
*/

public class SimpleDrag extends View {




private Paint mPaint;
private Rect mRect;

/**
 * @param context  
 * @return of type SimpleDrag
 * Constructor function
 * @since Feb 19, 2013 
 * @author rajeshcp
 */
public SimpleDrag(Context context) {
    super(context);
    init();
}

/**
 * @param context
 * @param attrs  
 * @return of type SimpleDrag
 * Constructor function
 * @since Feb 19, 2013 
 * @author rajeshcp
 */
public SimpleDrag(Context context, AttributeSet attrs) {
    super(context, attrs);
    init();
}

/**
 * @param context
 * @param attrs
 * @param defStyle  
 * @return of type SimpleDrag
 * Constructor function
 * @since Feb 19, 2013 
 * @author rajeshcp
 */
public SimpleDrag(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
    init();
}

/* (non-Javadoc)
 * @see android.view.View#onDraw(android.graphics.Canvas)
 * @since Feb 19, 2013
 * @author rajeshcp 
 */
@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    canvas.drawColor(Color.BLUE, PorterDuff.Mode.CLEAR);
    if( mRect != null )
    {
        mPaint.setColor(Color.RED);
        canvas.drawRect(mRect, mPaint);
    }
}


private void init()
{
    mRect  = new Rect(0, 0, 50, 50);
    mPaint = new Paint(Paint.FILTER_BITMAP_FLAG | Paint.DITHER_FLAG | Paint.ANTI_ALIAS_FLAG);
}

private Point mTouchPoint;

/* (non-Javadoc)
 * @see android.view.View#onTouchEvent(android.view.MotionEvent)
 * @since Feb 19, 2013
 * @author rajeshcp 
 */
@Override
public boolean onTouchEvent(MotionEvent event) {

    final int action = event.getAction();

    if( action == MotionEvent.ACTION_CANCEL || action == MotionEvent.ACTION_DOWN)
    {
        mTouchPoint = new Point((int)event.getX(), (int)event.getY());
        if( !mRect.contains(mTouchPoint.x, mTouchPoint.y) )
        {
            return false;
        }
    }

    if( action == MotionEvent.ACTION_MOVE )
    {
        final Point curretPoint = new Point((int)event.getX(), (int)event.getY());
        int xMoved = curretPoint.x - mTouchPoint.x;
        int yMoved = curretPoint.y - mTouchPoint.y;
        mRect.set(mRect.left + xMoved, mRect.top + yMoved, mRect.right + xMoved, mRect.bottom + yMoved);
        mTouchPoint = curretPoint;
        invalidate();
    }
    return true;
}



}

Call Invalidate when ever you want onDraw method to get called.