Question

With WinForms, is there a way to be alerted to a control changing location with respect to the screen?

Say you have a Form with a button on it, and you would like to know when the button is moved from its current pixel location on the screen. If the button is moved to a different location on its parent Form you could obviously use the LocationChanged event, but if the Form is moved by the user, how do you know the button has visually moved?

In this simplified case the quick answer is to monitor the Form’s LocationChanged and SizeChanged events, but there can be an arbitrary number of levels of nesting so monitoring those events for each parent up the chain to the primary form is not feasible. Using a timer to check if the location changed also seems like cheating (in a bad way).

Short version:
Given only an arbitrary Control object, is there a way to know when that Control’s location changes on the screen, without knowledge of the control’s parent hierarchy?

An illustration, by request:

Note that this “pinning” concept is an existing capability but it currently requires knowledge of the parent form and how the child control behaves; this is not the problem I am trying to solve. I would like to encapsulate this control tracking logic in an abstract Form that “pin-able” Forms can inherit from. Is there some message pump magic I can tap into to know when a control moves on the screen without having to deal with all the complicated parent tracking?

Answer 1

I’m not sure why you would say tracking the parent chain “is not feasible”. Not only is it feasible, it’s the right answer and the easy answer.

Just a quick hack at a solution:

private Control         _anchorControl;
private List<Control>   _parentChain = new List<Control>();
private void BuildChain()
{
    foreach(var item in _parentChain)
    {
        item.LocationChanged -= ControlLocationChanged;
        item.ParentChanged -= ControlParentChanged;
    }

    var current = _anchorControl;

    while( current != null )
    {
        _parentChain.Add(current);
        current = current.Parent;
    }

    foreach(var item in _parentChain)
    {
        item.LocationChanged += ControlLocationChanged;
        item.ParentChanged += ControlParentChanged;
    }
}

void ControlParentChanged(object sender, EventArgs e)
{
    BuildChain();
    ControlLocationChanged(sender, e);
}

void ControlLocationChanged(object sender, EventArgs e)
{
    // Update Location of Form
    if( _anchorControl.Parent != null )
    {
        var screenLoc = _anchorControl.Parent.PointToScreen(_anchorControl.Location);
        UpdateFormLocation(screenLoc);
    }
}