Algorithm for dragging objects on a fixed grid

I am working on a program for the mapping and playing of the popular tabletop game D&D :D Right now I am working on getting the basic functionality like dragging UI elements around, snapping to the grid and checking for collisions.

Right now every object when released from the mouse immediately snaps to the nearest grid point. This causes an issue when something like a player object snaps to a grid point that has a wall -or other- adjacent. So essentially when the player is dropped they wind up with some of the wall covering them. This is fine and working as intended, however the problem is that now my collision detection is tripped whenever you try to move this player because its sitting underneath a wall and because of this you cant drag the player anymore.

Here is the relevant code:

void UIObj_MouseMove(object sender, MouseEventArgs e)  
{  
            blocked = false;  
            if (dragging)  
            {  
                foreach (UIElement o in ((Floor)Parent).Children)  
                {  
                    if (o.GetType() != GetType() && o.GetType().BaseType == typeof(UIObj) &&  
                        Math.Sqrt(Math.Pow(((UIObj)o).cX - cX, 2) + Math.Pow(((UIObj)o).cY - cY, 2)) <  
                        Math.Max(r.Height + ((UIObj)o).r.Height, r.Width + ((UIObj)o).r.Width))  
                    {  
                        double Y = e.GetPosition((Floor)Parent).Y;  
                        double X = e.GetPosition((Floor)Parent).X;  
                        Geometry newRect = new RectangleGeometry(new Rect(Margin.Left + (X - prevX),  
                        Margin.Top + (Y - prevY), Margin.Right + (X - prevX), Margin.Bottom + (Y - prevY)));  
                        GeometryHitTestParameters ghtp = new GeometryHitTestParam开发者_Python百科eters(newRect);  
                        VisualTreeHelper.HitTest(o, null, new HitTestResultCallback(MyHitTestResultCallback), ghtp);  
                    }  
                }  
                if (!blocked)  
                {  
                    Margin = new Thickness(Margin.Left + (e.GetPosition((Floor)Parent).X - prevX),  
                        Margin.Top + (e.GetPosition((Floor)Parent).Y - prevY),  
                        Margin.Right + (e.GetPosition((Floor)Parent).X - prevX),  
                        Margin.Bottom + (e.GetPosition((Floor)Parent).Y - prevY));  
                    InvalidateVisual();  
                }
                prevX = e.GetPosition((Floor)Parent).X;  
                prevY = e.GetPosition((Floor)Parent).Y;  
                cX = Margin.Left + r.Width / 2;  
                cY = Margin.Top + r.Height / 2;  
            }  
        }  

internal virtual void SnapToGrid()  
        {  
            double xPos = Margin.Left;  
            double yPos = Margin.Top;  
            double xMarg =  xPos % ((Floor)Parent).cellDim;  
            double yMarg =  yPos % ((Floor)Parent).cellDim;  
            if (xMarg < ((Floor)Parent).cellDim / 2)  
            {  
                if (yMarg < ((Floor)Parent).cellDim / 2)  
                {  
                    Margin = new Thickness(xPos - xMarg, yPos - yMarg, xPos - xMarg + r.Width, yPos - yMarg + r.Height);
                    }  
                else  
                {  
                    Margin = new Thickness(xPos - xMarg, yPos - yMarg + ((Floor)Parent).cellDim, xPos - xMarg + r.Width,

                        yPos - yMarg + ((Floor)Parent).cellDim + r.Height);  
                }  
            }  
            else  
            {
                if (yMarg < ((Floor)Parent).cellDim / 2)
                {
                    Margin = new Thickness(xPos - xMarg + ((Floor)Parent).cellDim, yPos - yMarg,
                        xPos - xMarg + ((Floor)Parent).cellDim + r.Width, yPos - yMarg + r.Height);
                }
                else
                {
                    Margin = new Thickness(xPos - xMarg + ((Floor)Parent).cellDim, yPos - yMarg + ((Floor)Parent).cellDim,
                        xPos - xMarg + ((Floor)Parent).cellDim + r.Width, yPos - yMarg + ((Floor)Parent).cellDim + r.Height);
                }
            }
        }

Essentially I am looking for a simple way to modify the existing code to allow the movement of a UI element that has another one sitting on top of it. Thanks!

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I'd just like to take a moment to propose an alternate solution...
Use an existing well-tested implementation: MapTool!

Of course, this solution is useless if you particularly need or want to implement it yourself (definitely a fun personal project), but I figure it should be mentioned. 8 )

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Why not include the cases in which you would like to still be able to move the objects in your collision detection code. I am thinking of simple if statements in the collision detection code. I can't tell you mroe without seeing the collision detection code.

Also, what sets/changes "blocked"?

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If a wall tile overlaps an adjacent cell and this causes a collision to trigger, then you should use smaller collision shapes, which don't overlap.

Ideally the collision shapes should be exactly the size of the occupied grid cell(s), independent of the visual appearance, right?