B4J Code Snippet Snap and Show Guides when Dragging Controls

#Region Project Attributes
    #MainFormWidth: 600
    #MainFormHeight: 600
#End Region

Sub Process_Globals
    Private fx As JFX
    Private MainForm As Form
    Private xui As XUI
    Private Button1 As Button
    Private Button2 As Button
    Type PositionData (PressedX As Double, PressedY As Double,linesShown As Boolean)
    Type Line (X0 As Double, Y0 As Double, X1 As Double, Y1 As Double)
    Private ImageView1 As ImageView
    Private Pane1 As Pane
End Sub

Sub AppStart (Form1 As Form, Args() As String)
    MainForm = Form1
    MainForm.RootPane.LoadLayout("Layout1")
    MainForm.Show
    ImageView1.PickOnBounds = False
End Sub

Private Sub Button2_MouseDragged (EventData As MouseEvent)
    HandleButtonDragged(Sender,EventData)
End Sub

Private Sub Button2_MousePressed (EventData As MouseEvent)
    HandleButtonPressed(Sender,EventData)
End Sub


Private Sub Button1_MouseDragged (EventData As MouseEvent)
    HandleButtonDragged(Sender,EventData)
End Sub

Private Sub Button1_MousePressed (EventData As MouseEvent)
    HandleButtonPressed(Sender,EventData)
End Sub

Private Sub HandleButtonPressed(btn As Button,EventData As MouseEvent)
    Dim btn As Button = Sender
    Dim pd As PositionData

    Dim event As JavaObject = EventData
    pd.PressedX = event.RunMethod("getSceneX",Null)
    pd.PressedY = event.RunMethod("getSceneY",Null)
    pd.linesShown = False
    btn.Tag = pd
End Sub

Private Sub HandleButtonDragged(btn As Button,EventData As MouseEvent)
    Dim btn As Button = Sender
    Dim event As JavaObject = EventData
    Dim pd As PositionData = btn.Tag
    Dim XDiff As Double= event.RunMethod("getSceneX",Null) - pd.pressedX
    Dim YDiff As Double= event.RunMethod("getSceneY",Null) - pd.pressedY
    btn.Left = XDiff+btn.Left
    btn.Top = YDiff+btn.Top
    If pd.linesShown = True Then
        If XDiff < 0 Then
            XDiff = XDiff - 2
        Else
            XDiff = XDiff + 2
        End If
        If YDiff < 0 Then
            YDiff = YDiff - 2
        Else
            YDiff = YDiff + 2
        End If
        pd.linesShown = False
    End If
    pd.PressedX = event.RunMethod("getSceneX",Null)
    pd.PressedY = event.RunMethod("getSceneY",Null)
    Dim snapped As Boolean = SnapAndShowGuideLines(btn,XDiff,YDiff)
    If snapped Then
        pd.linesShown = True
    End If
End Sub

Private Sub SnapAndShowGuideLines(n As Node,XDiff As Double,YDiff As Double) As Boolean
    Dim snapped As Boolean
    Dim bc As BitmapCreator
    bc.Initialize(Pane1.Width,Pane1.Height)
    Dim left As Double = n.Left
    Dim top As Double = n.Top
    Dim right As Double = n.Left + n.PrefWidth
    Dim bottom As Double = n.top + n.PrefHeight
    Dim centerX As Double = n.Left + n.PrefWidth/2
    Dim centerX As Double = n.Left + n.PrefWidth/2
    Dim centerY As Double = n.top + n.PrefHeight/2
    Dim shapes As List = GetShapes(n)
    Dim linesToDraw As List
    linesToDraw.Initialize

    For Each shape As B4XRect In shapes
        If Abs(shape.Left - left) < 3 And Abs(XDiff) < 2 Then 'left
            If shape.Left <> left Then
                n.Left = shape.Left 'snap
                snapped = True
            End If
            Dim l As Line
            l.Initialize
            l.X0 = shape.Left
            l.Y0 = Min(n.Top,shape.Top)
            l.X1 = shape.Left
            l.Y1 = Max(n.Top+n.PrefHeight,shape.Bottom)
            linesToDraw.Add(l)
        End If
    
        If Abs(shape.Right - right) < 3 And Abs(XDiff) < 2 Then ' right
            If shape.Right <> right Then
                n.Left = shape.Right - n.PrefWidth 'snap
                snapped = True
            End If
            Dim l As Line
            l.Initialize
            l.X0 = shape.Right
            l.Y0 = Min(n.Top,shape.Top)
            l.X1 = shape.Right
            l.Y1 = Max(n.Top+n.PrefHeight,shape.Bottom)
            linesToDraw.Add(l)
        End If
        
        If Abs(shape.Right - left) < 3 And Abs(XDiff) < 2 Then ' left and right
            If shape.Right <> left Then
                n.Left = shape.Right 'snap
                snapped = True
            End If
            Dim l As Line
            l.Initialize
            l.X0 = shape.Right
            l.Y0 = Min(n.Top,shape.Top)
            l.X1 = shape.Right
            l.Y1 = Max(n.Top+n.PrefHeight,shape.Bottom)
            linesToDraw.Add(l)
        End If
        
        If Abs(shape.Left - right) < 3 And Abs(XDiff) < 2 Then ' right and left
            If shape.Left <> right Then
                n.Left = shape.Left - n.PrefWidth 'snap
                snapped = True
            End If
            Dim l As Line
            l.Initialize
            l.X0 = shape.Left
            l.Y0 = Min(n.Top,shape.Top)
            l.X1 = shape.Left
            l.Y1 = Max(n.Top+n.PrefHeight,shape.Bottom)
            linesToDraw.Add(l)
        End If

        If Abs(shape.CenterX - centerX) < 3 And Abs(XDiff) < 2 Then 'centerx
            If shape.CenterX <> centerX Then
                n.Left = shape.CenterX - n.PrefWidth/2 'snap
                snapped = True
            End If
            Dim l As Line
            l.Initialize
            l.X0 = shape.centerX
            l.Y0 = Min(n.Top,shape.Top)
            l.X1 = shape.centerX
            l.Y1 = Max(n.Top+n.PrefHeight,shape.Bottom)
            linesToDraw.Add(l)
        End If
    
        If Abs(shape.Top - top) < 3 And Abs(YDiff) < 2 Then 'top
            If shape.Top <> top Then
                n.Top = shape.Top 'snap
                snapped = True
            End If
            Dim l As Line
            l.Initialize
            l.X0 = Min(n.Left,shape.Left)
            l.Y0 = shape.Top
            l.X1 = Max(n.Left+n.PrefWidth,shape.Right)
            l.Y1 = shape.Top
            linesToDraw.Add(l)
        End If
    
        If Abs(shape.Bottom - bottom) < 3 And Abs(YDiff) < 2 Then 'bottom
            If shape.Bottom <> bottom Then
                n.Top = shape.Bottom - n.PrefHeight 'snap
                snapped = True
            End If
            Dim l As Line
            l.Initialize
            l.X0 = Min(n.Left,shape.Left)
            l.Y0 = shape.Bottom
            l.X1 = Max(n.Left+n.PrefWidth,shape.Right)
            l.Y1 = shape.Bottom
            linesToDraw.Add(l)
        End If
        
        If Abs(shape.Top - bottom) < 3 And Abs(YDiff) < 2 Then 'top and bottom
            If shape.Top <> bottom Then
                n.Top = shape.Top - n.PrefHeight 'snap
                snapped = True
            End If
            Dim l As Line
            l.Initialize
            l.X0 = Min(n.Left,shape.Left)
            l.Y0 = shape.Top
            l.X1 = Max(n.Left+n.PrefWidth,shape.Right)
            l.Y1 = shape.Top
            linesToDraw.Add(l)
        End If
        
        If Abs(shape.Bottom - top) < 3 And Abs(YDiff) < 2 Then 'bottom and top
            If shape.Bottom <> top Then
                n.Top = shape.Bottom 'snap
                snapped = True
            End If
            Dim l As Line
            l.Initialize
            l.X0 = Min(n.Left,shape.Left)
            l.Y0 = shape.Bottom
            l.X1 = Max(n.Left+n.PrefWidth,shape.Right)
            l.Y1 = shape.Bottom
            linesToDraw.Add(l)
        End If
    
        If Abs(shape.CenterY - centerY) < 3 And Abs(YDiff) < 2 Then 'centery
            If shape.CenterY <> centerY Then
                n.Top = shape.centerY - n.PrefHeight/2 'snap
                snapped = True
            End If
            Dim l As Line
            l.Initialize
            l.X0 = Min(n.Left,shape.Left)
            l.Y0 = shape.centerY
            l.X1 = Max(n.Left+n.PrefWidth,shape.Right)
            l.Y1 = shape.centerY
            linesToDraw.Add(l)
        End If

    Next

    
    If linesToDraw.Size>0 Then
        For Each l As Line In linesToDraw
            bc.DrawLine(l.X0,l.Y0,l.X1,l.Y1,xui.Color_Red,1)
        Next
    End If
    ImageView1.SetImage(bc.Bitmap)
    Return snapped
End Sub

Private Sub GetShapes(n As Node) As List
    Dim shapes As List
    shapes.Initialize
    Dim paneRect As B4XRect
    paneRect.Initialize(0,0,Pane1.Width,Pane1.Height)
    shapes.Add(paneRect)
    For Each nodeForComparing As Node In Array(Button1,Button2)
        If n <> nodeForComparing Then
            Dim r As B4XRect
            r.Initialize(nodeForComparing.Left,nodeForComparing.Top, _
            nodeForComparing.Left+nodeForComparing.PrefWidth, _
            nodeForComparing.Top+nodeForComparing.PrefHeight)
            shapes.Add(r)
        End If
    Next
    Return shapes
End Sub