From: ptkwt@...1.aracnet.com (Phil Tomson)
Date: 2003-05-21T04:33:54+09:00
Subject: Re: problem deleting item on a TkCanvas

In article <bacl9402479@enews4.newsguy.com>,
Phil Tomson <ptkwt@shell1.aracnet.com> wrote:
>I've got a TkCanvas where I initially draw a vertical line at the far left 
>of the canvas.  I've bound the left mouse button such that when pressed a 
>new vertical line is drawn at that x location, and the old line is 
>_supposed_ to be erased (deleted).  But the old line is not being deleted.
>
>Here's the code:
>
>require 'tk'
>class SigWave
>  def initialize(root,signame)
>    @frame = TkFrame.new(root).pack('anchor' => 'e','padx'=>1,'pady'=>1)
>    @name  = TkLabel.new(@frame){text "#{signame}"}.pack('side'=>'left')
>    @canvas = TkCanvas.new(@frame)
>    @canvas.configure  'height'=> 20 
>    @canvas.pack('side'=>'right', 'fill'=>'x')
>    @cursor=TkcLine.new(@canvas, 1,0,1,20).fill('blue')
>    @canvas.bind('1',proc{|e| draw_cursor(e.x) })
>    draw_cursor(1)
>  end
>
>  def draw_cursor(x)
>    @canvas.delete(@cursor) #<= this seems to do nothing
>    @cursor=TkcLine.new(@canvas, x,0,x,20).fill('blue')
>  end
>end
>
>root  = TkRoot.new() {title "Waveform" }
>SigWave.new(root,'signame')
>
>Tk.mainloop
>#end of code
>
>
>Am I doing the @canvas.delete correctly?  
>

I figured it out.  You've got to add a tag to the TkcLine, like:

  @cursor=TkcLine.new(@canvas, x,0,x,20,'tag'=>'cursor')

Then the draw_cursor method becomes:

  def draw_cursor(x)
    @canvas.delete('cursor')
    @cursor=TkcLine.new(@canvas, x,0,x,20,'tag'=>'cursor')
    @cursor.fill('blue')
  end


BTW: @cursor seems to be only an empty string.

Phil