Updated 2015-12-10 05:40:22 by APN

"Drag" is an action idiom common to several GUI canons, roughly co-ordinate with, say "pan". It most often appears in discussions of drag and drop facilities.

It only takes a few lines of Tk to exhibit dragging. Here is an example Michael Kraus offered in comp.lang.tcl:

This demo creates a small window consisting of a single button, that can be dragged around on the screen. Clicking the button quits the program.
  pack [button .b -text "The Button" -command exit]
  wm overrideredirect . 1

  bind .b <1> {
    set iX0 [expr %X-[winfo rootx .b]]
    set iY0 [expr %Y-[winfo rooty .b]]
    set bMoved 0

  bind .b <B1-Motion> {
    wm geometry . +[expr %X-$iX0]+[expr %Y-$iY0]
    set bMoved 1

  bind .b <ButtonRelease-1> {
    if { $bMoved } break

Lars H: The following demonstrates one way of dragging a canvas -- change which part of the coordinate system is visible in the window, by pressing and dragging button 1.
 pack [canvas .c] -expand 1 -fill both
 .c create text 10 10 -anchor nw -text "Hello world!"
 .c create line {-100 0 100 0} -arrow last
 .c create line {0 -100 0 100} -arrow last
 bind .c <ButtonPress-1> {%W scan mark %x %y}
 bind .c <B1-Motion> {%W scan dragto %x %y 1}

This is particularly useful with quick hacks, where one displays some piece of graphic in a canvas and doesn't know for sure whether all of it fits in the visible part of the canvas.

dzach: And if one wants to limit dragging to the occupied area of a canvas, then:
 .c configure -scrollregion [.c bbox all]

does it, provided it is called after the items have been drawn.

[Next up: exhibit dragging an item around a canvas. Check with CL if you're in a hurry.]