I ended up giving up on doing the scrollbar as a separate class, which
is probably for the best. This is the pertinent code for what I
wanted, and it does what I need it to do. I found the canvas-moving-w-
mouse.py program helpful in getting started with this; some of the
code is directly from that example. I'm posting my code on the off
chance someone will find something of use in it.
#Constants for the scrollbar icon range.
self.UPPER_LIMIT = 160
self.LOWER_LIMIT = 334
#Prepare pictures
self.upArrow = tk.PhotoImage ("upArrow", file =
"upArrow.gif")
self.scrollIcon = tk.PhotoImage ("scrollIcon", file =
"scrollIcon.gif")
self.downArrow = tk.PhotoImage ("downArrow", file =
"downArrow.gif")
#Main Canvas
self.shell = tk.Canvas (parent, width = 388, height = 408,
borderwidth = - 2)
self.shell.create_image (0, 0, image = self.shell_image,
anchor = tk.NW)
self.shell.create_image (361, 9, image = self.exit_image,
anchor = tk.NW, tags = "Exit")
#Inner frame that contains a canvas.
#This is what is scrolled by the scrollbar.
self.masteryFrame = tk.Frame (parent, borderwidth = -2)
self.masteryCanvas = tk.Canvas (self.masteryFrame, width =
326, height = 218,
scrollregion =
(0,0,326,439), borderwidth = -2)
self.masteryCanvas.pack()
self.masteryFrame.pack()
self.masteryCanvas.create_image (0, 0, image =
"masteryFrame", anchor = tk.NW)
self.shell.create_window (22, 135, window =
self.masteryFrame,
width = 326, height = 218,
anchor = tk.NW)
#Scrollbar
self.shell.create_image (350, 136, image = self.upArrow,
anchor = tk.NW,
tags = "upArrow")
self.shell.create_image (357, 160, image =
self.scrollIcon, tags = "scroll")
self.shell.create_image (350, 343, image = self.downArrow,
anchor = tk.NW,
tags = "downArrow")
self.shell.tag_bind ("scroll", "<Button-1>",
self.mouseDown)
self.shell.tag_bind ("scroll", "<B1-Motion>",
self.mouseMove)
self.shell.tag_bind ("upArrow", "<ButtonRelease-1>",
self.stepUp)
self.shell.tag_bind ("downArrow", "<ButtonRelease-1>",
self.stepDown)
self.shell.tag_bind ("Exit", "<Button-1>", self.close)
self.shell.pack (side = tk.LEFT)
def mouseDown (self, event):
#Scrollbar Function
#Remember where the mouse went down"
self.lastx = event.x
self.lasty = event.y
def mouseMove (self, event):
#Scrollbar Function
#Whatever the mouse is over is automatically tagged as
current by tk.
#Only moves vertically.
#Keep the cursor in bounds:
if event.y >= self.UPPER_LIMIT and event.y <=
self.LOWER_LIMIT:
self.shell.move(tk.CURRENT, 0, event.y - self.lasty)
self.lasty = event.y
elif event.y < self.UPPER_LIMIT:
self.shell.coords(tk.CURRENT, 357, self.UPPER_LIMIT )
self.lasty = event.y
elif event.y > 334:
self.shell.coords(tk.CURRENT, 357, self.LOWER_LIMIT)
self.lasty = event.y
self.masteryCanvas.yview_moveto (self.scrollbarPosition())
def stepUp (self, event):
#Scrollbar Function
#Move up one row or to the top, whichever is less.
#43.5 causes the canvas to move one row.
next = self.shell.coords ("scroll")[1] - 43.5
if next >= self.UPPER_LIMIT:
self.shell.coords ("scroll", 357, next)
else:
self.shell.coords ("scroll", 357, self.UPPER_LIMIT)
self.masteryCanvas.yview_moveto (self.scrollbarPosition())
def stepDown (self, event):
#Scrollbar Function
#Move down one row or to the bottom, whichever is less.
#43.5 causes the canvas to move one row.
next = self.shell.coords ("scroll")[1] + 43.5
if next <= self.LOWER_LIMIT:
self.shell.coords( "scroll", 357, next)
else:
self.shell.coords( "scroll", 357, self.LOWER_LIMIT)
self.masteryCanvas.yview_moveto (self.scrollbarPosition())
def scrollbarPosition (self):
#Scrollbar Function that computes movement
#Return a value between 0 and .5 for moving the canvas.
yCoord = self.shell.coords ("scroll")[1]
length = self.LOWER_LIMIT - self.UPPER_LIMIT
current = yCoord - self.UPPER_LIMIT
current /= 2
#print current / length
return (current / length)
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