Basj Basj - 5 months ago 40
Python Question

Move a tkinter canvas with Mouse

I would like to move a whole tkinter

Canvas
with Mouse Click (hold) + Motion of the mouse. I tried with
canvas.move
but it doesn't work unfortunately.

How can I scroll in the whole canvas ? (not move each element of the canvas, but rather scroll the displayed area of the canvas)

import Tkinter as Tk

oldx = 0
oldy = 0

def oldxyset(event):
global oldx, oldy
oldx = event.x
oldy = event.y

def callback(event):
# How to move the whole canvas here?
print oldx - event.x, oldy - event.y

root = Tk.Tk()

c = Tk.Canvas(root, width=400, height=400, bg='white')
o = c.create_oval(150, 10, 100, 60, fill='red')
c.pack()

c.bind("<ButtonPress-1>", oldxyset)
c.bind("<B1-Motion>", callback)

root.mainloop()

Answer

The canvas has built-in support for scrolling with the mouse, via the scan_mark and scan_dragto methods. The former remembers where you clicked the mouse, and the latter scrolls the window an appropriate amount of pixels.

Note: the gain attribute tells scan_moveto how many pixels to move for each pixel the mouse moves. By default it is 10, so if you want 1:1 correlation between the cursor and the canvas you will need to set this value to 1 (as shown in the example).

Here's an example:

import Tkinter as tk
import random

class Example(tk.Frame):
    def __init__(self, root):
        tk.Frame.__init__(self, root)
        self.canvas = tk.Canvas(self, width=400, height=400, background="bisque")
        self.xsb = tk.Scrollbar(self, orient="horizontal", command=self.canvas.xview)
        self.ysb = tk.Scrollbar(self, orient="vertical", command=self.canvas.yview)
        self.canvas.configure(yscrollcommand=self.ysb.set, xscrollcommand=self.xsb.set)
        self.canvas.configure(scrollregion=(0,0,1000,1000))

        self.xsb.grid(row=1, column=0, sticky="ew")
        self.ysb.grid(row=0, column=1, sticky="ns")
        self.canvas.grid(row=0, column=0, sticky="nsew")
        self.grid_rowconfigure(0, weight=1)
        self.grid_columnconfigure(0, weight=1)

        for n in range(50):
            x0 = random.randint(0, 900)
            y0 = random.randint(50, 900)
            x1 = x0 + random.randint(50, 100)
            y1 = y0 + random.randint(50,100)
            color = ("red", "orange", "yellow", "green", "blue")[random.randint(0,4)]
            self.canvas.create_rectangle(x0,y0,x1,y1, outline="black", fill=color)
        self.canvas.create_text(50,10, anchor="nw", 
                                text="Click and drag to move the canvas")

        # This is what enables scrolling with the mouse:
        self.canvas.bind("<ButtonPress-1>", self.scroll_start)
        self.canvas.bind("<B1-Motion>", self.scroll_move)

    def scroll_start(self, event):
        self.canvas.scan_mark(event.x, event.y)

    def scroll_move(self, event):
        self.canvas.scan_dragto(event.x, event.y, gain=1)


if __name__ == "__main__":
    root = tk.Tk()
    Example(root).pack(fill="both", expand=True)
    root.mainloop()