from gi.repository import GObject

timer = None
def cancel():
    global timer
    if timer:
        GObject.source_remove(timer)
        timer = None

LINEAR=1
DECELERATE=2

def animate_value(callback, current, target, policy=DECELERATE, speed=32):
    global timer
    timer = None
    if current != target:
        if policy == DECELERATE:
            delta = abs(target - current) / 2
        else:
            delta = speed
        if current > target:
            current -= delta
        else:
            current += delta
        if abs(current - target) < 2:
            current = target
        callback(current)
        timer = GObject.timeout_add(1000 / 25, lambda: animate_value(callback, current, target))

def scroll_to(scrollview, widget):
    result = widget.translate_coordinates(scrollview.get_child().get_child(), 0, 0)
    if result:
        adj = scrollview.get_vadjustment()
        animate_value(adj.set_value, adj.get_value(), result[1])