Select the last object of the group as the alignment target when using `Mobject.arrange()`

[zhanbao2000]

Description of proposed feature

My motivation

Hello, I want to be able to select the last object in a group as the target for alignment when using Mobject.arrange(), instead of the default first object.

Like this:

ReversedArrange.mp4

Why

Mobject.arrange() always uses the first object as the alignment target:

manim/manim/mobject/mobject.py

Lines 2368 to 2372 in 2ab99b5

	for m1, m2 in zip(self.submobjects, self.submobjects[1:]):
	m2.next_to(m1, direction, buff, **kwargs)
	if center:
	self.center()
	return self

If I want to implement reverse-arrange, I must use the Mobject.next_to() method one by one (in reverse order) for the objects within the group, as implemented in above code.

How can the new feature be used?

By adding a new param last in Mobject.arrange():

group.animate.arrange(center=False, last=True, direction=LEFT)

Additional comments

Here is my change of Mobject.arrange() to implement this feature:

def arrange(
    self,
    direction: Vector3D = RIGHT,
    buff: float = DEFAULT_MOBJECT_TO_MOBJECT_BUFFER,
    center: bool = True,
    last: bool = False,
    **kwargs,
) -> Self:
    if last:
        submobjects = list(reversed(self.submobjects))
    else:
        submobjects = self.submobjects
    
    for m1, m2 in zip(submobjects, submobjects[1:]):
        m2.next_to(m1, direction, buff, **kwargs)
    if center:
        self.center()
    return self

Here is my code to test this feature:

from manim import *

class ReversedArrange(Scene):

    def construct(self):
        square_r = Square(fill_color=RED, fill_opacity=1).move_to(ORIGIN + LEFT * 5)
        square_g = Square(fill_color=GREEN, fill_opacity=1).move_to(ORIGIN)
        square_b = Square(fill_color=BLUE, fill_opacity=1).move_to(ORIGIN + RIGHT * 5)

        group = Group(square_r, square_g, square_b)

        self.play(FadeIn(group))
        self.wait(1)

        self.play(group.animate.arrange(center=False, last=True, direction=LEFT))

If modify Mobject.arrange() as I did, you have to specify the direction param as LEFT when call this method, otherwise the other object in the group will move to the right of the target object (which is assumed to be the rightmost one). Like this:

self.play(group.animate.arrange(center=False, last=True))  # without direction=LEFT

ReversedArrange.mp4