Succession() act incorrectly when display Mobjs successively

[cckk4467]

Description of bug / unexpected behavior

when more than once Write() or FadeIn() sth within Succession(), it will show the mobjs to be displayed before the animation begin
look at the blue triangle at the very beginning of the video below. It's not supposed to be presented before the white line

Expected behavior

Just to display the mobjs one by one normally

How to reproduce the issue

Code for reproducing the problem

class Test(Scene):
    def construct(self):
        tri=Line(UP)
        a=Succession(
            FadeIn(Line()),
            FadeIn(tri),
        )
        self.play(a)

Additional media files

Images/GIFs

Test.mp4

[AliNear]

Yes I have a similar issue with Succession, it displays all the mobjects at fist then runs the animations successively.

[cobordism]

There is a more general problem with the way AnimationGroup works (Succession inherits from there).

To fix this properly we need to refactor all of that.

The cause of this issue is that an AnimationGroup is not a group of animations, but an animation on a Group mobject. All mobjects are added to the group, and the initialisation call at Animation.begin() happens before the first animation starts.

[ad-chaos]

Fixed via #2396 by the introduction of the introducer keyword argument.