Updaters and trails

TracedPath trails, tracker-driven positions, (m, dt) updaters, clear_updaters and the UpdateFromFunc family.

This page lists 6 examples. The code is imported from corpus/examples/updater/; the screenshots are end states rendered on real machines.

TracedPath follows a mobject

A trail line grows behind the moving dot and coincides with the path travelled.

Rendered

Code

"""TracedPath 跟随物体

@expect 点移动时身后拖出一条轨迹线，轨迹与走过的路径重合。
@note ⚠️ 与 CE 写法不同：CE 收函数 dot.get_center，manim-web 收物体本身——技能文档已注明
"""

from manim import *


class TracedPathScene(Scene):
    def construct(self):
        dot = Dot(point=[-3, 0, 0], color=RED)
        path = TracedPath(dot, stroke_color=YELLOW)
        self.add(path, dot)
        self.play(dot.animate.shift(RIGHT * 6))
        self.wait(0.3)

Position driven by a tracker

As the tracker's value is animated, the dot moves to the matching position (printed coordinates agree with the picture). Self-evidence: tick 2 on the NumberLine at the bottom coincides with where the red dot settles.

Rendered

Code

"""用 tracker 驱动位置

@expect tracker 的值被动画推进时，点跟着移到对应位置（print 出的坐标与画面一致）。
自证：底部 NumberLine 的刻度 2 与红点落定位置重合。"""

from manim import *


class ValueDrivenScene(Scene):
    def construct(self):
        tracker = ValueTracker(0)
        dot = Dot(color=RED)
        dot.add_updater(lambda m: m.move_to([tracker.get_value(), 0, 0]))
        # length=8：8 个单位格对应 8 个场景单位，数轴 1:1，刻度 2 就在场景 x=2
        line = NumberLine(x_range=[-4, 4, 1], include_numbers=True, length=8)
        label = Text("tracker = 2", font_size=24)
        self.add(tracker, dot, line)
        self.play(tracker.animate.set_value(2), run_time=1)
        self.wait(0.3)
        # 等 updater flush 后点已落在 (2,0)，label 才跟着点走
        label.next_to(dot, UP)
        self.play(FadeIn(label))
        self.wait(0.3)

Updater taking (m, dt)

A (m, dt) updater advances with real time during wait, not only during animations.

Rendered

Code

"""更新器收 (m, dt) 两个参数

@expect (m, dt) 更新器在 wait 期间按真实时间推进，不是只在动画里动。
@note manim-web 的 updater 签名是 (mobject, dt)
"""

from manim import *


class WithDtScene(Scene):
    def construct(self):
        dot = Dot(point=LEFT * 3, color=RED)
        def step(m, dt):
            m.shift(RIGHT * dt * 2)
        dot.add_updater(step)
        self.add(dot)
        self.play(dot.animate.shift(RIGHT * 0.01), run_time=1)
        self.wait(0.3)

clear_updaters stops the following

After clear_updaters the mobject stays put: further tracker changes no longer move it.

Rendered

Code

"""clear_updaters 停掉跟随

@expect clear_updaters 之后物体停住：tracker 再变它也不跟。"""

from manim import *


class ClearScene(Scene):
    def construct(self):
        tracker = ValueTracker(0)
        dot = Dot(color=RED)
        dot.add_updater(lambda m: m.move_to([tracker.get_value(), 0, 0]))
        self.add(tracker, dot)
        self.play(tracker.animate.set_value(2), run_time=1)
        dot.clear_updaters()
        self.play(tracker.animate.set_value(0), run_time=1)
        self.wait(0.3)

Number follows the tracker

The number follows the tracker live; it does not jump to the final value at the end.

Rendered

Code

"""数字跟着 tracker 变

@expect 数字实时跟着 tracker 变（不是播完才跳到终值）。"""

from manim import *


class DecimalScene(Scene):
    def construct(self):
        tracker = ValueTracker(0)
        num = DecimalNumber(0, num_decimal_places=1)
        num.to_corner(UR)
        num.add_updater(lambda m: m.set_value(tracker.get_value()))
        self.add(tracker, num)
        self.play(tracker.animate.set_value(5), run_time=1)
        self.wait(0.3)

Animations driving update functions (UpdateFromFunc / UpdateFromAlphaFunc)

The update function is driven during the animation and the m it receives answers to CE method names (a fixed wrapping incident).

Rendered

Code

"""用动画驱动更新函数（UpdateFromFunc / UpdateFromAlphaFunc）

@expect 更新函数在动画期间被驱动，回调里拿到的 m 能用 CE 方法名操作（修过的包装事故）。
@note 回调里的第一个参数是真 Mobject（适配层包过），CE 方法名照写即可：
      `lambda m: m.set_opacity(...)`。
      踩过的坑：这两个动画在 manim-web 里按回调(mobject, alpha) 调，而那个 mobject 是它自己的
      裸 JS 对象（只有 camelCase 方法）——不包一层就是 AttributeError，且抛在渲染循环里，
      播放的 promise 永不 resolve，页面一直转（真机事故）。适配层已在 _FUNC_ANIMATIONS 里包好。
      回调参数个数按自己的签名算：UpdateFromFunc 习惯写单参，写双参也能收下 alpha。
"""

from manim import *


class UpdateFromFuncScene(Scene):
    def construct(self):
        square = Square(side_length=2, color=BLUE)
        self.add(square)

        # 单参回调：整段动画期间反复调用（UpdateFromFunc 的 alpha 一般用不上）
        self.play(
            UpdateFromFunc(square, lambda m: m.set_stroke(color=YELLOW, width=8)),
            run_time=0.6,
        )

        # 双参回调：alpha 从 0 走到 1，适合「随进度改某个属性」
        self.play(
            UpdateFromAlphaFunc(square, lambda m, alpha: m.set_fill(BLUE, alpha)),
            run_time=0.8,
        )
        # 适配层在动画终态会丢掉回调刷出来的填充；play 外的 set_fill 不触发重绘，
        # 故再用一条 .animate 动画显式补上不透明蓝填充（终帧必须实心蓝）
        self.play(square.animate.set_fill(BLUE, 1.0), run_time=0.4)
        self.wait(0.3)