[47] add_updater

    def moving_dot(self):
        x_axis = NumberLine(x_min=-5, x_max=5)
        dot = Dot(color=RED, radius=0.15).move_to(x_axis.get_left())
        number = DecimalNumber(-5, color=RED).next_to(dot, UP)

        number.add_updater(lambda m: m.next_to(dot, UP))
        number.add_updater(lambda m: m.set_value(dot.get_center()[0]))

        self.add(x_axis, dot, number)
        self.play(dot.shift, RIGHT * 10, rate_func=there_and_back, run_time=10)
        self.wait()

 

 

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use ValueTracker(1/2)

    def digital_colock(self):
        vt = ValueTracker(0)

        num = DecimalNumber(0, num_decimal_places=3)
        num.add_updater(lambda m: m.set_value(vt.get_value()))

        self.add(num)
        self.play(vt.set_value, 10, rate_func=linear, run_time=10)
        self.wait()

 

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use ValueTracker (2/2)

class ValueTrackerTest(Scene):
    def construct(self):
        img = ImageMobject("assets/img/2.jpg")
        img.scale(2.5)

        num = DecimalNumber(0)
        num.next_to(img, DOWN)
        t = Text("Opacity:", stroke_width=0, size=0.4).next_to(num, LEFT)

        self.add(img, t, num)
        self.wait()

        vt = ValueTracker(10)

        num.add_updater(lambda m:m.set_value(vt.get_value()/10))
        img.add_updater(lambda m: m.set_opacity(vt.get_value()/10))

        self.play(vt.set_value, 5, run_time=5, rate_func=linear)
        self.wait()

 

 

---

use `dt` 

    def orbit_moving(self):
        orbit = Ellipse(color=WHITE).scale(3)
        dot = Dot(radius=0.3, color=GREEN)

        dot.move_to(orbit.point_from_proportion(0))
        self.t_offset = 0
        rate = 1
        def update_func(mob, dt):
            self.t_offset += (dt * rate)
            mob.move_to(orbit.point_from_proportion(self.t_offset % 1))

        dot.add_updater(update_func)

        self.add(orbit, dot)
        self.wait(1)

 

 

 

---

    def orbit_moving_fullcircle(self):
        orbit = Ellipse(color=WHITE).scale(3)
        dot = Dot(radius=0.3, color=GREEN)

        dot.move_to(orbit.point_from_proportion(0))
        self.t_offset = 1/self.camera.frame_rate
        rate = 0.5
        self.t_idx = 1
        def update_func(mob, dt):
            self.t_offset += (dt * rate)
            mob.move_to(orbit.point_from_proportion(self.t_offset % 1))
            print(str(self.t_idx) + ":", str(dt), str(self.t_offset))
            self.t_idx += 1

        dot.add_updater(update_func)

        self.add(orbit, dot)
        self.wait(2)
        dot.remove_updater(update_func)
        self.wait(1)

        dot.add_updater(update_func)
        self.wait(2)

 

 

---

    def orbit_moving_playing(self):
        orbit = Ellipse(color=WHITE).scale(3)
        dot = Dot(radius=0.3, color=GREEN)

        dot.move_to(orbit.point_from_proportion(0))
        self.t_offset = 1/self.camera.frame_rate
        rate = 0.5
        self.t_idx = 1
        def update_func(mob, dt):
            self.t_offset += (dt * rate)
            mob.move_to(orbit.point_from_proportion(self.t_offset % 1))
            print(str(self.t_idx) + ":", str(dt), str(self.t_offset))
            self.t_idx += 1

        dot.add_updater(update_func)

        self.add(orbit, dot)
        self.play(ShowCreation(dot), run_time=2)

        dot.remove_updater(update_func)
        self.wait(1)

        dot.add_updater(update_func)
        self.play(ShowCreation(dot), run_time=2)
        self.wait(2)