AnimatedBoundary. Perhaps a better name is in order, but this is meant to add a little visual interest to things like a framing rectangle.

big_ol_pile_of_manim_imports.py

@@ -35,6 +35,7 @@ from manimlib.camera.moving_camera import *
 from manimlib.camera.three_d_camera import *
 
 from manimlib.mobject.coordinate_systems import *
+from manimlib.mobject.changing import *
 from manimlib.mobject.frame import *
 from manimlib.mobject.functions import *
 from manimlib.mobject.geometry import *

manimlib/mobject/changing.py

@@ -0,0 +1,60 @@
+import warnings
+
+from manimlib.constants import *
+from manimlib.mobject.types.vectorized_mobject import VGroup
+from manimlib.mobject.types.vectorized_mobject import VMobject
+from manimlib.utils.rate_functions import smooth
+
+POINTLESS_VMOBJECT_WARNING = """
+
+Calling AnimatedBoundary on a VMobject with no points.
+"""
+
+
+class AnimatedBoundary(VGroup):
+    CONFIG = {
+        "colors": [BLUE_D, BLUE_B, BLUE_E, GREY_BROWN],
+        "max_stroke_width": 3,
+        "cycle_rate": 0.5,
+    }
+
+    def __init__(self, vmobject, **kwargs):
+        super().__init__(**kwargs)
+        if len(vmobject.points) == 0:
+            warnings.warn(POINTLESS_VMOBJECT_WARNING)
+        self.vmobject = vmobject
+        self.boundary_copies = [
+            VMobject(stroke_width=0, fill_opacity=0)
+            for x in range(2)
+        ]
+        self.add(*self.boundary_copies)
+        self.total_time = 0
+        self.add_updater(lambda m, dt: self.update(dt))
+
+    def update(self, dt):
+        # Not actual time, but something which passes at
+        # an altered rate to make the implementation below
+        # cleaner
+        time = self.total_time * self.cycle_rate
+        growing, fading = self.boundary_copies
+        colors = self.colors
+        msw = self.max_stroke_width
+
+        index = int(time % len(colors))
+        alpha = smooth(time % 1)
+
+        if int(time) % 2 == 0:
+            bounds = (0, alpha)
+        else:
+            bounds = (1 - alpha, 1)
+        growing.pointwise_become_partial(self.vmobject, *bounds)
+        growing.set_stroke(colors[index], width=msw)
+
+        if time > 1:
+            fading.pointwise_become_partial(self.vmobject, 0, 1)
+            fading.set_stroke(
+                color=colors[index - 1],
+                width=(1 - alpha) * msw
+            )
+
+        self.total_time += dt