Added docstrings for :class:~.ValueTracker (#1246)

* added docstings to valuetracker

* added more docstings

* Move pointer value addition to before animation

Could make it transition smoother.

* Add test for ValueTracker.__isub__()

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

Co-authored-by: Ricky Chon <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

manim/mobject/value_tracker.py

@@ -36,10 +36,15 @@ def construct(self):
                             )
                 )
                 self.add(number_line, pointer,label)
+                pointer_value += 1.5
+                self.wait(1)
+                pointer_value -= 4
+                self.wait(0.5)
                 self.play(pointer_value.animate.set_value(5)),
-                self.wait()
+                self.wait(0.5)
                 self.play(pointer_value.animate.set_value(3))
-
+                self.play(pointer_value.animate.increment_value(-2))
+                self.wait(0.5)
     """
 
     def __init__(self, value=0, **kwargs):
@@ -48,19 +53,28 @@ def __init__(self, value=0, **kwargs):
         self.set_value(value)
 
     def get_value(self):
+        """Get the current value of the ValueTracker. This value changes continuously when :attr:`animate` for the ValueTracker is called."""
         return self.points[0, 0]
 
     def set_value(self, value):
+        """Sets a new value (float, int) to the ValueTracker"""
         self.points[0, 0] = value
         return self
 
     def increment_value(self, d_value):
+        """Increments (adds) a value (float, int) to the ValueTracker"""
         self.set_value(self.get_value() + d_value)
 
     def __iadd__(self, d_value):
+        """adds `+=` syntax to increment the value of the ValueTracker"""
         self.increment_value(d_value)
         return self
 
+    def __isub__(self, d_value):
+        """adds `-=` syntax to decrement the value of the ValueTracker"""
+        self.increment_value(-d_value)
+        return self
+
     def interpolate(self, mobject1, mobject2, alpha, path_func=straight_path):
         """
         Turns self into an interpolation between mobject1

tests/test_value_tracker.py

@@ -26,3 +26,10 @@ def test_value_tracker_iadd():
     tracker = ValueTracker(0.0)
     tracker += 10.0
     assert tracker.get_value() == 10.0
+
+
+def test_value_tracker_isub():
+    """Test ValueTracker.__isub__()"""
+    tracker = ValueTracker(20.0)
+    tracker -= 10.0
+    assert tracker.get_value() == 10.0