"""A self-contained Manim Community 0.21.0 lesson; no LaTeX required.
Render: manim -qm --disable_caching slope_scene.py SlopeLesson
"""
from manim import (
    BLUE, ORANGE, WHITE, Axes, Dot, Line, Scene, Text,
    ValueTracker, always_redraw, linear,
)


class SlopeLesson(Scene):
    def construct(self):
        self.camera.background_color = "#111827"
        axes = Axes(
            x_range=[-3, 3, 1], y_range=[-5, 5, 1],
            x_length=9, y_length=4.5,
            axis_config={"include_tip": False, "color": WHITE},
        ).shift([0, -0.1, 0])
        title = Text("Changing slope: y = m x", font_size=34).move_to([0, 3.25, 0])
        scale_note = Text("Read the values: the x and y screen scales differ.", font_size=20).move_to([0, 2.65, 0])
        labels = [Text("x", font_size=23).next_to(axes.x_axis.get_end(), [1, 0, 0]),
                  Text("y", font_size=23).next_to(axes.y_axis.get_end(), [0, 1, 0])]
        for x in [-2, -1, 1, 2]:
            labels.append(Text(str(x), font_size=19).move_to(axes.c2p(x, 0) + [0, -0.28, 0]))
        for y in [-4, -2, 2, 4]:
            labels.append(Text(str(y), font_size=19).move_to(axes.c2p(0, y) + [-0.3, 0, 0]))
        m = ValueTracker(0)
        graph = always_redraw(lambda: Line(
            axes.c2p(-2, -2 * m.get_value()),
            axes.c2p(2, 2 * m.get_value()), color=BLUE, stroke_width=5,
        ))
        point = always_redraw(lambda: Dot(axes.c2p(2, 2 * m.get_value()), color=ORANGE, radius=0.09))
        readout = always_redraw(lambda: Text(
            f"m = {m.get_value():.1f}     x = 2     y = {2 * m.get_value():.1f}",
            font_size=27, color=ORANGE,
        ).move_to([0, -2.9, 0]))
        self.add(axes, title, scale_note, *labels, graph, point, readout)
        self.wait(2)
        for target in [1, 2, -1]:
            self.play(m.animate.set_value(target), run_time=1, rate_func=linear)
            self.wait(2)
        self.wait(1)