Gizmo Arrows (#10550)

## Objective

- Add an arrow gizmo as suggested by #9400 

## Solution

(excuse my Protomen music)


https://github.com/bevyengine/bevy/assets/14184826/192adf24-079f-4a4b-a17b-091e892974ec

Wasn't horribly hard when i remembered i can change coordinate systems
whenever I want. Gave them four tips (as suggested by @alice-i-cecile in
discord) instead of trying to decide what direction the tips should
point.

Made the tip length default to 1/10 of the arrow's length, which looked
good enough to me. Hard-coded the angle from the body to the tips to 45
degrees.

## Still TODO

- [x] actual doc comments
- [x] doctests
- [x] `ArrowBuilder.with_tip_length()`

---

## Changelog

- Added `gizmos.arrow()` and `gizmos.arrow_2d()`
- Added arrows to `2d_gizmos` and `3d_gizmos` examples

## Migration Guide

N/A

---------

Co-authored-by: Nicola Papale <nicopap@users.noreply.github.com>

crates/bevy_gizmos/src/arrows.rs

@@ -0,0 +1,106 @@
+//! Additional Gizmo Functions -- Arrows
+
+use crate::prelude::Gizmos;
+use bevy_math::{Quat, Vec2, Vec3};
+use bevy_render::color::Color;
+
+pub struct ArrowBuilder<'a, 's> {
+    gizmos: &'a mut Gizmos<'s>,
+    start: Vec3,
+    end: Vec3,
+    color: Color,
+    tip_length: f32,
+}
+
+/// A builder returned by [`Gizmos::arrow`] and [`Gizmos::arrow_2d`]
+impl ArrowBuilder<'_, '_> {
+    /// Change the length of the tips to be `length`.
+    /// The default tip length is [length of the arrow]/10.
+    ///
+    /// # Example
+    /// ```
+    /// # use bevy_gizmos::prelude::*;
+    /// # use bevy_render::prelude::*;
+    /// # use bevy_math::prelude::*;
+    /// fn system(mut gizmos: Gizmos) {
+    ///     gizmos.arrow(Vec3::ZERO, Vec3::ONE, Color::GREEN)
+    ///         .with_tip_length(3.);
+    /// }
+    /// # bevy_ecs::system::assert_is_system(system);
+    /// ```
+    #[doc(alias = "arrow_head_length")]
+    pub fn with_tip_length(&mut self, length: f32) {
+        self.tip_length = length;
+    }
+}
+
+impl Drop for ArrowBuilder<'_, '_> {
+    /// Draws the arrow, by drawing lines with the stored [`Gizmos`]
+    fn drop(&mut self) {
+        // first, draw the body of the arrow
+        self.gizmos.line(self.start, self.end, self.color);
+        // now the hard part is to draw the head in a sensible way
+        // put us in a coordinate system where the arrow is pointing towards +x and ends at the origin
+        let pointing = (self.end - self.start).normalize();
+        let rotation = Quat::from_rotation_arc(Vec3::X, pointing);
+        let tips = [
+            Vec3::new(-1., 1., 0.),
+            Vec3::new(-1., 0., 1.),
+            Vec3::new(-1., -1., 0.),
+            Vec3::new(-1., 0., -1.),
+        ];
+        // - extend the vectors so their length is `tip_length`
+        // - rotate the world so +x is facing in the same direction as the arrow
+        // - translate over to the tip of the arrow
+        let tips = tips.map(|v| rotation * (v.normalize() * self.tip_length) + self.end);
+        for v in tips {
+            // then actually draw the tips
+            self.gizmos.line(self.end, v, self.color);
+        }
+    }
+}
+
+impl<'s> Gizmos<'s> {
+    /// Draw an arrow in 3D, from `start` to `end`. Has four tips for convienent viewing from any direction.
+    ///
+    /// This should be called for each frame the arrow needs to be rendered.
+    ///
+    /// # Example
+    /// ```
+    /// # use bevy_gizmos::prelude::*;
+    /// # use bevy_render::prelude::*;
+    /// # use bevy_math::prelude::*;
+    /// fn system(mut gizmos: Gizmos) {
+    ///     gizmos.arrow(Vec3::ZERO, Vec3::ONE, Color::GREEN);
+    /// }
+    /// # bevy_ecs::system::assert_is_system(system);
+    /// ```
+    pub fn arrow(&mut self, start: Vec3, end: Vec3, color: Color) -> ArrowBuilder<'_, 's> {
+        let length = (end - start).length();
+        ArrowBuilder {
+            gizmos: self,
+            start,
+            end,
+            color,
+            tip_length: length / 10.,
+        }
+    }
+
+    /// Draw an arrow in 2D (on the xy plane), from `start` to `end`.
+    ///
+    /// This should be called for each frame the arrow needs to be rendered.
+    ///
+    /// # Example
+    /// ```
+    /// # use bevy_gizmos::prelude::*;
+    /// # use bevy_render::prelude::*;
+    /// # use bevy_math::prelude::*;
+    /// fn system(mut gizmos: Gizmos) {
+    ///     gizmos.arrow_2d(Vec2::ZERO, Vec2::X, Color::GREEN);
+    /// }
+    /// # bevy_ecs::system::assert_is_system(system);
+    /// ```
+    pub fn arrow_2d(&mut self, start: Vec2, end: Vec2, color: Color) -> ArrowBuilder<'_, 's> {
+        self.arrow(start.extend(0.), end.extend(0.), color)
+    }
+}

crates/bevy_gizmos/src/lib.rs

@@ -16,6 +16,7 @@
 //!
 //! See the documentation on [`Gizmos`] for more examples.
 
+mod arrows;
 pub mod gizmos;
 
 #[cfg(feature = "bevy_sprite")]

examples/2d/2d_gizmos.rs

@@ -53,6 +53,12 @@ fn system(mut gizmos: Gizmos, time: Res<Time>) {
     // Arcs default amount of segments is linearly interpolated between
     // 1 and 32, using the arc length as scalar.
     gizmos.arc_2d(Vec2::ZERO, sin / 10., PI / 2., 350., Color::ORANGE_RED);
+
+    gizmos.arrow_2d(
+        Vec2::ZERO,
+        Vec2::from_angle(sin / -10. + PI / 2.) * 50.,
+        Color::YELLOW,
+    );
 }
 
 fn update_config(mut config: ResMut<GizmoConfig>, keyboard: Res<Input<KeyCode>>, time: Res<Time>) {

examples/3d/3d_gizmos.rs

@@ -96,6 +96,8 @@ fn system(mut gizmos: Gizmos, time: Res<Time>) {
     gizmos
         .sphere(Vec3::ZERO, Quat::IDENTITY, 3.2, Color::BLACK)
         .circle_segments(64);
+
+    gizmos.arrow(Vec3::ZERO, Vec3::ONE * 1.5, Color::YELLOW);
 }
 
 fn rotate_camera(mut query: Query<&mut Transform, With<Camera>>, time: Res<Time>) {