Add example for making the camera follow a target (#49)

examples/follow_target.rs

@@ -0,0 +1,97 @@
+//! Demonstrates how to have the camera follow a target object
+
+use bevy::prelude::*;
+use bevy_panorbit_camera::{PanOrbitCamera, PanOrbitCameraPlugin};
+use std::f32::consts::TAU;
+
+fn main() {
+    App::new()
+        .add_plugins(DefaultPlugins)
+        .add_plugins(PanOrbitCameraPlugin)
+        .add_systems(Startup, setup)
+        .add_systems(Update, (animate_cube, cam_follow).chain())
+        .run();
+}
+
+#[derive(Component)]
+struct Cube;
+
+fn setup(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+) {
+    // Ground
+    commands.spawn(PbrBundle {
+        mesh: meshes.add(shape::Plane::from_size(5.0).into()),
+        material: materials.add(Color::rgb(0.3, 0.5, 0.3).into()),
+        ..default()
+    });
+    // Cube
+    commands
+        .spawn(PbrBundle {
+            mesh: meshes.add(Mesh::from(shape::Cube { size: 1.0 })),
+            material: materials.add(Color::rgb(0.8, 0.7, 0.6).into()),
+            transform: Transform::from_xyz(0.0, 0.5, 0.0),
+            ..default()
+        })
+        .insert(Cube);
+    // Light
+    commands.spawn(PointLightBundle {
+        point_light: PointLight {
+            intensity: 1500.0,
+            shadows_enabled: true,
+            ..default()
+        },
+        transform: Transform::from_xyz(4.0, 8.0, 4.0),
+        ..default()
+    });
+    // Camera
+    commands.spawn((
+        Camera3dBundle {
+            transform: Transform::from_translation(Vec3::new(0.0, 1.5, 5.0)),
+            ..default()
+        },
+        PanOrbitCamera {
+            // Panning the camera changes the focus, and so you most likely want to disable
+            // panning when setting the focus manually
+            pan_sensitivity: 0.0,
+            // If you want to fully control the camera's focus, set smoothness to 0 so it
+            // immediately snaps to that location. If you want the 'follow' to be smoothed,
+            // leave this at default or set it to something between 0 and 1.
+            pan_smoothness: 0.0,
+            ..default()
+        },
+    ));
+}
+
+#[derive(Default)]
+struct AnimateAngle(f32);
+
+/// Move the cube in a circle around the Y axis
+fn animate_cube(
+    time: Res<Time>,
+    mut cube_q: Query<&mut Transform, With<Cube>>,
+    mut angle: Local<AnimateAngle>,
+) {
+    if let Ok(mut cube_tfm) = cube_q.get_single_mut() {
+        // Rotate 20 degrees a second, wrapping around to 0 after a full rotation
+        angle.0 += 20f32.to_radians() * time.delta_seconds() % TAU;
+        // Convert angle to position
+        let pos = Vec3::new(angle.0.sin() * 1.5, 0.5, angle.0.cos() * 1.5);
+        cube_tfm.translation = pos;
+    }
+}
+
+/// Set the camera's focus to the cube's position
+fn cam_follow(mut pan_orbit_q: Query<&mut PanOrbitCamera>, cube_q: Query<&Transform, With<Cube>>) {
+    if let Ok(mut pan_orbit) = pan_orbit_q.get_single_mut() {
+        if let Ok(cube_tfm) = cube_q.get_single() {
+            pan_orbit.target_focus = cube_tfm.translation;
+            // Whenever changing properties manually like this, it's necessary to force
+            // PanOrbitCamera to update this frame (by default it only updates when there are
+            // input events).
+            pan_orbit.force_update = true;
+        }
+    }
+}