Update touch.rs docs (#4523)

# Objective

- Part of the splitting process of #3692.

## Solution

- Document `touch.rs` inside of `bevy_input`.

crates/bevy_input/src/touch.rs

@@ -3,38 +3,45 @@ use bevy_ecs::system::ResMut;
 use bevy_math::Vec2;
 use bevy_utils::HashMap;
 
-/// Represents a touch event
+/// A touch input event.
 ///
-/// Every time the user touches the screen, a new `Start` event with an unique
+/// ## Logic
-/// identifier for the finger is generated. When the finger is lifted, an `End`
+///
+/// Every time the user touches the screen, a new [`TouchPhase::Started`] event with an unique
+/// identifier for the finger is generated. When the finger is lifted, the [`TouchPhase::Ended`]
 /// event is generated with the same finger id.
 ///
-/// After a `Start` event has been emitted, there may be zero or more `Move`
+/// After a [`TouchPhase::Started`] event has been emitted, there may be zero or more [`TouchPhase::Moved`]
 /// events when the finger is moved or the touch pressure changes.
 ///
-/// The finger id may be reused by the system after an `End` event. The user
+/// The finger id may be reused by the system after an [`TouchPhase::Ended`] event. The user
-/// should assume that a new `Start` event received with the same id has nothing
+/// should assume that a new [`TouchPhase::Started`] event received with the same id has nothing
 /// to do with the old finger and is a new finger.
 ///
-/// A `Cancelled` event is emitted when the system has canceled tracking this
+/// A [`TouchPhase::Cancelled`] event is emitted when the system has canceled tracking this
 /// touch, such as when the window loses focus, or on iOS if the user moves the
 /// device against their face.
+///
+/// ## Note
+///
+/// This event is the translated version of the `WindowEvent::Touch` from the `winit` crate.
+/// It is available to the end user and can be used for game logic.
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub struct TouchInput {
+    /// The phase of the touch input.
     pub phase: TouchPhase,
+    /// The position of the finger on the touchscreen.
     pub position: Vec2,
-    /// Describes how hard the screen was pressed. May be `None` if the platform
+    /// Describes how hard the screen was pressed.
-    /// does not support pressure sensitivity.
     ///
-    /// ## Platform-specific
+    /// May be [`None`] if the platform does not support pressure sensitivity.
-    ///
+    /// This feature is only available on **iOS** 9.0+ and **Windows** 8+.
-    /// - Only available on **iOS** 9.0+ and **Windows** 8+.
     pub force: Option<ForceTouch>,
-    /// Unique identifier of a finger.
+    /// The unique identifier of the finger.
     pub id: u64,
 }
 
-/// Describes the force of a touch event
+/// A force description of a [`Touch`](crate::touch::Touch) input.
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub enum ForceTouch {
     /// On iOS, the force is calibrated so that the same number corresponds to
@@ -63,65 +70,107 @@ pub enum ForceTouch {
     /// If the platform reports the force as normalized, we have no way of
     /// knowing how much pressure 1.0 corresponds to – we know it's the maximum
     /// amount of force, but as to how much force, you might either have to
-    /// press really really hard, or not hard at all, depending on the device.
+    /// press really hard, or not hard at all, depending on the device.
     Normalized(f64),
 }
 
-/// Describes touch-screen input state.
+/// A phase of a [`TouchInput`](crate::touch::TouchInput).
+///
+/// ## Usage
+///
+/// It is used to describe the phase of the touch input that is currently active.
+/// This includes a phase that indicates that a touch input has started or ended,
+/// or that a finger has moved. There is also a cancelled phase that indicates that
+/// the system cancelled the tracking of the finger.
 #[derive(Debug, Hash, PartialEq, Eq, Clone, Copy)]
 #[cfg_attr(feature = "serialize", derive(serde::Serialize, serde::Deserialize))]
 pub enum TouchPhase {
+    /// A finger started to touch the touchscreen.
     Started,
+    /// A finger moved over the touchscreen.
     Moved,
+    /// A finger stopped touching the touchscreen.
     Ended,
+    /// The system cancelled the tracking of the finger.
+    ///
+    /// This occurs when the window loses focus, or on iOS if the user moves the
+    /// device against their face.
     Cancelled,
 }
 
+/// A touch input.
+///
+/// ## Usage
+///
+/// It is used to store the position and force of a touch input and also the `id` of the finger.
+/// The data of the touch input comes from the [`TouchInput`] event and is being stored
+/// inside of the [`Touches`] `bevy` resource.
 #[derive(Debug, Clone, Copy)]
 pub struct Touch {
+    /// The id of the touch input.
     id: u64,
+    /// The starting position of the touch input.
     start_position: Vec2,
+    /// The starting force of the touch input.
     start_force: Option<ForceTouch>,
+    /// The previous position of the touch input.
     previous_position: Vec2,
+    /// The previous force of the touch input.
     previous_force: Option<ForceTouch>,
+    /// The current position of the touch input.
     position: Vec2,
+    /// The current force of the touch input.
     force: Option<ForceTouch>,
 }
 
 impl Touch {
+    /// The delta of the current `position` and the `previous_position`.
     pub fn delta(&self) -> Vec2 {
         self.position - self.previous_position
     }
 
+    /// The distance of the `start_position` and the current `position`.
     pub fn distance(&self) -> Vec2 {
         self.position - self.start_position
     }
 
+    /// Returns the `id` of the touch.
     #[inline]
     pub fn id(&self) -> u64 {
         self.id
     }
 
+    /// Returns the `start_position` of the touch.
     #[inline]
     pub fn start_position(&self) -> Vec2 {
         self.start_position
     }
 
+    /// Returns the `start_force` of the touch.
     #[inline]
     pub fn start_force(&self) -> Option<ForceTouch> {
         self.start_force
     }
 
+    /// Returns the `previous_position` of the touch.
     #[inline]
     pub fn previous_position(&self) -> Vec2 {
         self.previous_position
     }
 
+    /// Returns the `previous_force` of the touch.
+    #[inline]
+    pub fn previous_force(&self) -> Option<ForceTouch> {
+        self.previous_force
+    }
+
+    /// Returns the current `position` of the touch.
     #[inline]
     pub fn position(&self) -> Vec2 {
         self.position
     }
 
+    /// Returns the current `force` of the touch.
     #[inline]
     pub fn force(&self) -> Option<ForceTouch> {
         self.force
@@ -142,51 +191,76 @@ impl From<&TouchInput> for Touch {
     }
 }
 
+/// A collection of [`Touch`]es.
+///
+/// ## Usage
+///
+/// It is used to create a `bevy` resource that stores the data of the touches on a touchscreen
+/// and can be accessed inside of a system.
+///
+/// ## Updating
+///
+/// The resource is updated inside of the [`touch_screen_input_system`](crate::touch::touch_screen_input_system).
 #[derive(Debug, Clone, Default)]
 pub struct Touches {
+    /// A collection of every [`Touch`] that is currently being pressed.
     pressed: HashMap<u64, Touch>,
+    /// A collection of every [`Touch`] that just got pressed.
     just_pressed: HashMap<u64, Touch>,
+    /// A collection of every [`Touch`] that just got released.
     just_released: HashMap<u64, Touch>,
+    /// A collection of every [`Touch`] that just got cancelled.
     just_cancelled: HashMap<u64, Touch>,
 }
 
 impl Touches {
+    /// An iterator visiting every pressed [`Touch`] input in arbitrary order.
     pub fn iter(&self) -> impl Iterator<Item = &Touch> + '_ {
         self.pressed.values()
     }
 
+    /// Returns the [`Touch`] input corresponding to the `id` if it is being pressed.
     pub fn get_pressed(&self, id: u64) -> Option<&Touch> {
         self.pressed.get(&id)
     }
 
+    /// Returns `true` if the input corresponding to the `id` has just been pressed.
     pub fn just_pressed(&self, id: u64) -> bool {
         self.just_pressed.contains_key(&id)
     }
 
+    /// An iterator visiting every just pressed [`Touch`] input in arbitrary order.
     pub fn iter_just_pressed(&self) -> impl Iterator<Item = &Touch> {
         self.just_pressed.values()
     }
 
+    /// Returns the [`Touch`] input corresponding to the `id` if it has just been released.
     pub fn get_released(&self, id: u64) -> Option<&Touch> {
         self.just_released.get(&id)
     }
 
+    /// Returns `true` if the input corresponding to the `id` has just been released.
     pub fn just_released(&self, id: u64) -> bool {
         self.just_released.contains_key(&id)
     }
 
+    /// An iterator visiting every just released [`Touch`] input in arbitrary order.
     pub fn iter_just_released(&self) -> impl Iterator<Item = &Touch> {
         self.just_released.values()
     }
 
+    /// Returns `true` if the input corresponding to the `id` has just been cancelled.
     pub fn just_cancelled(&self, id: u64) -> bool {
         self.just_cancelled.contains_key(&id)
     }
 
+    /// An iterator visiting every just cancelled [`Touch`] input in arbitrary order.
     pub fn iter_just_cancelled(&self) -> impl Iterator<Item = &Touch> {
         self.just_cancelled.values()
     }
 
+    /// Processes a [`TouchInput`] event by updating the `pressed`, `just_pressed`,
+    /// `just_released`, and `just_cancelled` collections.
     fn process_touch_event(&mut self, event: &TouchInput) {
         match event.phase {
             TouchPhase::Started => {
@@ -213,6 +287,13 @@ impl Touches {
         };
     }
 
+    /// Clears the `just_pressed`, `just_released`, and `just_cancelled` collections.
+    ///
+    /// This is not clearing the `pressed` collection, because it could incorrectly mark
+    /// a touch input as not pressed eventhough it is pressed. This could happen if the
+    /// touch input is not moving for a single frame and would therefore be marked as
+    /// not pressed, because this function is called on every single frame no matter
+    /// if there was an event or not.
     fn update(&mut self) {
         self.just_pressed.clear();
         self.just_released.clear();
@@ -220,7 +301,12 @@ impl Touches {
     }
 }
 
-/// Updates the `Touches` resource with the latest `TouchInput` events
+/// Updates the [`Touches`] resource with the latest [`TouchInput`] events.
+///
+/// ## Differences
+///
+/// The main difference between the [`TouchInput`] event and the [`Touches`] resource is that
+/// the latter has convenient functions like [`Touches::just_pressed`] and [`Touches::just_released`].
 pub fn touch_screen_input_system(
     mut touch_state: ResMut<Touches>,
     mut touch_input_events: EventReader<TouchInput>,