Fixed bevy_ui touch input (#4099)

# Objective

`bevy_ui` doesn't support correctly touch inputs because of two problems in the focus system:
- It attempts to retrieve touch input with a specific `0` id
- It doesn't retrieve touch positions and bases its focus solely on mouse position, absent from mobile devices

## Solution

I added a few methods to the `Touches` resource, allowing to check if **any** touch input was pressed, released or cancelled and to retrieve the *position* of the first pressed touch input and adapted the focus system.

I added a test button to the *iOS* example and it works correclty on emulator. I did not test on a real touch device as:
- Android is not working (https://github.com/bevyengine/bevy/issues/3249)
- I don't have an iOS device

crates/bevy_input/src/touch.rs

@@ -224,6 +224,11 @@ impl Touches {
         self.pressed.get(&id)
     }
 
+    /// Checks if any touch input was just pressed.
+    pub fn any_just_pressed(&self) -> bool {
+        !self.just_pressed.is_empty()
+    }
+
     /// 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)
@@ -239,6 +244,11 @@ impl Touches {
         self.just_released.get(&id)
     }
 
+    /// Checks if any touch input was just released.
+    pub fn any_just_released(&self) -> bool {
+        !self.just_released.is_empty()
+    }
+
     /// 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)
@@ -249,6 +259,11 @@ impl Touches {
         self.just_released.values()
     }
 
+    /// Checks if any touch input was just cancelled.
+    pub fn any_just_cancelled(&self) -> bool {
+        !self.just_cancelled.is_empty()
+    }
+
     /// 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)
@@ -259,6 +274,11 @@ impl Touches {
         self.just_cancelled.values()
     }
 
+    /// Retrieves the position of the first currently pressed touch, if any
+    pub fn first_pressed_position(&self) -> Option<Vec2> {
+        self.pressed.values().next().map(|t| t.position)
+    }
+
     /// Processes a [`TouchInput`] event by updating the `pressed`, `just_pressed`,
     /// `just_released`, and `just_cancelled` collections.
     fn process_touch_event(&mut self, event: &TouchInput) {

crates/bevy_ui/src/focus.rs

@@ -71,10 +71,6 @@ pub fn ui_focus_system(
         Option<&CalculatedClip>,
     )>,
 ) {
-    let cursor_position = windows
-        .get_primary()
-        .and_then(|window| window.cursor_position());
-
     // reset entities that were both clicked and released in the last frame
     for entity in state.entities_to_reset.drain(..) {
         if let Ok(mut interaction) = node_query.get_component_mut::<Interaction>(entity) {
@@ -83,7 +79,7 @@ pub fn ui_focus_system(
     }
 
     let mouse_released =
-        mouse_button_input.just_released(MouseButton::Left) || touches_input.just_released(0);
+        mouse_button_input.just_released(MouseButton::Left) || touches_input.any_just_released();
     if mouse_released {
         for (_entity, _node, _global_transform, interaction, _focus_policy, _clip) in
             node_query.iter_mut()
@@ -97,7 +93,12 @@ pub fn ui_focus_system(
     }
 
     let mouse_clicked =
-        mouse_button_input.just_pressed(MouseButton::Left) || touches_input.just_pressed(0);
+        mouse_button_input.just_pressed(MouseButton::Left) || touches_input.any_just_pressed();
+
+    let cursor_position = windows
+        .get_primary()
+        .and_then(|window| window.cursor_position())
+        .or_else(|| touches_input.first_pressed_position());
 
     let mut moused_over_z_sorted_nodes = node_query
         .iter_mut()

examples/ios/src/lib.rs

@@ -13,13 +13,14 @@ fn main() {
         .add_startup_system(setup_scene)
         .add_startup_system(setup_music)
         .add_system(touch_camera)
+        .add_system(button_handler)
         .run();
 }
 
 fn touch_camera(
     windows: ResMut<Windows>,
     mut touches: EventReader<TouchInput>,
-    mut camera: Query<&mut Transform, With<Camera>>,
+    mut camera: Query<&mut Transform, With<Camera3d>>,
     mut last_position: Local<Option<Vec2>>,
 ) {
     for touch in touches.iter() {
@@ -47,6 +48,7 @@ fn setup_scene(
     mut commands: Commands,
     mut meshes: ResMut<Assets<Mesh>>,
     mut materials: ResMut<Assets<StandardMaterial>>,
+    asset_server: Res<AssetServer>,
 ) {
     // plane
     commands.spawn_bundle(PbrBundle {
@@ -86,6 +88,65 @@ fn setup_scene(
         transform: Transform::from_xyz(-2.0, 2.5, 5.0).looking_at(Vec3::ZERO, Vec3::Y),
         ..default()
     });
+
+    // Test ui
+    commands.spawn_bundle(Camera2dBundle::default());
+    commands
+        .spawn_bundle(ButtonBundle {
+            style: Style {
+                justify_content: JustifyContent::Center,
+                align_items: AlignItems::Center,
+                position_type: PositionType::Absolute,
+                position: UiRect {
+                    left: Val::Px(50.0),
+                    right: Val::Px(50.0),
+                    top: Val::Auto,
+                    bottom: Val::Px(50.0),
+                },
+                ..default()
+            },
+            ..default()
+        })
+        .with_children(|b| {
+            b.spawn_bundle(TextBundle {
+                text: Text {
+                    sections: vec![TextSection {
+                        value: "Test Button".to_string(),
+                        style: TextStyle {
+                            font: asset_server.load("fonts/FiraSans-Bold.ttf"),
+                            font_size: 30.0,
+                            color: Color::BLACK,
+                        },
+                    }],
+                    alignment: TextAlignment {
+                        vertical: VerticalAlign::Center,
+                        horizontal: HorizontalAlign::Center,
+                    },
+                },
+                ..default()
+            });
+        });
+}
+
+fn button_handler(
+    mut interaction_query: Query<
+        (&Interaction, &mut UiColor),
+        (Changed<Interaction>, With<Button>),
+    >,
+) {
+    for (interaction, mut color) in interaction_query.iter_mut() {
+        match *interaction {
+            Interaction::Clicked => {
+                *color = Color::BLUE.into();
+            }
+            Interaction::Hovered => {
+                *color = Color::GRAY.into();
+            }
+            Interaction::None => {
+                *color = Color::WHITE.into();
+            }
+        }
+    }
 }
 
 fn setup_music(asset_server: Res<AssetServer>, audio: Res<Audio>) {