Make state private and only accessible through getter for State resource (#8009)

# Objective
State requires a kind of awkward `state.0` to get the current state and
exposes the field directly to manipulation.

## Solution
Make it accessible through a getter method as well as privatize the
field to make sure false assumptions about setting the state aren't
made.

## Migration Guide
- Use `State::get` instead of accessing the tuple field directly.

crates/bevy_ecs/src/schedule/condition.rs

@@ -685,14 +685,14 @@ pub mod common_conditions {
     /// app.run(&mut world);
     /// assert_eq!(world.resource::<Counter>().0, 1);
     ///
-    /// *world.resource_mut::<State<GameState>>() = State(GameState::Paused);
+    /// *world.resource_mut::<State<GameState>>() = State::new(GameState::Paused);
     ///
     /// // Now that we are in `GameState::Pause`, `pause_system` will run
     /// app.run(&mut world);
     /// assert_eq!(world.resource::<Counter>().0, 0);
     /// ```
     pub fn in_state<S: States>(state: S) -> impl FnMut(Res<State<S>>) -> bool + Clone {
-        move |current_state: Res<State<S>>| current_state.0 == state
+        move |current_state: Res<State<S>>| *current_state == state
     }
 
     /// Generates a [`Condition`](super::Condition)-satisfying closure that returns `true`
@@ -741,7 +741,7 @@ pub mod common_conditions {
     /// app.run(&mut world);
     /// assert_eq!(world.resource::<Counter>().0, 1);
     ///
-    /// *world.resource_mut::<State<GameState>>() = State(GameState::Paused);
+    /// *world.resource_mut::<State<GameState>>() = State::new(GameState::Paused);
     ///
     /// // Now that we are in `GameState::Pause`, `pause_system` will run
     /// app.run(&mut world);
@@ -751,7 +751,7 @@ pub mod common_conditions {
         state: S,
     ) -> impl FnMut(Option<Res<State<S>>>) -> bool + Clone {
         move |current_state: Option<Res<State<S>>>| match current_state {
-            Some(current_state) => current_state.0 == state,
+            Some(current_state) => *current_state == state,
             None => false,
         }
     }
@@ -803,7 +803,7 @@ pub mod common_conditions {
     /// app.run(&mut world);
     /// assert_eq!(world.resource::<Counter>().0, 1);
     ///
-    /// *world.resource_mut::<State<GameState>>() = State(GameState::Paused);
+    /// *world.resource_mut::<State<GameState>>() = State::new(GameState::Paused);
     ///
     /// // Now that `GameState` has been updated `my_system` will run
     /// app.run(&mut world);

crates/bevy_ecs/src/schedule/state.rs

@@ -73,7 +73,27 @@ pub struct OnTransition<S: States> {
 ///
 /// The starting state is defined via the [`Default`] implementation for `S`.
 #[derive(Resource, Default, Debug)]
-pub struct State<S: States>(pub S);
+pub struct State<S: States>(S);
+
+impl<S: States> State<S> {
+    /// Creates a new state with a specific value.
+    ///
+    /// To change the state use [`NextState<S>`] rather than using this to modify the `State<S>`.
+    pub fn new(state: S) -> Self {
+        Self(state)
+    }
+
+    /// Get the current state.
+    pub fn get(&self) -> &S {
+        &self.0
+    }
+}
+
+impl<S: States> PartialEq<S> for State<S> {
+    fn eq(&self, other: &S) -> bool {
+        self.get() == other
+    }
+}
 
 /// The next state of [`State<S>`].
 ///