1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
#[cfg(feature = "reflect")]
use bevy_ecs::reflect::{ReflectComponent, ReflectMapEntities};
use bevy_ecs::{
    component::Component,
    entity::{Entity, EntityMapper, MapEntities},
    prelude::FromWorld,
    world::World,
};
use core::slice;
use smallvec::SmallVec;
use std::ops::Deref;

/// Contains references to the child entities of this entity.
///
/// Each child must contain a [`Parent`] component that points back to this entity.
/// This component rarely needs to be created manually,
/// consider using higher level utilities like [`BuildChildren::with_children`]
/// which are safer and easier to use.
///
/// See [`HierarchyQueryExt`] for hierarchy related methods on [`Query`].
///
/// [`HierarchyQueryExt`]: crate::query_extension::HierarchyQueryExt
/// [`Query`]: bevy_ecs::system::Query
/// [`Parent`]: crate::components::parent::Parent
/// [`BuildChildren::with_children`]: crate::child_builder::BuildChildren::with_children
#[derive(Component, Debug)]
#[cfg_attr(feature = "reflect", derive(bevy_reflect::Reflect))]
#[cfg_attr(feature = "reflect", reflect(Component, MapEntities))]
pub struct Children(pub(crate) SmallVec<[Entity; 8]>);

impl MapEntities for Children {
    fn map_entities<M: EntityMapper>(&mut self, entity_mapper: &mut M) {
        for entity in &mut self.0 {
            *entity = entity_mapper.map_entity(*entity);
        }
    }
}

// TODO: We need to impl either FromWorld or Default so Children can be registered as Reflect.
// This is because Reflect deserialize by creating an instance and apply a patch on top.
// However Children should only ever be set with a real user-defined entities. Its worth looking
// into better ways to handle cases like this.
impl FromWorld for Children {
    #[inline]
    fn from_world(_world: &mut World) -> Self {
        Children(SmallVec::new())
    }
}

impl Children {
    /// Constructs a [`Children`] component with the given entities.
    #[inline]
    pub(crate) fn from_entities(entities: &[Entity]) -> Self {
        Self(SmallVec::from_slice(entities))
    }

    /// Swaps the child at `a_index` with the child at `b_index`.
    #[inline]
    pub fn swap(&mut self, a_index: usize, b_index: usize) {
        self.0.swap(a_index, b_index);
    }

    /// Sorts children [stably](https://en.wikipedia.org/wiki/Sorting_algorithm#Stability)
    /// in place using the provided comparator function.
    ///
    /// For the underlying implementation, see [`slice::sort_by`].
    ///
    /// For the unstable version, see [`sort_unstable_by`](Children::sort_unstable_by).
    ///
    /// See also [`sort_by_key`](Children::sort_by_key), [`sort_by_cached_key`](Children::sort_by_cached_key).
    #[inline]
    pub fn sort_by<F>(&mut self, compare: F)
    where
        F: FnMut(&Entity, &Entity) -> std::cmp::Ordering,
    {
        self.0.sort_by(compare);
    }

    /// Sorts children [stably](https://en.wikipedia.org/wiki/Sorting_algorithm#Stability)
    /// in place using the provided key extraction function.
    ///
    /// For the underlying implementation, see [`slice::sort_by_key`].
    ///
    /// For the unstable version, see [`sort_unstable_by_key`](Children::sort_unstable_by_key).
    ///
    /// See also [`sort_by`](Children::sort_by), [`sort_by_cached_key`](Children::sort_by_cached_key).
    #[inline]
    pub fn sort_by_key<K, F>(&mut self, compare: F)
    where
        F: FnMut(&Entity) -> K,
        K: Ord,
    {
        self.0.sort_by_key(compare);
    }

    /// Sorts children [stably](https://en.wikipedia.org/wiki/Sorting_algorithm#Stability)
    /// in place using the provided key extraction function. Only evaluates each key at most
    /// once per sort, caching the intermediate results in memory.
    ///
    /// For the underlying implementation, see [`slice::sort_by_cached_key`].
    ///
    /// See also [`sort_by`](Children::sort_by), [`sort_by_key`](Children::sort_by_key).
    #[inline]
    pub fn sort_by_cached_key<K, F>(&mut self, compare: F)
    where
        F: FnMut(&Entity) -> K,
        K: Ord,
    {
        self.0.sort_by_cached_key(compare);
    }

    /// Sorts children [unstably](https://en.wikipedia.org/wiki/Sorting_algorithm#Stability)
    /// in place using the provided comparator function.
    ///
    /// For the underlying implementation, see [`slice::sort_unstable_by`].
    ///
    /// For the stable version, see [`sort_by`](Children::sort_by).
    ///
    /// See also [`sort_unstable_by_key`](Children::sort_unstable_by_key).
    #[inline]
    pub fn sort_unstable_by<F>(&mut self, compare: F)
    where
        F: FnMut(&Entity, &Entity) -> std::cmp::Ordering,
    {
        self.0.sort_unstable_by(compare);
    }

    /// Sorts children [unstably](https://en.wikipedia.org/wiki/Sorting_algorithm#Stability)
    /// in place using the provided key extraction function.
    ///
    /// For the underlying implementation, see [`slice::sort_unstable_by_key`].
    ///
    /// For the stable version, see [`sort_by_key`](Children::sort_by_key).
    ///
    /// See also [`sort_unstable_by`](Children::sort_unstable_by).
    #[inline]
    pub fn sort_unstable_by_key<K, F>(&mut self, compare: F)
    where
        F: FnMut(&Entity) -> K,
        K: Ord,
    {
        self.0.sort_unstable_by_key(compare);
    }
}

impl Deref for Children {
    type Target = [Entity];

    #[inline(always)]
    fn deref(&self) -> &Self::Target {
        &self.0[..]
    }
}

impl<'a> IntoIterator for &'a Children {
    type Item = <Self::IntoIter as Iterator>::Item;

    type IntoIter = slice::Iter<'a, Entity>;

    #[inline(always)]
    fn into_iter(self) -> Self::IntoIter {
        self.0.iter()
    }
}