egui/
id.rs

1// TODO(emilk): have separate types `PositionId` and `UniqueId`. ?
2
3use std::num::NonZeroU64;
4
5/// egui tracks widgets frame-to-frame using [`Id`]s.
6///
7/// For instance, if you start dragging a slider one frame, egui stores
8/// the sliders [`Id`] as the current active id so that next frame when
9/// you move the mouse the same slider changes, even if the mouse has
10/// moved outside the slider.
11///
12/// For some widgets [`Id`]s are also used to persist some state about the
13/// widgets, such as Window position or whether not a collapsing header region is open.
14///
15/// This implies that the [`Id`]s must be unique.
16///
17/// For simple things like sliders and buttons that don't have any memory and
18/// doesn't move we can use the location of the widget as a source of identity.
19/// For instance, a slider only needs a unique and persistent ID while you are
20/// dragging the slider. As long as it is still while moving, that is fine.
21///
22/// For things that need to persist state even after moving (windows, collapsing headers)
23/// the location of the widgets is obviously not good enough. For instance,
24/// a collapsing region needs to remember whether or not it is open even
25/// if the layout next frame is different and the collapsing is not lower down
26/// on the screen.
27///
28/// Then there are widgets that need no identifiers at all, like labels,
29/// because they have no state nor are interacted with.
30///
31/// This is niche-optimized to that `Option<Id>` is the same size as `Id`.
32#[derive(Clone, Copy, Hash, Eq, PartialEq)]
33#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
34pub struct Id(NonZeroU64);
35
36impl nohash_hasher::IsEnabled for Id {}
37
38impl Id {
39    /// A special [`Id`], in particular as a key to [`crate::Memory::data`]
40    /// for when there is no particular widget to attach the data.
41    ///
42    /// The null [`Id`] is still a valid id to use in all circumstances,
43    /// though obviously it will lead to a lot of collisions if you do use it!
44    pub const NULL: Self = Self(NonZeroU64::MAX);
45
46    #[inline]
47    const fn from_hash(hash: u64) -> Self {
48        if let Some(nonzero) = NonZeroU64::new(hash) {
49            Self(nonzero)
50        } else {
51            Self(NonZeroU64::MIN) // The hash was exactly zero (very bad luck)
52        }
53    }
54
55    /// Generate a new [`Id`] by hashing some source (e.g. a string or integer).
56    pub fn new(source: impl std::hash::Hash) -> Self {
57        Self::from_hash(ahash::RandomState::with_seeds(1, 2, 3, 4).hash_one(source))
58    }
59
60    /// Generate a new [`Id`] by hashing the parent [`Id`] and the given argument.
61    pub fn with(self, child: impl std::hash::Hash) -> Self {
62        use std::hash::{BuildHasher as _, Hasher as _};
63        let mut hasher = ahash::RandomState::with_seeds(1, 2, 3, 4).build_hasher();
64        hasher.write_u64(self.0.get());
65        child.hash(&mut hasher);
66        Self::from_hash(hasher.finish())
67    }
68
69    /// Short and readable summary
70    pub fn short_debug_format(&self) -> String {
71        format!("{:04X}", self.value() as u16)
72    }
73
74    /// The inner value of the [`Id`].
75    ///
76    /// This is a high-entropy hash, or [`Self::NULL`].
77    #[inline(always)]
78    pub fn value(&self) -> u64 {
79        self.0.get()
80    }
81
82    pub fn accesskit_id(&self) -> accesskit::NodeId {
83        self.value().into()
84    }
85
86    /// Create a new [`Id`] from a high-entropy value. No hashing is done.
87    ///
88    /// This can be useful if you have an [`Id`] that was converted to some other type
89    /// (e.g. accesskit::NodeId) and you want to convert it back to an [`Id`].
90    ///
91    /// # Safety
92    /// You need to ensure that the value is high-entropy since it might be used in
93    /// a [`IdSet`] or [`IdMap`], which rely on the assumption that [`Id`]s have good entropy.
94    ///
95    /// The method is not unsafe in terms of memory safety.
96    ///
97    /// # Panics
98    /// If the value is zero, this will panic.
99    #[doc(hidden)]
100    #[expect(unsafe_code)]
101    pub unsafe fn from_high_entropy_bits(value: u64) -> Self {
102        Self(NonZeroU64::new(value).expect("Id must be non-zero."))
103    }
104}
105
106impl std::fmt::Debug for Id {
107    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
108        write!(f, "{:04X}", self.value() as u16)
109    }
110}
111
112/// Convenience
113impl From<&'static str> for Id {
114    #[inline]
115    fn from(string: &'static str) -> Self {
116        Self::new(string)
117    }
118}
119
120impl From<String> for Id {
121    #[inline]
122    fn from(string: String) -> Self {
123        Self::new(string)
124    }
125}
126
127#[test]
128fn id_size() {
129    assert_eq!(std::mem::size_of::<Id>(), 8);
130    assert_eq!(std::mem::size_of::<Option<Id>>(), 8);
131}
132
133// ----------------------------------------------------------------------------
134
135/// `IdSet` is a `HashSet<Id>` optimized by knowing that [`Id`] has good entropy, and doesn't need more hashing.
136pub type IdSet = nohash_hasher::IntSet<Id>;
137
138/// `IdMap<V>` is a `HashMap<Id, V>` optimized by knowing that [`Id`] has good entropy, and doesn't need more hashing.
139pub type IdMap<V> = nohash_hasher::IntMap<Id, V>;
egui/id.rs

egui/
id.rs
