Struct EntityGeneration 

pub struct EntityGeneration(/* private fields */);

This tracks different versions or generations of an EntityRow. Importantly, this can wrap, meaning each generation is not necessarily unique per EntityRow.

This should be treated as a opaque identifier, and its internal representation may be subject to change.

Aliasing

Internally EntityGeneration wraps a u32, so it can’t represent every possible generation. Eventually, generations can (and do) wrap or alias. This can cause Entity and EntityGeneration values to be equal while still referring to different conceptual entities. This can cause some surprising behavior:

let (aliased, did_alias) = EntityGeneration::FIRST.after_versions(1u32 << 31).after_versions_and_could_alias(1u32 << 31);
assert!(did_alias);
assert!(EntityGeneration::FIRST == aliased);

This can cause some unintended side effects. See Entity docs for practical concerns and how to minimize any risks.

Implementations

impl EntityGeneration

pub const FIRST: Self

Represents the first generation of an EntityRow.

pub const fn to_bits(self) -> u32

Gets some bits that represent this value. The bits are opaque and should not be regarded as meaningful.

pub const fn from_bits(bits: u32) -> Self

Reconstruct an EntityGeneration previously destructured with EntityGeneration::to_bits.

Only useful when applied to results from to_bits in the same instance of an application.

pub const fn after_versions(self, versions: u32) -> Self

Returns the EntityGeneration that would result from this many more versions of the corresponding EntityRow from passing.

pub const fn after_versions_and_could_alias(self, versions: u32) -> (Self, bool)

Identical to after_versions but also returns a bool indicating if, after these versions, one such version could conflict with a previous one.

If this happens, this will no longer uniquely identify a version of an EntityRow. This is called entity aliasing.

pub const fn cmp_approx(&self, other: &Self) -> Ordering

Compares two generations.

Generations that are later will be Greater than earlier ones.

let later_generation = EntityGeneration::FIRST.after_versions(400);
assert_eq!(EntityGeneration::FIRST.cmp_approx(&later_generation), Ordering::Less);

let (aliased, did_alias) = EntityGeneration::FIRST.after_versions(400).after_versions_and_could_alias(u32::MAX);
assert!(did_alias);
assert_eq!(EntityGeneration::FIRST.cmp_approx(&aliased), Ordering::Less);

Ordering will be incorrect and non-transitive for distant generations:

let later_generation = EntityGeneration::FIRST.after_versions(3u32 << 31);
let much_later_generation = later_generation.after_versions(3u32 << 31);

// while these orderings are correct and pass assertions...
assert_eq!(EntityGeneration::FIRST.cmp_approx(&later_generation), Ordering::Less);
assert_eq!(later_generation.cmp_approx(&much_later_generation), Ordering::Less);

// ... this ordering is not and the assertion fails!
assert_eq!(EntityGeneration::FIRST.cmp_approx(&much_later_generation), Ordering::Less);

Because of this, EntityGeneration does not implement Ord/PartialOrd.

Trait Implementations

impl Clone for EntityGeneration

fn clone(&self) -> EntityGeneration

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for EntityGeneration

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Display for EntityGeneration

fn fmt(&self, __derive_more_f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Hash for EntityGeneration

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl PartialEq for EntityGeneration

fn eq(&self, other: &EntityGeneration) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl Copy for EntityGeneration

impl Eq for EntityGeneration

impl StructuralPartialEq for EntityGeneration