Struct FocusSequenceNumber

pub struct FocusSequenceNumber(pub u64);

A sequence number generated by a script thread for its pipelines. The constellation attaches the target pipeline’s last seen FocusSequenceNumber to every focus-related message it sends.

This is used to resolve the inconsistency that occurs due to bidirectional focus state synchronization and provide eventual consistency. Example:

script                            constellation
-----------------------------------------------------------------------
send ActivateDocument ----------> receive ActivateDocument
                            ,---- send FocusDocument
                            |
focus an iframe             |
send Focus -----------------|---> receive Focus
                            |     focus the iframe's content document
receive FocusDocument <-----'     send FocusDocument to the content pipeline --> ...
unfocus the iframe
focus the document

Final state:                      Final state:
 the iframe is not focused         the iframe is focused

When the above sequence completes, from the script thread’s point of view, the iframe is unfocused, but from the constellation’s point of view, the iframe is still focused.

This inconsistency can be resolved by associating a sequence number to each message. Whenever a script thread initiates a focus operation, it generates and sends a brand new sequence number. The constellation attaches the last-received sequence number to each message it sends. This way, the script thread can discard out-dated incoming focus messages, and eventually, all actors converge to the consistent state which is determined based on the last focus message received by the constellation.

script                            constellation
-----------------------------------------------------------------------
send ActivateDocument ----------> receive ActivateDocument
                            ,---- send FocusDocument (0)
                            |
seq_number += 1             |
focus an iframe             |
send Focus (1) -------------|---> receive Focus (1)
                            |     focus the iframe's content document
receive FocusDocument (0) <-'     send FocusDocument to the content pipeline --> ...
ignore it because 0 < 1

Final state:                      Final state:
 the iframe is focused             the iframe is focused

Tuple Fields

0: u64

Trait Implementations

impl Clone for FocusSequenceNumber

fn clone(&self) -> FocusSequenceNumber

Returns a copy of the value.

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

Performs copy-assignment from source.

impl Debug for FocusSequenceNumber

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

Formats the value using the given formatter.

impl Default for FocusSequenceNumber

fn default() -> FocusSequenceNumber

Returns the “default value” for a type.

impl<'de> Deserialize<'de> for FocusSequenceNumber

fn deserialize<__D>( __deserializer: __D, ) -> Result<FocusSequenceNumber, <__D as Deserializer<'de>>::Error>

where __D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl Display for FocusSequenceNumber

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

Formats the value using the given formatter.

impl Hash for FocusSequenceNumber

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

where __H: Hasher,

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 MallocSizeOf for FocusSequenceNumber

fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize

Measure the heap usage of all descendant heap-allocated structures, but not the space taken up by the value itself.

impl PartialEq for FocusSequenceNumber

fn eq(&self, other: &FocusSequenceNumber) -> 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 PartialOrd for FocusSequenceNumber

fn partial_cmp(&self, other: &FocusSequenceNumber) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

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

Tests less than (for self and other) and is used by the < operator.

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

Tests less than or equal to (for self and other) and is used by the <= operator.

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

Tests greater than (for self and other) and is used by the > operator.

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

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl Serialize for FocusSequenceNumber

fn serialize<__S>( &self, __serializer: __S, ) -> Result<<__S as Serializer>::Ok, <__S as Serializer>::Error>

where __S: Serializer,

Serialize this value into the given Serde serializer.

impl Copy for FocusSequenceNumber

impl Eq for FocusSequenceNumber

impl StructuralPartialEq for FocusSequenceNumber