Enum FieldValue 

pub enum FieldValue<'a> {
    Null,
    Text(&'a str),
}

The value of one field of one item.

TLCP distinguishes null from the empty string, and so does this type: # on the wire produces FieldValue::Null and $ produces FieldValue::Text wrapping "" [docs/spec/04-notifications.md §2.2]. They are not interchangeable. A quote adapter that has no bid to report sends #; one whose status line is deliberately blank sends $.

Examples

An ItemUpdate is obtained from a live subscription, so the example is written as the function you would pass one to — which compiles, and so keeps saying something true:

use lightstreamer_rs::{FieldValue, ItemUpdate};

fn report_bid(update: &ItemUpdate) {
    match update.field_by_name("bid") {
        Some(FieldValue::Text(bid)) => println!("bid is {bid}"),
        Some(FieldValue::Null) => println!("there is no bid"),
        None => println!("this subscription has no `bid` field"),
    }
}

The three arms are the three answers, and they are not interchangeable: Text("") is a fourth thing again — a value that happens to be empty. Use is_empty_text to ask.

Variants

Null

The field has no value — the # marker [docs/spec/04-notifications.md §2.2].

Also what a field reads as before any update has ever set it, which is only observable between the subscription being activated and its first U: the first update of a subscription carries a token for every field [docs/spec/04-notifications.md §2.3].

Text(&'a str)

The field holds this text, which may be the empty string — the $ marker sets it to exactly that [docs/spec/04-notifications.md §2.2].

Implementations

impl<'a> FieldValue<'a>

pub const fn is_null(self) -> bool

Whether the field is null, i.e. has no value at all.

A field holding the empty string is not null.

pub fn is_empty_text(self) -> bool

Whether the field holds the empty string.

A null field is not empty text: it has no text.

pub const fn text(self) -> Option<&'a str>

The text, or None if the field is null.

Collapsing the distinction is a deliberate act here, which is the point: text() reads as “I am choosing to treat null as absent”, whereas an accessor that returned &str directly would make the same choice silently.

pub fn text_or(self, default: &'a str) -> &'a str

The text, substituting default if the field is null.

This is how a field value is rendered for a human: Display is deliberately not implemented, because every rendering of a null has to choose a spelling and no choice is safe to make on the caller’s behalf — the empty string collapses null into $, and any placeholder is a string a field could legitimately hold. Naming the substitute at the call site is a one-word cost and it keeps println!("{}", value.text_or("(null)")) honest.

Trait Implementations

impl<'a> Clone for FieldValue<'a>

fn clone(&self) -> FieldValue<'a>

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl<'a> Copy for FieldValue<'a>

impl<'a> Debug for FieldValue<'a>

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

Formats the value using the given formatter.

impl<'a> Eq for FieldValue<'a>

impl<'a> Hash for FieldValue<'a>

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<'a> PartialEq for FieldValue<'a>

fn eq(&self, other: &FieldValue<'a>) -> bool

Equality operator ==.

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

Inequality operator !=.

impl<'a> StructuralPartialEq for FieldValue<'a>