rx_rust/operators/conditional_boolean/
default_if_empty.rs

1use crate::utils::types::MaybeSend;
2use crate::{
3    observable::{Observable, Subscription},
4    observer::{Flow, Observer, Termination},
5};
6use educe::Educe;
7
8/// Emits a specified item if the source Observable completes without emitting any items.
9/// See <https://reactivex.io/documentation/operators/defaultifempty.html>
10///
11/// # Examples
12/// ```rust
13/// use rx_rust::{
14///     observable::ObservableExt,
15///     observer::Termination,
16///     operators::{
17///         conditional_boolean::default_if_empty::DefaultIfEmpty,
18///         creating::from_iter::FromIter,
19///     },
20/// };
21///
22/// let mut values = Vec::new();
23/// let mut terminations = Vec::new();
24///
25/// let observable = DefaultIfEmpty::new(FromIter::new(Vec::<i32>::new()), 42);
26/// observable.subscribe_with_callback(
27///     |value| values.push(value),
28///     |termination| terminations.push(termination),
29/// );
30///
31/// assert_eq!(values, vec![42]);
32/// assert_eq!(terminations, vec![Termination::Completed]);
33/// ```
34#[derive(Educe)]
35#[educe(Debug, Clone)]
36pub struct DefaultIfEmpty<T, OE> {
37    source: OE,
38    default_value: T,
39}
40
41impl<T, OE> DefaultIfEmpty<T, OE> {
42    pub fn new<'or, E>(source: OE, item: T) -> Self
43    where
44        OE: Observable<'or, T, E>,
45    {
46        Self {
47            source,
48            default_value: item,
49        }
50    }
51}
52
53impl<'or, T, E, OE> Observable<'or, T, E> for DefaultIfEmpty<T, OE>
54where
55    OE: Observable<'or, T, E>,
56    T: MaybeSend + 'or,
57{
58    type D = OE::D;
59
60    fn subscribe(self, observer: impl Observer<T, E> + MaybeSend + 'or) -> Subscription<Self::D> {
61        let observer = DefaultIfEmptyObserver {
62            observer,
63            default_value: Some(self.default_value),
64        };
65        self.source.subscribe(observer)
66    }
67}
68
69struct DefaultIfEmptyObserver<T, OR> {
70    observer: OR,
71    default_value: Option<T>, // None means the source's got at least one value already.
72}
73
74impl<T, E, OR> Observer<T, E> for DefaultIfEmptyObserver<T, OR>
75where
76    OR: Observer<T, E>,
77{
78    fn on_next(&mut self, value: T) -> Flow {
79        self.default_value = None;
80        self.observer.on_next(value)
81    }
82
83    fn on_termination(mut self, termination: Termination<E>) {
84        // The final value ends the stream, so a downstream that stopped on it is not completed
85        // on top of that: it has already ended itself.
86        if matches!(termination, Termination::Completed)
87            && let Some(default_value) = self.default_value.take()
88            && self.observer.on_next(default_value).is_stop()
89        {
90            return;
91        }
92        self.observer.on_termination(termination);
93    }
94}
rx_rust/operators/conditional_boolean/default_if_empty.rs

rx_rust/operators/conditional_boolean/
default_if_empty.rs
