typst_utils/
deferred.rs

1use std::sync::Arc;
2
3use once_cell::sync::OnceCell;
4
5/// A value that is lazily executed on another thread.
6///
7/// Execution will be started in the background and can be waited on.
8pub struct Deferred<T>(Arc<OnceCell<T>>);
9
10impl<T: Send + Sync + 'static> Deferred<T> {
11    /// Creates a new deferred value.
12    ///
13    /// The closure will be called on a secondary thread such that the value
14    /// can be initialized in parallel.
15    pub fn new<F>(f: F) -> Self
16    where
17        F: FnOnce() -> T + Send + Sync + 'static,
18    {
19        let inner = Arc::new(OnceCell::new());
20        let cloned = Arc::clone(&inner);
21        rayon::spawn(move || {
22            // Initialize the value if it hasn't been initialized yet.
23            // We do this to avoid panicking in case it was set externally.
24            cloned.get_or_init(f);
25        });
26        Self(inner)
27    }
28
29    /// Waits on the value to be initialized.
30    ///
31    /// If the value has already been initialized, this will return
32    /// immediately. Otherwise, this will block until the value is
33    /// initialized in another thread.
34    pub fn wait(&self) -> &T {
35        // Fast path if the value is already available. We don't want to yield
36        // to rayon in that case.
37        if let Some(value) = self.0.get() {
38            return value;
39        }
40
41        // Ensure that we yield to give the deferred value a chance to compute
42        // single-threaded platforms (for WASM compatibility).
43        while let Some(rayon::Yield::Executed) = rayon::yield_now() {}
44
45        self.0.wait()
46    }
47}
48
49impl<T> Clone for Deferred<T> {
50    fn clone(&self) -> Self {
51        Self(Arc::clone(&self.0))
52    }
53}
typst_utils/deferred.rs

typst_utils/
deferred.rs
