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//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2023 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
/// A lazily initializable atomic strong reference.
///
/// These values can be set (initialized) exactly once, but read many
/// times.
@available(SwiftStdlib 6.0, *)
@frozen
@_staticExclusiveOnly
public struct AtomicLazyReference<Instance: AnyObject>: ~Copyable {
@usableFromInline
let storage: Atomic<Unmanaged<Instance>?>
@available(SwiftStdlib 6.0, *)
@inlinable
public init() {
storage = Atomic<Unmanaged<Instance>?>(nil)
}
@inlinable
deinit {
if let unmanaged = storage.load(ordering: .acquiring) {
unmanaged.release()
}
}
}
@available(SwiftStdlib 6.0, *)
extension AtomicLazyReference {
/// Atomically initializes this reference if its current value is nil, then
/// returns the initialized value. If this reference is already initialized,
/// then `storeIfNil(_:)` discards its supplied argument and returns the
/// current value without updating it.
///
/// The following example demonstrates how this can be used to implement a
/// thread-safe lazily initialized reference:
///
/// class Image {
/// let _histogram = AtomicLazyReference<Histogram>()
///
/// // This is safe to call concurrently from multiple threads.
/// var atomicLazyHistogram: Histogram {
/// if let histogram = _histogram.load() { return histogram }
/// // Note that code here may run concurrently on
/// // multiple threads, but only one of them will get to
/// // succeed setting the reference.
/// let histogram = ...
/// return _histogram.storeIfNil(histogram)
/// }
/// }
///
/// - Note: This operation uses acquiring-and-releasing memory ordering.
///
/// - Parameter desired: A value of `Instance` that we will attempt to store
/// if the lazy reference is currently nil.
/// - Returns: The value of `Instance` that was successfully stored within the
/// lazy reference. This may or may not be the same value of `Instance` that
/// was passed to this function.
@available(SwiftStdlib 6.0, *)
public func storeIfNil(_ desired: consuming Instance) -> Instance {
let desiredUnmanaged = Unmanaged.passRetained(desired)
let (exchanged, current) = storage.compareExchange(
expected: nil,
desired: desiredUnmanaged,
ordering: .acquiringAndReleasing
)
if !exchanged {
// The reference has already been initialized. Balance the retain that we
// performed on 'desired'.
desiredUnmanaged.release()
return current!.takeUnretainedValue()
}
return desiredUnmanaged.takeUnretainedValue()
}
/// Atomically loads and returns the current value of this reference.
///
/// - Note: The load operation is performed with the memory ordering
/// `AtomicLoadOrdering.acquiring`.
///
/// - Returns: A value of `Instance` if the lazy reference was written to, or
/// `nil` if it has not been written to yet.
@available(SwiftStdlib 6.0, *)
public func load() -> Instance? {
let value = storage.load(ordering: .acquiring)
return value?.takeUnretainedValue()
}
}
@available(SwiftStdlib 6.0, *)
extension AtomicLazyReference: @unchecked Sendable where Instance: Sendable {}
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