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Box.swift
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Box.swift
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import Dispatch
enum Sealant<R> {
case pending(Handlers<R>)
case resolved(R)
}
final class Handlers<R> {
var bodies: [(R) -> Void] = []
func append(_ item: @escaping(R) -> Void) { bodies.append(item) }
}
/// - Remark: not protocol ∵ http://www.russbishop.net/swift-associated-types-cont
class Box<T> {
func inspect() -> Sealant<T> { fatalError() }
func inspect(_: (Sealant<T>) -> Void) { fatalError() }
func seal(_: T) {}
}
final class SealedBox<T>: Box<T> {
let value: T
init(value: T) {
self.value = value
}
override func inspect() -> Sealant<T> {
return .resolved(value)
}
}
final class EmptyBox<T>: Box<T> {
private var sealant = Sealant<T>.pending(.init())
private let barrier = DispatchQueue(label: "org.promisekit.barrier", attributes: .concurrent)
override func seal(_ value: T) {
var handlers: Handlers<T>!
barrier.sync(flags: .barrier) {
guard case .pending(let _handlers) = self.sealant else {
return // already fulfilled!
}
handlers = _handlers
self.sealant = .resolved(value)
}
//FIXME we are resolved so should `pipe(to:)` be called at this instant, “thens are called in order” would be invalid
//NOTE we don’t do this in the above `sync` because that could potentially deadlock
//THOUGH since `then` etc. typically invoke after a run-loop cycle, this issue is somewhat less severe
if let handlers = handlers {
handlers.bodies.forEach{ $0(value) }
}
//TODO solution is an unfortunate third state “sealed” where then's get added
// to a separate handler pool for that state
// any other solution has potential races
}
override func inspect() -> Sealant<T> {
var rv: Sealant<T>!
barrier.sync {
rv = self.sealant
}
return rv
}
override func inspect(_ body: (Sealant<T>) -> Void) {
var sealed = false
barrier.sync(flags: .barrier) {
switch sealant {
case .pending:
// body will append to handlers, so we must stay barrier’d
body(sealant)
case .resolved:
sealed = true
}
}
if sealed {
// we do this outside the barrier to prevent potential deadlocks
// it's safe because we never transition away from this state
body(sealant)
}
}
}
extension Optional where Wrapped: DispatchQueue {
@inline(__always)
func async(flags: DispatchWorkItemFlags?, _ body: @escaping() -> Void) {
switch self {
case .none:
body()
case .some(let q):
if let flags = flags {
q.async(flags: flags, execute: body)
} else {
q.async(execute: body)
}
}
}
}