Perform listener invocations synchronously if we are on the target q

Sources/Pwomise/Pwomise.swift

@@ -18,6 +18,11 @@ public protocol PromiseAbuseDelegate {
 
 public var SharedPromiseAbuseDelegate: PromiseAbuseDelegate?
 
+typealias dispatch_get_current_queue_t = @convention(c) () -> Unmanaged<DispatchQueue>
+private let dispatch_get_current_queue = dlsym(dlopen(nil, RTLD_GLOBAL), "dispatch_get_current_queue").map {
+ unsafeBitCast($0, to: dispatch_get_current_queue_t.self)
+}
+
 public class Promise<Output>: CustomDebugStringConvertible {
  internal typealias Pending = PendingPromise<Output, Error>
  public typealias Completion = Result<Output, Error>
@@ -109,7 +114,9 @@ public class Promise<Output>: CustomDebugStringConvertible {
  }
  }
 
- internal var listeners: [(Completion) -> ()] = [] {
+ typealias Listener = (Completion) -> ()
+
+ internal var listeners: [Listener] = [] {
  didSet {
  guard listeners.count > 0 else {
  return
@@ -156,6 +163,31 @@ public class Promise<Output>: CustomDebugStringConvertible {
  result != .pending
  }
 
+ private func createExecutorFunction(_ result: Completion) -> ((@escaping Listener) -> ())? {
+ switch resolveQueue {
+ case is OS_dispatch_queue_concurrent:
+ // this should be scheduled concurrently, it must have an executor
+ break
+ case let queue as DispatchQueue:
+ if dispatch_get_current_queue?().takeUnretainedValue() == queue {
+ // same queue, passthrough
+ return nil
+ }
+ case let runLoop as RunLoop:
+ if RunLoop.current == runLoop {
+ // same runloop, passthrough
+ return nil
+ }
+ default:
+ break
+ }
+ return { listener in
+ self.resolveQueue.schedule {
+ listener(result)
+ }
+ }
+ }
+
  private func emit() {
  guard case .resolved(let result) = result, listeners.count > 0 else {
  // Not resolved or no listeners
@@ -166,13 +198,12 @@ public class Promise<Output>: CustomDebugStringConvertible {
  let listeners = listeners
  self.listeners = []
 
- for listener in listeners {
- resolveQueue.schedule {
- listener(result)
- }
+ if let executor = createExecutorFunction(result) {
+ listeners.forEach(executor)
+ resolveQueue.wake()
+ } else {
+ listeners.forEach { $0(result) }
  }
-
- resolveQueue.wake()
  }
 
  // Changes the RunLoop downstream listeners are invoked on