Add AsyncCallback.Mutex

Closes #815

Author: japgolly

core/src/main/scala/japgolly/scalajs/react/AsyncCallback.scala

@@ -327,6 +327,44 @@ object AsyncCallback {
       _ <- b.complete.when_(count <= 0)
     } yield new CountDownLatch(count, b)
 
+  // ===================================================================================================================
+
+  final class Mutex private[AsyncCallback]() {
+
+    private var mutex: Option[Barrier] =
+      None
+
+    private val release: Callback =
+      CallbackTo {
+        val old = mutex
+        mutex = None
+        old
+      }.flatMap(Callback.traverseOption(_)(_.complete))
+
+    /** Wrap a [[AsyncCallback]] so that it executes in the mutex.
+      *
+      * Note: THIS IS NOT RE-ENTRANT. Calling this from within the mutex will block.
+      */
+    def apply[A](ac: AsyncCallback[A]): AsyncCallback[A] =
+      byName {
+
+        mutex match {
+          case None =>
+            // Mutex empty
+            val b = barrier.runNow()
+            mutex = Some(b)
+            ac.finallyRunSync(release)
+
+          case Some(b) =>
+            // Mutex in use
+            b.await >> apply(ac)
+        }
+      }
+  }
+
+  /** Creates a new (non-reentrant) mutex. */
+  def mutex: CallbackTo[Mutex] =
+    CallbackTo(new Mutex)
 }
 
 // █████████████████████████████████████████████████████████████████████████████████████████████████████████████████████

doc/changelog/1.7.7.md

@@ -36,6 +36,18 @@ This entire release is focused on `AsyncCallback`.
     }
     ```
 
+  * Added `mutex` which returns a `AsyncCallback.Mutex`:
+
+    ```scala
+    AsyncCallback.Mutex {
+      /** Wrap a AsyncCallback so that it executes in the mutex.
+        *
+        * Note: THIS IS NOT RE-ENTRANT. Calling this from within the mutex will block.
+        */
+      def apply[A](ac: AsyncCallback[A]): AsyncCallback[A]
+    }
+    ```
+
   * You can now add a `_` suffix to the following to return an `AsyncCallback[Unit]` and be more efficient under-the-hood:
 
     * `traverse`

test/src/test/scala/japgolly/scalajs/react/core/AsyncCallbackTest.scala

@@ -236,5 +236,16 @@ object AsyncCallbackTest extends TestSuite {
       }
     }
 
+    "mutex" - asyncTest {
+      for {
+        mutex <- AsyncCallback.mutex.asAsyncCallback
+        b     <- AsyncCallback.barrier.asAsyncCallback
+        _     <- mutex(b.await).fork_.asAsyncCallback // mutex start
+        t     <- mutex(AsyncCallback.unit).timeoutMs(500).delayMs(1)
+        _     <- b.complete.asAsyncCallback // mutex end
+        _     <- mutex(AsyncCallback.unit)
+      } yield assert(t.isEmpty)
+    }
+
   }
 }