Add isComplete to AsyncCallback.Barrier

Closes #818

Author: japgolly

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

@@ -31,7 +31,15 @@ object AsyncCallback {
       p <- SyncPromise[A]
     } yield (AsyncCallback(p.onComplete), p.complete)
 
-  final case class Barrier(await: AsyncCallback[Unit], complete: Callback) {
+  final class Barrier(val await: AsyncCallback[Unit], completePromise: Callback) {
+
+    private var _complete = false
+
+    def complete: Callback =
+      completePromise.finallyRun(Callback { _complete = true })
+
+    def isComplete: CallbackTo[Boolean] =
+      CallbackTo(_complete)
 
     @inline
     @deprecated("Use .await", "1.7.7")
@@ -43,7 +51,7 @@ object AsyncCallback {
   lazy val barrier: CallbackTo[Barrier] =
     for {
       (promise, complete) <- promise[Unit]
-    } yield Barrier(promise, complete(tryUnit))
+    } yield new Barrier(promise, complete(tryUnit))
 
   def first[A](f: (Try[A] => Callback) => Callback): AsyncCallback[A] =
     new AsyncCallback(g => CallbackTo {

doc/changelog/1.7.7.md

@@ -1,13 +1,17 @@
-## 1.7.7
+# 1.7.7
 
 This entire release is focused on `AsyncCallback`.
 
-* In `AsyncCallback.Barrier`, renamed `waitForCompletion` to `await`.
-  `waitForCompletion` is still available but will result in a deprecation warning.
-  Automatic migration is below.
+* `AsyncCallback.Barrier`:
 
+  * Added `isComplete: CallbackTo[Boolean]` to synchronously query whether the barrier is complete or not.
 
-### Migration
+  * Renamed `waitForCompletion` to `await`.
+    `waitForCompletion` is still available but will result in a deprecation warning.
+    Automatic migration is below.
+
+
+## Migration
 
 ```sh
 find . -type f -name '*.scala' -exec perl -pi -e '

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

@@ -108,6 +108,13 @@ object AsyncCallbackTest extends TestSuite {
       }
     }
 
+    "barrier" - {
+      val b = AsyncCallback.barrier.runNow()
+      assertEq(b.isComplete.runNow(), false)
+      b.complete.runNow()
+      assertEq(b.isComplete.runNow(), true)
+    }
+
     "debounce" - {
       val t = new TestTimer
       var i = 0