Fix #1785 CancellationToken.Register

src/Fable.Transforms/Replacements.fs

@@ -2371,6 +2371,7 @@ let cancels (_: ICompiler) (ctx: Context) r t (i: CallInfo) (thisArg: Expr optio
         Helper.CoreCall("Async", Naming.removeGetSetPrefix i.CompiledName |> Naming.lowerFirst, t, args, argTypes, ?loc=r) |> Some
     // TODO: Add check so CancellationTokenSource cannot be cancelled after disposed?
     | "Dispose" -> Null Type.Unit |> makeValue r |> Some
+    | "Register" -> Helper.InstanceCall(thisArg.Value, "register", t, args, i.SignatureArgTypes, ?loc=r) |> Some
     | _ -> None
 
 let monitor (_: ICompiler) (ctx: Context) r t (i: CallInfo) (thisArg: Expr option) (args: Expr list) =

src/fable-library/AsyncBuilder.ts

@@ -36,6 +36,11 @@ export class CancellationToken {
     removeListener(id: number) {
         return this._listeners.delete(id);
     }
+    register(f: (state?: any)=>void, state?: any) {
+        const $ = this;
+        const id = this.addListener(state == null ? f : () => f(state));
+        return { Dispose() { $.removeListener(id); } }
+    }
 }
 
 export class OperationCanceledError extends Error {

tests/Main/AsyncTests.fs

@@ -22,6 +22,12 @@ type Get =
     | GetZero of replyChannel: AsyncReplyChannel<int>
     | GetOne of replyChannel: AsyncReplyChannel<int>
 
+let sleepAndAssign token res =
+    Async.StartImmediate(async {
+        do! Async.Sleep 200
+        res := true
+    }, token)
+
 let tests =
   testList "Async" [
     testCase "Simple async translates without exception" <| fun () ->
@@ -112,11 +118,6 @@ let tests =
     #if FABLE_COMPILER
     testCaseAsync "Async cancellation works" <| fun () ->
         async {
-            let sleepAndAssign token res =
-                Async.StartImmediate(async {
-                    do! Async.Sleep 200
-                    res := true
-                }, token)
             let res1, res2, res3 = ref false, ref false, ref false
             let tcs1 = new System.Threading.CancellationTokenSource(50)
             let tcs2 = new System.Threading.CancellationTokenSource()
@@ -131,6 +132,19 @@ let tests =
             equal false !res2
             equal true !res3
         }
+
+    testCaseAsync "CancellationTokenSource.Register works" <| fun () ->
+        async {
+            let mutable x = 0
+            let res1 = ref false
+            let tcs1 = new System.Threading.CancellationTokenSource(50)
+            let foo = tcs1.Token.Register(fun () ->
+                x <- x + 1)
+            sleepAndAssign tcs1.Token res1
+            do! Async.Sleep 500
+            equal false !res1
+            equal 1 x
+        }
     #endif
 
     testCase "Async.StartWithContinuations works" <| fun () ->