Fix #1863: Binding event to a value

src/Fable.Transforms/FSharp2Fable.Util.fs

@@ -415,16 +415,25 @@ module Patterns =
 
     /// This matches the boilerplate generated to wrap .NET events from F#
     let (|CreateEvent|_|) = function
-        | Call(Some(Call(None, createEvent,_,_,
-                        [Lambda(_eventDelegate, Call(Some callee, addEvent,[],[],[Value _eventDelegate']));
-                         Lambda(_eventDelegate2, Call(Some _callee2, _removeEvent,[],[],[Value _eventDelegate2']));
-                         Lambda(_callback, NewDelegate(_, Lambda(_delegateArg0, Lambda(_delegateArg1, Application(Value _callback',[],[Value _delegateArg0'; Value _delegateArg1'])))))])),
-                memb, typArgs, methTypArgs, args)
-                when createEvent.FullName = Types.createEvent ->
+        | Call(None,createEvent,_,_,
+               [Lambda(_eventDelegate, Call(Some callee, addEvent,[],[],[Value _eventDelegate']));
+                Lambda(_eventDelegate2, Call(Some _callee2, _removeEvent,[],[],[Value _eventDelegate2']));
+                Lambda(_callback, NewDelegate(_, Lambda(_delegateArg0, Lambda(_delegateArg1, Application(Value _callback',[],[Value _delegateArg0'; Value _delegateArg1'])))))])
+          when createEvent.FullName = Types.createEvent ->
             let eventName = addEvent.CompiledName.Replace("add_","")
+            Some (callee, eventName)
+        | _ -> None
+
+    let (|CallCreateEvent|_|) = function
+        | Call(Some(CreateEvent(callee, eventName)), memb, typArgs, methTypArgs, args) ->
             Some (callee, eventName, memb, typArgs, methTypArgs, args)
         | _ -> None
 
+    let (|BindCreateEvent|_|) = function
+        | Let((var, CreateEvent(value, eventName)), body) ->
+            Some (var, value, eventName, body)
+        | _ -> None
+
     let (|ConstructorCall|_|) = function
         | NewObject(baseCall, genArgs, baseArgs) -> Some(baseCall, genArgs, baseArgs)
         | Call(None, baseCall, genArgs1, genArgs2, baseArgs) when baseCall.IsConstructor ->

src/Fable.Transforms/FSharp2Fable.fs

@@ -367,14 +367,21 @@ let private transformExpr (com: IFableCompiler) (ctx: Context) fsExpr =
         let body = Fable.Let([ident1, id1Expr], Fable.Let([ident2, id2Expr], restExpr))
         return Fable.Let([tupleIdent, tupleExpr], body)
 
-    | CreateEvent (callee, eventName, memb, ownerGenArgs, membGenArgs, membArgs) ->
+    | CallCreateEvent (callee, eventName, memb, ownerGenArgs, membGenArgs, membArgs) ->
         let! callee = transformExpr com ctx callee
         let! args = transformExprList com ctx membArgs
         let callee = get None Fable.Any callee eventName
         let genArgs = ownerGenArgs @ membGenArgs |> Seq.map (makeType com ctx.GenericArgs)
         let typ = makeType com ctx.GenericArgs fsExpr.Type
         return makeCallFrom com ctx (makeRangeFrom fsExpr) typ false genArgs (Some callee) args memb
 
+    | BindCreateEvent (var, value, eventName, body) ->
+        let! value = transformExpr com ctx value
+        let value = get None Fable.Any value eventName
+        let ctx, ident = putBindingInScope com ctx var value
+        let! body = transformExpr com ctx body
+        return Fable.Let([ident, value], body)
+
     // TODO: Detect if it's ResizeArray and compile as FastIntegerForLoop?
     | ForOf (PutArgInScope com ctx (newContext, ident), value, body) ->
         let! value = transformExpr com ctx value

tests/Main/DateTimeTests.fs

@@ -543,4 +543,34 @@ let tests =
             equal 10 !res
             t.Stop()
         }
+
+    testCaseAsync "Assigning an event to a variable works" <| fun () -> // See #1863
+        let createTimerAndObservable timerInterval =
+            // setup a timer
+            let timer = new System.Timers.Timer(float timerInterval)
+            timer.AutoReset <- true
+            // events are automatically IObservable
+            let observable = timer.Elapsed
+            // return an async task
+            let task = async {
+                timer.Start()
+                do! Async.Sleep 200
+                timer.Stop()
+            }
+            // return a async task and the observable
+            (task,observable)
+        // create the timer and the corresponding observable
+        let basicTimer2 , timerEventStream = createTimerAndObservable 50
+
+        let mutable acc = 1
+        // register that everytime something happens on the
+        // event stream, print the time.
+        timerEventStream |> Observable.subscribe (fun _ ->
+            acc <- acc + 1) |>ignore
+
+        async {
+            do! basicTimer2
+            printfn "%i" acc
+            acc > 2 |> equal true
+        }
   ]