Merge pull request #224 from TimLariviere/cmd-option

Optional commands (Cmd.ofMsgOption / Cmd.ofAsyncMsgOption)

docs/update.md

@@ -127,3 +127,45 @@ let backgroundCmd =
         return msg
     })
 ```
+
+Optional commands
+------
+
+There might be cases where before a message is sent, you need to check if you want to send it (e.g. check user's preferences, ask user's permission, ...)
+
+Fabulous has 2 helper functions for this:
+
+- `Cmd.ofMsgOption`
+
+```fsharp
+let autoSaveCmd =
+    match userPreference.IsAutoSaveEnabled with
+    | false -> None
+    | true ->
+        autoSave()
+        Some Msg.AutoSaveDone
+
+let update msg model =
+    match msg with
+    | TimedTick -> model, (Cmd.ofMsgOption autoSaveCmd)
+    | AutoSaveDone -> ...
+```
+
+- `Cmd.ofAsyncMsgOption`
+
+```fsharp
+let takePictureCmd = async {
+    try
+        let! picture = takePictureAsync()
+        Some (Msg.PictureTaken picture)
+    with
+    | exn ->
+        do! displayAlert("Exception: " + exn.Message)
+        None
+}
+
+let update msg model =
+    match msg with
+    | TakePicture -> model, (Cmd.ofAsyncMsgOption takePictureCmd)
+    | PictureTaken -> ...
+```

src/Fabulous.Core/ElmishCmd.fs

@@ -21,6 +21,13 @@ module Cmd =
     let ofMsg (msg:'msg) : Cmd<'msg> =
         [fun dispatch -> dispatch msg]
 
+    /// Command to issue a specific message, only when Option.IsSome = true
+    let ofMsgOption (msg:'msg option) : Cmd<'msg> =
+        [ fun dispatch ->
+             match msg with
+             | None -> ()
+             | Some msg -> dispatch msg ]
+
     /// When emitting the message, map to another type
     let map (f: 'a -> 'msg) (cmd: Cmd<'a>) : Cmd<'msg> =
         cmd |> List.map (fun g -> (fun dispatch -> f >> dispatch) >> g)
@@ -35,8 +42,17 @@ module Cmd =
 
     let dispatch d (cmd: Cmd<_>) = for sub in cmd do sub d
 
+    /// Command to issue a message at the end of an asynchronous task
     let ofAsyncMsg (p: Async<'msg>) : Cmd<'msg> =
         [ fun dispatch -> async { let! msg = p in dispatch msg } |> Async.StartImmediate ]
+
+    /// Command to issue a message at the end of an asynchronous task, only when Option.IsSome = true
+    let ofAsyncMsgOption (p: Async<'msg option>) : Cmd<'msg> =
+        [ fun dispatch -> async { 
+            let! msg = p
+            match msg with
+            | None -> ()
+            | Some msg -> dispatch msg } |> Async.StartImmediate ]
 
     //let ofAsyncMsgs p : Cmd<_> =
     //    [ fun dispatch -> p |> AsyncSeq.iter dispatch |> Async.StartImmediate ]