From e5f29a83690072e92d1beecd3eea9a697fa8e3a3 Mon Sep 17 00:00:00 2001
From: Andrii Chebukin <xperiandri@live.ru>
Date: Tue, 31 Aug 2021 22:47:05 +0300
Subject: [PATCH] Improved OneWaySeq incremental loading sample

---
 src/Samples/Samples/OneWaySeq.fs | 46 +++++++++++++++++++++++---------
 1 file changed, 34 insertions(+), 12 deletions(-)

diff --git a/src/Samples/Samples/OneWaySeq.fs b/src/Samples/Samples/OneWaySeq.fs
index 1b897ab0..9ce17d5e 100644
--- a/src/Samples/Samples/OneWaySeq.fs
+++ b/src/Samples/Samples/OneWaySeq.fs
@@ -9,32 +9,54 @@ open Elmish.Uno
 type Model =
   { OneWaySeqNumbers: int list
     OneWayNumbers: int list
-    IncrementalLoadingNumbers: int FlatList }
+    IncrementalLoadingNumbers: int FlatList
+    IsLoading: bool }
 
 let initial =
   { OneWaySeqNumbers = [ 1000..-1..1 ]
     OneWayNumbers = [ 1000..-1..1 ]
-    IncrementalLoadingNumbers = [ 1..1..10 ] |> FlatList.ofSeq }
+    IncrementalLoadingNumbers = [ 1..1..10 ] |> FlatList.ofSeq
+    IsLoading = false }
 
-let init () = initial
+let init () = initial, Cmd.none
 
 type Msg =
   | AddOneWaySeqNumber
   | AddOneWayNumber
   | LoadMore of count: uint * tcs: TaskCompletionSource<uint>
+  | LoadedMore of allItems: FlatList<int>
+  | ErrorLoadingMore
 
-let update msg m =
-  match msg with
-  | AddOneWaySeqNumber -> { m with OneWaySeqNumbers = m.OneWaySeqNumbers.Head + 1 :: m.OneWaySeqNumbers }
-  | AddOneWayNumber -> { m with OneWayNumbers = m.OneWayNumbers.Head + 1 :: m.OneWayNumbers }
-  | LoadMore (count, tcs) ->
+let asyncLoadItems (tcs: uint TaskCompletionSource) (items: FlatList<int>) count = async {
+  try
     let intCount = int count
-    let builder = m.IncrementalLoadingNumbers.ToBuilder()
-    let max = FlatList.last m.IncrementalLoadingNumbers
+    let builder = items.ToBuilder()
+    let max = FlatList.last items
     for i = max + 1 to max + intCount do
       builder.Add(i)
+    // Must be called to complete Task and unblock UI to update
+    // Otherwise it will block loading async operaiton and it will never be called again
     tcs.SetResult(count)
-    { m with IncrementalLoadingNumbers = builder.ToImmutable() }
+    return LoadedMore <| builder.ToImmutable ()
+  with ex ->
+    // Must be called to complete Task and unblock UI to update
+    // Otherwise it will block loading async operaiton and it will never be called again
+    tcs.SetException ex
+    return ErrorLoadingMore
+ }
+
+let update msg m =
+  match msg with
+  | AddOneWaySeqNumber -> { m with OneWaySeqNumbers = m.OneWaySeqNumbers.Head + 1 :: m.OneWaySeqNumbers }, Cmd.none
+  | AddOneWayNumber -> { m with OneWayNumbers = m.OneWayNumbers.Head + 1 :: m.OneWayNumbers }, Cmd.none
+  | LoadMore (count, tcs) ->
+                        // If error hapened earlier it may be worth to do someting with that instead
+                        { m with IsLoading = true }, // Display some hint (indetermined progress bar)
+                        asyncLoadItems tcs m.IncrementalLoadingNumbers count |> Cmd.OfAsync.result
+  | LoadedMore items -> { m with
+                            IsLoading = false // Hide loading indicator
+                            IncrementalLoadingNumbers = items }, Cmd.none
+  | ErrorLoadingMore -> m, Cmd.none // add error to model that will appear on IU
 
 let bindings : Binding<Model, Msg> list = [
   "OneWaySeqNumbers" |> Binding.oneWaySeq ((fun m -> m.OneWaySeqNumbers), (=), id)
@@ -49,7 +71,7 @@ let designModel = initial
 
 [<CompiledName("Program")>]
 let program =
-  Program.mkSimpleUno init update bindings
+  Program.mkProgramUno init update bindings
   |> Program.withConsoleTrace
 
 [<CompiledName("Config")>]