Add parallel exporting

Closes #264

DiscordChatExporter.Gui/ViewModels/Dialogs/SettingsViewModel.cs

@@ -1,5 +1,6 @@
 using DiscordChatExporter.Core.Services;
 using DiscordChatExporter.Gui.ViewModels.Framework;
+using Tyrrrz.Extensions;
 
 namespace DiscordChatExporter.Gui.ViewModels.Dialogs
 {
@@ -25,6 +26,12 @@ namespace DiscordChatExporter.Gui.ViewModels.Dialogs
             set => _settingsService.IsTokenPersisted = value;
         }
 
+        public int ParallelLimit
+        {
+            get => _settingsService.ParallelLimit;
+            set => _settingsService.ParallelLimit = value.Clamp(1, 10);
+        }
+
         public SettingsViewModel(SettingsService settingsService)
         {
             _settingsService = settingsService;

DiscordChatExporter.Gui/ViewModels/RootViewModel.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using System.Net;
+using System.Threading;
 using System.Threading.Tasks;
 using DiscordChatExporter.Core.Models;
 using DiscordChatExporter.Core.Models.Exceptions;
@@ -260,10 +261,13 @@ namespace DiscordChatExporter.Gui.ViewModels
 
             // Export channels
             var successfulExportCount = 0;
-            for (var i = 0; i < dialog.Channels.Count; i++)
+            using var semaphore = new SemaphoreSlim(_settingsService.ParallelLimit.ClampMin(1));
+
+            await Task.WhenAll(dialog.Channels.Select(async (channel, i) =>
             {
                 var operation = operations[i];
-                var channel = dialog.Channels[i];
+
+                await semaphore.WaitAsync();
 
                 try
                 {
@@ -288,8 +292,9 @@ namespace DiscordChatExporter.Gui.ViewModels
                 finally
                 {
                     operation.Dispose();
+                    semaphore.Release();
                 }
-            }
+            }));
 
             // Notify of overall completion
             if (successfulExportCount > 0)

DiscordChatExporter.Gui/Views/Dialogs/SettingsView.xaml

@@ -56,6 +56,21 @@
                 IsChecked="{Binding IsTokenPersisted}" />
         </DockPanel>
 
+        <!--  Parallel limit  -->
+        <StackPanel Background="Transparent" ToolTip="How many channels can be exported at the same time">
+            <TextBlock Margin="16,8">
+                <Run Text="Parallel limit:" />
+                <Run Foreground="{DynamicResource PrimaryTextBrush}" Text="{Binding ParallelLimit, Mode=OneWay}" />
+            </TextBlock>
+            <Slider
+                Margin="16,8"
+                IsSnapToTickEnabled="True"
+                Maximum="10"
+                Minimum="1"
+                TickFrequency="1"
+                Value="{Binding ParallelLimit}" />
+        </StackPanel>
+
         <!--  Save button  -->
         <Button
             Margin="8"