OffScreen - Add async example

- Old example is still there for reference, just not used by default
- Add AsyncContext/SingleThreadSynchronizationContext to ensure async calls continue on main thread.

CefSharp.MinimalExample.OffScreen/AsyncContext.cs

@@ -0,0 +1,38 @@
+/// https://devblogs.microsoft.com/pfxteam/await-synchronizationcontext-and-console-apps/
+
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace CefSharp.MinimalExample.OffScreen
+{
+    public static class AsyncContext
+    {
+        public static void Run(Func<Task> func)
+        {
+            var prevCtx = SynchronizationContext.Current;
+
+            try
+            {
+                var syncCtx = new SingleThreadSynchronizationContext();
+
+                SynchronizationContext.SetSynchronizationContext(syncCtx);
+
+                var t = func();
+
+                t.ContinueWith(delegate
+                {
+                    syncCtx.Complete();
+                }, TaskScheduler.Default);
+
+                syncCtx.RunOnCurrentThread();
+
+                t.GetAwaiter().GetResult();
+            }
+            finally
+            {
+                SynchronizationContext.SetSynchronizationContext(prevCtx);
+            }
+        }
+    }
+}

CefSharp.MinimalExample.OffScreen/CefSharp.MinimalExample.OffScreen.csproj

@@ -84,6 +84,9 @@
   <PropertyGroup>
     <ApplicationManifest>app.manifest</ApplicationManifest>
   </PropertyGroup>
+  <PropertyGroup>
+    <StartupObject>CefSharp.MinimalExample.OffScreen.Program</StartupObject>
+  </PropertyGroup>
   <ItemGroup>
     <Reference Include="CefSharp, Version=95.7.141.0, Culture=neutral, PublicKeyToken=40c4b6fc221f4138">
       <HintPath>..\packages\CefSharp.Common.95.7.141\lib\net452\CefSharp.dll</HintPath>
@@ -103,8 +106,10 @@
     <Reference Include="Microsoft.CSharp" />
   </ItemGroup>
   <ItemGroup>
+    <Compile Include="AsyncContext.cs" />
     <Compile Include="Program.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
+    <Compile Include="SingleThreadSynchronizationContext.cs" />
   </ItemGroup>
   <ItemGroup>
     <None Include="app.config" />

CefSharp.MinimalExample.OffScreen/Program.cs

@@ -1,4 +1,4 @@
-// Copyright © 2010-2015 The CefSharp Authors. All rights reserved.
+// Copyright © 2010-2021 The CefSharp Authors. All rights reserved.
 //
 // Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.
 
@@ -11,10 +11,19 @@ using System.Threading.Tasks;
 
 namespace CefSharp.MinimalExample.OffScreen
 {
+    /// <summary>
+    /// CefSharp.OffScreen Minimal Example
+    /// </summary>
     public class Program
     {
-        private static ChromiumWebBrowser browser;
+        /// <summary>
-
+        /// Asynchronous demo using CefSharp.OffScreen
+        /// Loads google.com, uses javascript to fill out the search box then takes a screenshot which is opened
+        /// in the default image viewer.
+        /// For a synchronous demo see <see cref="MainSync(string[])"/> below.
+        /// </summary>
+        /// <param name="args">args</param>
+        /// <returns>exit code</returns>
         public static int Main(string[] args)
         {
 #if ANYCPU
@@ -28,6 +37,106 @@ namespace CefSharp.MinimalExample.OffScreen
             Console.WriteLine("You may see Chromium debugging output, please wait...");
             Console.WriteLine();
 
+            //Console apps don't have a SynchronizationContext, so to ensure our await calls continue on the main thread we use a super simple implementation from
+            //https://devblogs.microsoft.com/pfxteam/await-synchronizationcontext-and-console-apps/
+            //Continuations will happen on the main thread. Cef.Initialize/Cef.Shutdown must be called on the same Thread.
+            //The Nito.AsyncEx.Context Nuget package has a more advanced implementation
+            //should you wish to use a pre-build implementation.
+            //https://github.com/StephenCleary/AsyncEx/blob/8a73d0467d40ca41f9f9cf827c7a35702243abb8/doc/AsyncContext.md#console-example-using-asynccontext
+            //NOTE: This is only required if you use await
+
+            AsyncContext.Run(async delegate
+            {
+                var settings = new CefSettings()
+                {
+                    //By default CefSharp will use an in-memory cache, you need to specify a Cache Folder to persist data
+                    CachePath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData), "CefSharp\\Cache")
+                };
+
+                //Perform dependency check to make sure all relevant resources are in our output directory.
+                var success = await Cef.InitializeAsync(settings, performDependencyCheck: true, browserProcessHandler: null);
+
+                if (!success)
+                {
+                    throw new Exception("Unable to initialize CEF, check the log file.");
+                }
+
+                // Create the CefSharp.OffScreen.ChromiumWebBrowser instance
+                using (var browser = new ChromiumWebBrowser(testUrl))
+                {
+                    var initialLoadResponse = await browser.WaitForInitialLoadAsync();
+
+                    if (!initialLoadResponse.Success)
+                    {
+                        throw new Exception(string.Format("Page load failed with ErrorCode:{0}, HttpStatusCode:{1}", initialLoadResponse.ErrorCode, initialLoadResponse.HttpStatusCode));
+                    }
+
+                    var response = await browser.EvaluateScriptAsync("document.querySelector('[name=q]').value = 'CefSharp Was Here!'");
+
+                    //Give the browser a little time to render
+                    await Task.Delay(500);
+                    // Wait for the screenshot to be taken.
+                    var bitmap = await browser.ScreenshotAsync();
+
+                    // File path to save our screenshot e.g. C:\Users\{username}\Desktop\CefSharp screenshot.png
+                    var screenshotPath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Desktop), "CefSharp screenshot.png");
+
+                    Console.WriteLine();
+                    Console.WriteLine("Screenshot ready. Saving to {0}", screenshotPath);
+
+                    // Save the Bitmap to the path.
+                    // The image type is auto-detected via the ".png" extension.
+                    bitmap.Save(screenshotPath);
+
+                    // We no longer need the Bitmap.
+                    // Dispose it to avoid keeping the memory alive. Especially important in 32-bit applications.
+                    bitmap.Dispose();
+
+                    Console.WriteLine("Screenshot saved. Launching your default image viewer...");
+
+                    // Tell Windows to launch the saved image.
+                    Process.Start(new ProcessStartInfo(screenshotPath)
+                    {
+                        // UseShellExecute is false by default on .NET Core.
+                        UseShellExecute = true
+                    });
+
+                    Console.WriteLine("Image viewer launched. Press any key to exit.");
+                }
+
+                // Wait for user to press a key before exit
+                Console.ReadKey();
+
+                // Clean up Chromium objects. You need to call this in your application otherwise
+                // you will get a crash when closing.
+                Cef.Shutdown();
+            });
+
+            return 0;
+        }
+
+        /// <summary>
+        /// Synchronous demo using CefSharp.OffScreen
+        /// Loads google.com, uses javascript to fill out the search box then takes a screenshot which is opened
+        /// in the default image viewer.
+        /// For a asynchronous demo see <see cref="Main(string[])"/> above.
+        /// To use this demo simply delete the <see cref="Main(string[])"/> method and rename this method to Main.
+        /// </summary>
+        /// <param name="args">args</param>
+        /// <returns>exit code</returns>
+        public static int MainSync(string[] args)
+        {
+#if ANYCPU
+            //Only required for PlatformTarget of AnyCPU
+            CefRuntime.SubscribeAnyCpuAssemblyResolver();
+#endif
+
+            const string testUrl = "https://www.google.com/";
+
+            Console.WriteLine("This example application will load {0}, take a screenshot, and save it to your desktop.", testUrl);
+            Console.WriteLine("You may see Chromium debugging output, please wait...");
+            Console.WriteLine();
+
             var settings = new CefSettings()
             {
                 //By default CefSharp will use an in-memory cache, you need to specify a Cache Folder to persist data
@@ -38,69 +147,76 @@ namespace CefSharp.MinimalExample.OffScreen
             Cef.Initialize(settings, performDependencyCheck: true, browserProcessHandler: null);
 
             // Create the offscreen Chromium browser.
-            browser = new ChromiumWebBrowser(testUrl);
+            var browser = new ChromiumWebBrowser(testUrl);
+
+            EventHandler<LoadingStateChangedEventArgs> handler = null;
+
+            handler = (s, e) =>
+            {
+                // Check to see if loading is complete - this event is called twice, one when loading starts
+                // second time when it's finished
+                if (!e.IsLoading)
+                {
+                    // Remove the load event handler, because we only want one snapshot of the page.
+                    browser.LoadingStateChanged -= handler;
+
+                    var scriptTask = browser.EvaluateScriptAsync("document.querySelector('[name=q]').value = 'CefSharp Was Here!'");
+
+                    scriptTask.ContinueWith(t =>
+                    {
+                        if(!t.Result.Success)
+                        {
+                            throw new Exception("EvaluateScriptAsync failed:" + t.Result.Message);
+                        }
+
+                        //Give the browser a little time to render
+                        Thread.Sleep(500);
+                        // Wait for the screenshot to be taken.
+                        var task = browser.ScreenshotAsync();
+                        task.ContinueWith(x =>
+                        {
+                            // File path to save our screenshot e.g. C:\Users\{username}\Desktop\CefSharp screenshot.png
+                            var screenshotPath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Desktop), "CefSharp screenshot.png");
+
+                            Console.WriteLine();
+                            Console.WriteLine("Screenshot ready. Saving to {0}", screenshotPath);
+
+                            // Save the Bitmap to the path.
+                            // The image type is auto-detected via the ".png" extension.
+                            task.Result.Save(screenshotPath);
+
+                            // We no longer need the Bitmap.
+                            // Dispose it to avoid keeping the memory alive.  Especially important in 32-bit applications.
+                            task.Result.Dispose();
+
+                            Console.WriteLine("Screenshot saved.  Launching your default image viewer...");
+
+                            // Tell Windows to launch the saved image.
+                            Process.Start(new ProcessStartInfo(screenshotPath)
+                            {
+                                // UseShellExecute is false by default on .NET Core.
+                                UseShellExecute = true
+                            });
+
+                            Console.WriteLine("Image viewer launched.  Press any key to exit.");
+                        }, TaskScheduler.Default);
+                    });
+                }
+            };
 
             // An event that is fired when the first page is finished loading.
             // This returns to us from another thread.
-            browser.LoadingStateChanged += BrowserLoadingStateChanged;
+            browser.LoadingStateChanged += handler;
 
             // We have to wait for something, otherwise the process will exit too soon.
             Console.ReadKey();
 
-            // Clean up Chromium objects.  You need to call this in your application otherwise
+            // Clean up Chromium objects. You need to call this in your application otherwise
             // you will get a crash when closing.
+            //The ChromiumWebBrowser instance will be disposed
             Cef.Shutdown();
 
             return 0;
         }
-
-        private static void BrowserLoadingStateChanged(object sender, LoadingStateChangedEventArgs e)
-        {
-            // Check to see if loading is complete - this event is called twice, one when loading starts
-            // second time when it's finished
-            // (rather than an iframe within the main frame).
-            if (!e.IsLoading)
-            {
-                // Remove the load event handler, because we only want one snapshot of the initial page.
-                browser.LoadingStateChanged -= BrowserLoadingStateChanged;
-
-                var scriptTask = browser.EvaluateScriptAsync("document.querySelector('[name=q]').value = 'CefSharp Was Here!'");
-
-                scriptTask.ContinueWith(t =>
-                {
-                    //Give the browser a little time to render
-                    Thread.Sleep(500);
-                    // Wait for the screenshot to be taken.
-                    var task = browser.ScreenshotAsync();
-                    task.ContinueWith(x =>
-                    {
-                        // Make a file to save it to (e.g. C:\Users\jan\Desktop\CefSharp screenshot.png)
-                        var screenshotPath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Desktop), "CefSharp screenshot.png");
-
-                        Console.WriteLine();
-                        Console.WriteLine("Screenshot ready. Saving to {0}", screenshotPath);
-
-                        // Save the Bitmap to the path.
-                        // The image type is auto-detected via the ".png" extension.
-                        task.Result.Save(screenshotPath);
-
-                        // We no longer need the Bitmap.
-                        // Dispose it to avoid keeping the memory alive.  Especially important in 32-bit applications.
-                        task.Result.Dispose();
-
-                        Console.WriteLine("Screenshot saved.  Launching your default image viewer...");
-
-                        // Tell Windows to launch the saved image.
-                        Process.Start(new ProcessStartInfo(screenshotPath)
-                        {
-                            // UseShellExecute is false by default on .NET Core.
-                            UseShellExecute = true
-                        });
-
-                        Console.WriteLine("Image viewer launched.  Press any key to exit.");
-                    }, TaskScheduler.Default);
-                });
-            }
-        }
     }
 }

CefSharp.MinimalExample.OffScreen/SingleThreadSynchronizationContext.cs

@@ -0,0 +1,32 @@
+/// https://devblogs.microsoft.com/pfxteam/await-synchronizationcontext-and-console-apps/
+
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Threading;
+
+namespace CefSharp.MinimalExample.OffScreen
+{
+    public sealed class SingleThreadSynchronizationContext : SynchronizationContext
+    {
+        private readonly BlockingCollection<KeyValuePair<SendOrPostCallback, object>> queue =
+            new BlockingCollection<KeyValuePair<SendOrPostCallback, object>>();
+
+        public override void Post(SendOrPostCallback d, object state)
+        {
+            queue.Add(new KeyValuePair<SendOrPostCallback, object>(d, state));
+        }
+
+        public void RunOnCurrentThread()
+        {
+            while (queue.TryTake(out var workItem, Timeout.Infinite))
+            {
+                workItem.Key(workItem.Value);
+            }
+        }
+
+        public void Complete()
+        {
+            queue.CompleteAdding();
+        }
+    }
+}