Add support for alternate screen buffers

Closes #250

src/Spectre.Console/AnsiConsole.Screen.cs

@@ -0,0 +1,19 @@
+using System;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// A console capable of writing ANSI escape sequences.
+    /// </summary>
+    public static partial class AnsiConsole
+    {
+        /// <summary>
+        /// Switches to an alternate screen buffer if the terminal supports it.
+        /// </summary>
+        /// <param name="action">The action to execute within the alternate screen buffer.</param>
+        public static void AlternateScreen(Action action)
+        {
+            Console.AlternateScreen(action);
+        }
+    }
+}

src/Spectre.Console/AnsiConsoleFactory.cs

@@ -55,6 +55,7 @@ namespace Spectre.Console
             profile.Capabilities.Legacy = legacyConsole;
             profile.Capabilities.Interactive = interactive;
             profile.Capabilities.Unicode = encoding.EncodingName.ContainsExact("Unicode");
+            profile.Capabilities.AlternateBuffer = supportsAnsi && !legacyConsole;
 
             // Enrich the profile
             ProfileEnricher.Enrich(

src/Spectre.Console/Capabilities.cs

@@ -55,6 +55,12 @@ namespace Spectre.Console
         /// </summary>
         public bool Unicode { get; set; }
 
+        /// <summary>
+        /// Gets or sets a value indicating whether
+        /// or not the console supports alternate buffers.
+        /// </summary>
+        public bool AlternateBuffer { get; set; }
+
         /// <summary>
         /// Initializes a new instance of the
         /// <see cref="Capabilities"/> class.

src/Spectre.Console/Extensions/AnsiConsoleExtensions.Exclusive.cs

@@ -29,7 +29,7 @@ namespace Spectre.Console
         /// <returns>The result of the function.</returns>
         public static Task<T> RunExclusive<T>(this IAnsiConsole console, Func<Task<T>> func)
         {
-            return console.ExclusivityMode.Run(func);
+            return console.ExclusivityMode.RunAsync(func);
         }
     }
 }

src/Spectre.Console/Extensions/AnsiConsoleExtensions.Screen.cs

@@ -0,0 +1,48 @@
+using System;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// Contains extension methods for <see cref="IAnsiConsole"/>.
+    /// </summary>
+    public static partial class AnsiConsoleExtensions
+    {
+        /// <summary>
+        /// Switches to an alternate screen buffer if the terminal supports it.
+        /// </summary>
+        /// <param name="console">The console.</param>
+        /// <param name="action">The action to execute within the alternate screen buffer.</param>
+        public static void AlternateScreen(this IAnsiConsole console, Action action)
+        {
+            if (console is null)
+            {
+                throw new ArgumentNullException(nameof(console));
+            }
+
+            if (!console.Profile.Capabilities.Ansi)
+            {
+                throw new NotSupportedException("Alternate buffers are not supported since your terminal does not support ANSI.");
+            }
+
+            if (!console.Profile.Capabilities.AlternateBuffer)
+            {
+                throw new NotSupportedException("Alternate buffers are not supported by your terminal.");
+            }
+
+            console.ExclusivityMode.Run<object?>(() =>
+            {
+                // Switch to alternate screen
+                console.Write(new ControlCode("\u001b[?1049h\u001b[H"));
+
+                // Execute custom action
+                action();
+
+                // Switch back to primary screen
+                console.Write(new ControlCode("\u001b[?1049l"));
+
+                // Dummy result
+                return null;
+            });
+        }
+    }
+}

src/Spectre.Console/IAnsiConsoleInput.cs

@@ -9,6 +9,13 @@ namespace Spectre.Console
     /// </summary>
     public interface IAnsiConsoleInput
     {
+        /// <summary>
+        /// Gets a value indicating whether or not
+        /// there is a key available.
+        /// </summary>
+        /// <returns><c>true</c> if there's a key available, otherwise <c>false</c>.</returns>
+        bool IsKeyAvailable();
+
         /// <summary>
         /// Reads a key from the console.
         /// </summary>

src/Spectre.Console/IExclusivityMode.cs

@@ -22,6 +22,6 @@ namespace Spectre.Console
         /// <typeparam name="T">The result type.</typeparam>
         /// <param name="func">The func to run in exclusive mode.</param>
         /// <returns>The result of the function.</returns>
-        Task<T> Run<T>(Func<Task<T>> func);
+        Task<T> RunAsync<T>(Func<Task<T>> func);
     }
 }

src/Spectre.Console/Internal/DefaultExclusivityMode.cs

@@ -31,7 +31,7 @@ namespace Spectre.Console.Internal
             }
         }
 
-        public async Task<T> Run<T>(Func<Task<T>> func)
+        public async Task<T> RunAsync<T>(Func<Task<T>> func)
         {
             // Try acquiring the exclusivity semaphore
             if (!await _semaphore.WaitAsync(0).ConfigureAwait(false))

src/Spectre.Console/Internal/DefaultInput.cs

@@ -13,6 +13,16 @@ namespace Spectre.Console
             _profile = profile ?? throw new ArgumentNullException(nameof(profile));
         }
 
+        public bool IsKeyAvailable()
+        {
+            if (!_profile.Capabilities.Interactive)
+            {
+                throw new InvalidOperationException("Failed to read input in non-interactive mode.");
+            }
+
+            return System.Console.KeyAvailable;
+        }
+
         public ConsoleKeyInfo? ReadKey(bool intercept)
         {
             if (!_profile.Capabilities.Interactive)
@@ -20,16 +30,16 @@ namespace Spectre.Console
                 throw new InvalidOperationException("Failed to read input in non-interactive mode.");
             }
 
-            if (!System.Console.KeyAvailable)
-            {
-                return null;
-            }
-
             return System.Console.ReadKey(intercept);
         }
 
         public async Task<ConsoleKeyInfo?> ReadKeyAsync(bool intercept, CancellationToken cancellationToken)
         {
+            if (!_profile.Capabilities.Interactive)
+            {
+                throw new InvalidOperationException("Failed to read input in non-interactive mode.");
+            }
+
             while (true)
             {
                 if (cancellationToken.IsCancellationRequested)