Add support for displaying the version with wt --version (#5501)

## Summary of the Pull Request
Here's 3000 words:

![image](https://user-images.githubusercontent.com/18356694/80125877-e2af2700-8557-11ea-809a-63e50d76fe2b.png)
![image](https://user-images.githubusercontent.com/18356694/80125882-e5aa1780-8557-11ea-8f73-2e50c409b76d.png)
![image](https://user-images.githubusercontent.com/18356694/80125886-e8a50800-8557-11ea-8d28-6d1694c57c0c.png)

## References
* #4632 

## PR Checklist
* [x] Closes #5494
* [x] I work here
* [ ] Tests added/passed
* [n/a] Requires documentation to be updated

src/cascadia/TerminalApp/AppCommandlineArgs.cpp

@@ -2,6 +2,7 @@
 // Licensed under the MIT license.
 
 #include "pch.h"
+#include "AppLogic.h"
 #include "AppCommandlineArgs.h"
 #include "ActionArgs.h"
 #include <LibraryResources.h>
@@ -140,6 +141,11 @@ int AppCommandlineArgs::_handleExit(const CLI::App& command, const CLI::Error& e
     {
         _exitMessage = err.str();
     }
+
+    // We're displaying an error message - we should always exit instead of
+    // actually starting the Terminal.
+    _shouldExitEarly = true;
+
     return result;
 }
 
@@ -151,6 +157,22 @@ int AppCommandlineArgs::_handleExit(const CLI::App& command, const CLI::Error& e
 // - <none>
 void AppCommandlineArgs::_buildParser()
 {
+    auto versionCallback = [this](int64_t /*count*/) {
+        if (const auto appLogic{ winrt::TerminalApp::implementation::AppLogic::Current() })
+        {
+            // Set our message to display the application name and the current version.
+            _exitMessage = fmt::format("{0}\n{1}",
+                                       til::u16u8(appLogic->ApplicationDisplayName()),
+                                       til::u16u8(appLogic->ApplicationVersion()));
+            // Theoretically, we don't need to exit now, since this isn't really
+            // an error case. However, in practice, it feels weird to have `wt
+            // -v` open a new tab, and makes enough sense that `wt -v ;
+            // split-pane` (or whatever) just displays the version and exits.
+            _shouldExitEarly = true;
+        }
+    };
+    _app.add_flag_function("-v,--version", versionCallback, RS_A(L"CmdVersionDesc"));
+
     _buildNewTabParser();
     _buildSplitPaneParser();
     _buildFocusTabParser();
@@ -540,6 +562,19 @@ const std::string& AppCommandlineArgs::GetExitMessage()
     return _exitMessage;
 }
 
+// Method Description:
+// - Returns true if we should exit the application before even starting the
+//   window. We might want to do this if we're displaying an error message or
+//   the version string, or if we want to open the settings file.
+// Arguments:
+// - <none>
+// Return Value:
+// - true iff we should exit the application before even starting the window
+bool AppCommandlineArgs::ShouldExitEarly() const noexcept
+{
+    return _shouldExitEarly;
+}
+
 // Method Description:
 // - Ensure that the first command in our list of actions is a NewTab action.
 //   This makes sure that if the user passes a commandline like "wt split-pane

src/cascadia/TerminalApp/AppCommandlineArgs.h

@@ -36,6 +36,7 @@ public:
     void ValidateStartupCommands();
     std::deque<winrt::TerminalApp::ActionAndArgs>& GetStartupActions();
     const std::string& GetExitMessage();
+    bool ShouldExitEarly() const noexcept;
 
 private:
     static const std::wregex _commandDelimiterRegex;
@@ -79,6 +80,7 @@ private:
 
     std::deque<winrt::TerminalApp::ActionAndArgs> _startupActions;
     std::string _exitMessage;
+    bool _shouldExitEarly{ false };
 
     winrt::TerminalApp::NewTerminalArgs _getNewTerminalArgs(NewTerminalSubcommand& subcommand);
     void _addNewTerminalArgs(NewTerminalSubcommand& subcommand);

src/cascadia/TerminalApp/AppLogic.cpp

@@ -902,15 +902,24 @@ namespace winrt::TerminalApp::implementation
         return 0;
     }
 
-    winrt::hstring AppLogic::EarlyExitMessage()
+    winrt::hstring AppLogic::ParseCommandlineMessage()
     {
         if (_root)
         {
-            return _root->EarlyExitMessage();
+            return _root->ParseCommandlineMessage();
         }
         return { L"" };
     }
 
+    bool AppLogic::ShouldExitEarly()
+    {
+        if (_root)
+        {
+            return _root->ShouldExitEarly();
+        }
+        return false;
+    }
+
     winrt::hstring AppLogic::ApplicationDisplayName() const
     {
         try

src/cascadia/TerminalApp/AppLogic.h

@@ -28,7 +28,8 @@ namespace winrt::TerminalApp::implementation
         [[nodiscard]] std::shared_ptr<::TerminalApp::CascadiaSettings> GetSettings() const noexcept;
 
         int32_t SetStartupCommandline(array_view<const winrt::hstring> actions);
-        winrt::hstring EarlyExitMessage();
+        winrt::hstring ParseCommandlineMessage();
+        bool ShouldExitEarly();
 
         winrt::hstring ApplicationDisplayName() const;
         winrt::hstring ApplicationVersion() const;

src/cascadia/TerminalApp/AppLogic.idl

@@ -13,7 +13,8 @@ namespace TerminalApp
         MaximizedMode,
     };
 
-    [default_interface] runtimeclass AppLogic: IF7Listener {
+    [default_interface] runtimeclass AppLogic : IF7Listener
+    {
         AppLogic();
 
         // For your own sanity, it's better to do setup outside the ctor.
@@ -28,7 +29,8 @@ namespace TerminalApp
         Boolean IsElevated();
 
         Int32 SetStartupCommandline(String[] commands);
-        String EarlyExitMessage { get; };
+        String ParseCommandlineMessage { get; };
+        Boolean ShouldExitEarly { get; };
 
         void LoadSettings();
         Windows.UI.Xaml.UIElement GetRoot();

src/cascadia/TerminalApp/Resources/en-US/Resources.resw

@@ -232,6 +232,9 @@
     <value>Open in the given directory instead of the profile's set "startingDirectory"</value>
     <comment>{Locked="\"startingDirectory\""}</comment>
   </data>
+  <data name="CmdVersionDesc" xml:space="preserve">
+    <value>Display the application version</value>
+  </data>
   <data name="NewTabSplitButton.[using:Windows.UI.Xaml.Automation]AutomationProperties.HelpText" xml:space="preserve">
     <value>Press the button to open a new terminal tab with your default profile. Open the flyout to select which profile you want to open.</value>
   </data>

src/cascadia/TerminalApp/TerminalPage.cpp

@@ -1739,16 +1739,31 @@ namespace winrt::TerminalApp::implementation
     //   message. If there were no errors, this message will be blank.
     // - If the user requested help on any command (using --help), this will
     //   contain the help message.
+    // - If the user requested the version number (using --version), this will
+    //   contain the version string.
     // Arguments:
     // - <none>
     // Return Value:
     // - the help text or error message for the provided commandline, if one
     //   exists, otherwise the empty string.
-    winrt::hstring TerminalPage::EarlyExitMessage()
+    winrt::hstring TerminalPage::ParseCommandlineMessage()
     {
         return winrt::to_hstring(_appArgs.GetExitMessage());
     }
 
+    // Method Description:
+    // - Returns true if we should exit the application before even starting the
+    //   window. We might want to do this if we're displaying an error message or
+    //   the version string, or if we want to open the settings file.
+    // Arguments:
+    // - <none>
+    // Return Value:
+    // - true iff we should exit the application before even starting the window
+    bool TerminalPage::ShouldExitEarly()
+    {
+        return _appArgs.ShouldExitEarly();
+    }
+
     // Method Description:
     // - Returns a com_ptr to the implementation type of the tab at the given index
     // Arguments:

src/cascadia/TerminalApp/TerminalPage.h

@@ -50,7 +50,8 @@ namespace winrt::TerminalApp::implementation
         void CloseWindow();
 
         int32_t SetStartupCommandline(winrt::array_view<const hstring> args);
-        winrt::hstring EarlyExitMessage();
+        winrt::hstring ParseCommandlineMessage();
+        bool ShouldExitEarly();
 
         // -------------------------------- WinRT Events ---------------------------------
         DECLARE_EVENT_WITH_TYPED_EVENT_HANDLER(TitleChanged, _titleChangeHandlers, winrt::Windows::Foundation::IInspectable, winrt::hstring);

src/cascadia/TerminalApp/TerminalPage.idl

@@ -11,7 +11,8 @@ namespace TerminalApp
         TerminalPage();
 
         Int32 SetStartupCommandline(String[] commands);
-        String EarlyExitMessage { get; };
+        String ParseCommandlineMessage { get; };
+        Boolean ShouldExitEarly { get; };
 
         // XAML bound properties
         String ApplicationDisplayName { get; };

src/cascadia/WindowsTerminal/AppHost.cpp

@@ -102,7 +102,7 @@ void AppHost::_HandleCommandlineArgs()
             }
 
             const auto result = _logic.SetStartupCommandline({ args });
-            const auto message = _logic.EarlyExitMessage();
+            const auto message = _logic.ParseCommandlineMessage();
             if (!message.empty())
             {
                 const auto displayHelp = result == 0;
@@ -115,7 +115,10 @@ void AppHost::_HandleCommandlineArgs()
                             GetStringResource(messageTitle).data(),
                             MB_OK | messageIcon);
 
-                ExitProcess(result);
+                if (_logic.ShouldExitEarly())
+                {
+                    ExitProcess(result);
+                }
             }
         }
     }