Add getWebViewVersion() method (#197)

* Add method WebviewController.getWebViewVersion

* Use Utf8FromUtf16 instead of CW2A

* Update README.md

Author: Doflatango

README.md

@@ -7,7 +7,8 @@ A [Flutter](https://flutter.dev/) WebView plugin for Windows built on [Microsoft
 
 
 ### Target platform requirements
-- [WebView2 Runtime](https://developer.microsoft.com/en-us/microsoft-edge/webview2/)
+- [WebView2 Runtime](https://developer.microsoft.com/en-us/microsoft-edge/webview2/)  
+  Before initializing the webview, call `getWebViewVersion()` to check whether the required **WebView2 Runtime** is installed or not on the current system. If `getWebViewVersion()` returns null, guide your user to install **WebView2 Runtime** from this [page](https://developer.microsoft.com/en-us/microsoft-edge/webview2/).
 - Windows 10 1809+
 
 ### Development platform requirements

lib/src/webview.dart

@@ -83,6 +83,13 @@ class WebviewController extends ValueNotifier<WebviewValue> {
     });
   }
 
+  /// Get the browser version info including channel name if it is not the
+  /// WebView2 Runtime.
+  /// Returns [null] if the webview2 runtime is not installed.
+  static Future<String?> getWebViewVersion() async {
+    return _pluginChannel.invokeMethod<String>('getWebViewVersion');
+  }
+
   late Completer<void> _creatingCompleter;
   int _textureId = 0;
   bool _isDisposed = false;

windows/webview_windows_plugin.cc

@@ -12,6 +12,7 @@
 #include "webview_bridge.h"
 #include "webview_host.h"
 #include "webview_platform.h"
+#include "util/string_converter.h"
 
 #pragma comment(lib, "dxgi.lib")
 #pragma comment(lib, "d3d11.lib")
@@ -21,6 +22,7 @@ namespace {
 constexpr auto kMethodInitialize = "initialize";
 constexpr auto kMethodDispose = "dispose";
 constexpr auto kMethodInitializeEnvironment = "initializeEnvironment";
+constexpr auto kMethodGetWebViewVersion = "getWebViewVersion";
 
 constexpr auto kErrorCodeInvalidId = "invalid_id";
 constexpr auto kErrorCodeEnvironmentCreationFailed =
@@ -140,6 +142,16 @@ void WebviewWindowsPlugin::HandleMethodCall(
     return result->Success();
   }
 
+  if (method_call.method_name().compare(kMethodGetWebViewVersion) == 0) {
+    LPWSTR version_info = nullptr;
+    auto hr = GetAvailableCoreWebView2BrowserVersionString(nullptr, &version_info);
+    if (SUCCEEDED(hr) && version_info != nullptr) {
+      return result->Success(flutter::EncodableValue(util::Utf8FromUtf16(version_info)));
+    } else {
+      return result->Success();
+    }
+  }
+
   if (method_call.method_name().compare(kMethodInitialize) == 0) {
     return CreateWebviewInstance(std::move(result));
   }