jsaddle-webview2

Run jsaddle apps in a native Windows window hosting a Microsoft Edge WebView2 control — the Windows sibling of jsaddle-wkwebview (macOS/iOS, WebKit) and jsaddle-webkit2gtk (Linux, WebKitGTK).

import qualified Language.Javascript.JSaddle.WebView2 as WV2

main :: IO ()
main = WV2.run myJsmApp

How it works

The C shim (cbits/WebView2Shim.c, plain C COM against the official WebView2.h) creates a Win32 window, loads WebView2Loader.dll dynamically, creates the WebView2 environment/controller, and pumps the message loop. The jsaddle transport mirrors jsaddle-wkwebview exactly:

• Haskell → JS: batches are ExecuteScripted (marshalled to the UI thread with PostMessage, since WebView2 is single-threaded).
• JS → Haskell (async): window.chrome.webview.postMessage(...) → WebMessageReceived.
• JS → Haskell (sync): window.prompt("JSaddleSync", <json>). Default script dialogs are disabled, so the ScriptDialogOpening event fires on the UI thread while the page's JS is blocked in prompt(); the handler calls into Haskell, which computes the reply batch, and completes the dialog with put_ResultText + Accept. (Same trick as wkwebview's runJavaScriptTextInputPanelWithPrompt — which is why the WKWebView uiDelegate must never be overridden there, and why default script dialogs must stay disabled here.)

Build requirements

Only WebView2.h (and the headers it includes) is needed at compile time; nothing from the WebView2 SDK is linked. Get it from the Microsoft.Web.WebView2 NuGet package (it's a plain zip): build/native/include/WebView2.h. Point the include path at it, e.g. in cabal.project:

package jsaddle-webview2
  extra-include-dirs: C:\path\to\webview2-sdk\build\native\include

With nix, a small fetchzip derivation does it (license: unfree but freely redistributable):

webview2-sdk = pkgs.fetchzip {
  url = "https://www.nuget.org/api/v2/package/Microsoft.Web.WebView2/1.0.4022.49";
  extension = "zip";
  stripRoot = false;
  hash = "...";
};
# then: extra-include-dirs = [ "${webview2-sdk}/build/native/include" ];

The header is MIDL-generated flat COM and compiles fine under mingw-w64 (GHC's Windows toolchain) — no MSVC needed.

Runtime requirements

• The WebView2 runtime — preinstalled on Windows 11, Evergreen-installed on most Windows 10 machines (bootstrapper for the rest).
• WebView2Loader.dll next to the executable — from the same NuGet package (runtimes/win-x64/native/WebView2Loader.dll). It is LoadLibrary'd at startup, so no import library is needed at link time.

Notes and limitations

• runHTML uses NavigateToString: page origin is about:blank and the HTML is capped at 2MB. Use runURL (e.g. a file:// URL or SetVirtualHostNameToFolderMapping in a future version) for real apps.
• Page reloads are not supported (jsaddle state does not survive them); only the first completed navigation starts the jsaddle app.
• The user-data folder defaults to %LOCALAPPDATA%\jsaddle-webview2.
• The sync bridge rides window.prompt, so apps must not intercept ScriptDialogOpening themselves, and very large sync payloads depend on Chromium passing dialog text through unclamped.