Overview

These pages show one way to satisfy the Contract on each platform. They are non-normative: where they differ from the Contract, the Contract wins. Treat the code as a starting point, not a copy-paste drop-in.

A conforming host, on any platform, must:

1. Expose the todayWebViewBridge channel as an object with a postMessage(message): Promise surface.
2. Dispatch on the message type and implement track, headers, and refreshToken per Capabilities.
3. Resolve undefined for any unknown type — never reject.
4. Reject with a non-empty error string only where the Contract allows it (refreshToken).
5. Coalesce concurrent refreshToken calls into a single real refresh.

The platform difference is entirely in how the promise-returning channel is constructed. Apple WebViews get it for free; Android and Windows wrap their transport in a thin shim that restores the postMessage(message): Promise surface.

• iOS — WKScriptMessageHandlerWithReply.
• macOS — the same WKWebView handler, hosted in AppKit.
• Android — WebViewCompat.addWebMessageListener or a @JavascriptInterface + callback shim.
• Windows — WebView2 host object or chrome.webview.postMessage + request-id shim.

Two platform pairs share a transport and so cannot be told apart from the channel shape alone — iOS vs macOS (both WKWebView) and Android vs Windows (both expose a global channel object). Each host MUST inject the platform tag to disambiguate its half of the pair.