Web view JavaScript evaluation

Bind a JsHandle to the web view, then evaluate after its page loads:

let js = JsHandle::new();
let view = web_view_inline(res::assets::site).js(js);
// In an event handler:
day::task(async move {
    match js.eval("document.title").await {
        Ok(json) => { /* JSON string, including its quotes */ }
        Err(error) => { /* unavailable view, script error, or engine failure */ }
    }
});

The caller supplies trusted code. Do not concatenate untrusted strings into scripts; serialize values with a JSON encoder. Returned values are serialized as JSON. undefined becomes null; exceptions and values that cannot be serialized become errors. This is synchronous JavaScript evaluation, not an API for awaiting JavaScript promises.

Each request has its own numeric ID. A reply contains either 1␟<JSON> or 0␟<error name>␟<message> (␟ denotes U+001F). Backends remove their native string/JSON wrapper before delivering this envelope. Replies are separate from URL changes and external-link events. Disposing the Day node invalidates its handle; callers must handle errors when navigating away.

eval_support() reports whether an evaluation implementation exists. Browser same-origin rules still apply: a bundled site works; an inaccessible cross-origin frame returns an error. Remote sites can also refuse iframe embedding. This API does not bypass those restrictions.

The Windows GTK/Qt child hosts are not runtime-verified on Windows. They require the WebView2 Runtime. The common Rust host cross-checks against x86_64-pc-windows-msvc; the Qt hosting code has a C++ syntax check. Linux GTK’s API is checked against the installed webkit6 crate source. Harmony requires a compatible ArkWeb engine; a successful Rust build alone does not verify it.

Testing

In the day-piece-webview repository, cargo test --lib covers encoding, error replies and native path handling. node --test tests/browser.mjs checks browser link routing and same-origin evaluation/error replies. The demo’s dayscript/webview.yaml checks the actual bundled page and return path:

- web_eval:
    id: webview
    script: "document.getElementById('script-status').textContent"
    text: "The bundled script ran."
    timeout_secs: 30

Use text for an exact result or contains for a substring. An omitted assertion only checks that evaluation succeeds. For rendered-content tests, also assert dimensions and inspect a screenshot: a page can execute JavaScript while its content has no visible layout space.