In-App-Routing

const std = @import("std");
const ui = @import("ui");

const Page = enum { welcome, editor, settings };
const Router = ui.fx.Router(Page);

fn mountApp(cx: *ui.Cx, scope: *ui.Scope) anyerror!*ui.Node {
    // The router's two Signals are created in `scope`, so pass a scope that
    // lives as long as the window. The router value itself is large (~290 KB of
    // inline history), so keep it in Cx-owned state and pass the pointer around.
    const router = try cx.bindState(Router, try Router.init(scope, .welcome));

    const shell = try ui.box(cx, .{
        .width = .fill(),
        .height = .fill(),
        .direction = .column,
    }, .{});
    // ...append a toolbar and the page host (see "Rendering pages")
    _ = router;
    return shell;
}

var params: ui.fx.router.RouteParams = .{};
try params.set("file", "draft.md"); // copies the bytes; errors on overflow

router.navigate(.editor, params); // page + params
router.navigateTo(.settings); // page, empty params

_ = router.back(); // false when already at the oldest entry
_ = router.forward(); // false when there is no forward branch

if (router.canGoBack()) {
    // e.g. enable a Back button
}

const Nav = struct {
    router: *Router,

    fn goBack(self: *Nav) void {
        _ = self.router.back();
    }
};

const nav = try cx.bindState(Nav, .{ .router = router });
const back_button = try ui.widgets.Button(.{
    .label = "Back",
    .on_click = cx.on(Nav, nav, Nav.goBack),
}).mount(scope, cx);

const page_host = try ui.box(cx, .{
    .width = .fill(),
    .height = .fill(),
}, .{});

// Match appends the page to page_host and rebuilds it when router.current
// changes; each page gets a fresh child Scope that is disposed on leave.
_ = try ui.Match(Page, scope, page_host, router.current, cx, struct {
    fn build(child_scope: *ui.Scope, c: *ui.Cx, page: Page) anyerror!*ui.Node {
        return switch (page) {
            .welcome => mountWelcome(c, child_scope),
            .editor => mountEditor(c, child_scope),
            .settings => mountSettings(c, child_scope),
        };
    }
}.build);

// editor -> editor with other params does not rebuild the page,
// so subscribe to the params Signal inside the page.
try scope.createEffect(.{ .params = router.params }, struct {
    fn run(ctx: anytype) void {
        // get() returns a copy; keep it in a local so the slices stay valid.
        const params = ctx.params.get();
        const file = params.get("file") orelse "untitled.md";
        std.log.info("editor shows {s}", .{file});
    }
}.run);

router.navigate(.editor, params);
cx.console().scoped("router").debug("navigate -> {s}", .{@tagName(Page.editor)});