/**
 * Core Web Vitals, measured by Google's `web-vitals` library. The SDK does
 * not depend on the library: the app passes it in (`webVitals` option), so
 * web vitals are opt-in and cost nothing for apps that do not use them.
 * Only the shape below is read, so a 5.x or 6.x build works, including
 * the IIFE build's `window.webVitals`.
 *
 * - One report per hard page load, sent when the page is first hidden (tab
 *   switch, close, navigation away).
 * - The library is called with `reportAllChanges`, so it reports every new
 *   CLS, INP and LCP value as it happens and the collector always holds the
 *   latest one. Without it the library reports those only on
 *   `visibilitychange`, and on a navigation or a close the browser fires
 *   `pagehide` first (beforeunload, pagehide, visibilitychange, unload): a
 *   report made at `pagehide` would lose them.
 * - On a tab switch, the library's own final reports come from its hidden
 *   listener on `window` in the capture phase (5.x and 6.x). Ours is on
 *   `window` in the capture phase too, added after the library's, so it
 *   runs right after it, and before the page-leave flush (a `document`
 *   listener): the report goes out in the same keepalive request as the
 *   buffered events. The capture phase also works where `visibilitychange`
 *   does not bubble. The SDK needs web-vitals 5.x or later.
 * - SPA route changes and back/forward cache restores are not reported on
 *   their own; CLS and INP cover the whole life of the page, as CrUX does.
 * - A metric the browser does not support is left out, not reported as 0.
 */
/** The fields of a `web-vitals` Metric that the SDK reads. */
export type WebVitalsMetric = {
    name: string;
    value: number;
    navigationType?: string;
    navigationURL?: string;
};
type OnMetric = (onReport: (metric: WebVitalsMetric) => void, opts?: Record<string, unknown>) => unknown;
/** The `web-vitals` module, or any object with its `on*` functions. */
export type WebVitalsLibrary = {
    onLCP?: OnMetric;
    onINP?: OnMetric;
    onCLS?: OnMetric;
    onFCP?: OnMetric;
    onTTFB?: OnMetric;
};
export type WebVitalsMetrics = {
    lcp?: number;
    inp?: number;
    cls?: number;
    fcp?: number;
    ttfb?: number;
};
/** Receives the report. Returning exactly `false` means it was not accepted (not sent). */
export type WebVitalsReporter = (report: WebVitalsReport) => unknown;
export type WebVitalsReport = {
    metrics: WebVitalsMetrics;
    /** The library's navigationType: navigate, reload, back-forward, prerender, restore. */
    navigationType: string;
    /** The URL the page was loaded with, before any SPA route change. */
    url: string;
    /** Epoch ms of the navigation start. */
    startTime: number;
};
/** True when `value` has at least one of the library's `on*` functions. */
export declare const isWebVitalsLibrary: (value: unknown) => value is WebVitalsLibrary;
export declare class WebVitalsCollector {
    private readonly key;
    private readonly metrics;
    private readonly disposers;
    private readonly startTime;
    private url;
    private navigationType;
    private reported;
    /** Cleared by stop(): the library keeps its callbacks, which must not keep the SDK alive. */
    private onReport?;
    /**
     * Start measuring with the library the app passed in. Returns undefined
     * when there is nothing to measure with (no library, no browser).
     */
    static start(library: unknown, onReport: WebVitalsReporter, 
    /** One report per document for each key (the SDK passes its write key). */
    key?: string): WebVitalsCollector | undefined;
    /**
     * The URL the document was loaded with. The navigation entry keeps it even
     * when an SPA changed the route before the SDK started; `location` is the
     * fallback for browsers without Navigation Timing 2.
     */
    private static landingUrl;
    /**
     * Epoch ms of the navigation start: `performance.timeOrigin`, else the
     * legacy `performance.timing.navigationStart` (older Safari and WebViews),
     * and only then the current time.
     */
    private static navigationStart;
    private constructor();
    /** Stop without reporting. The library keeps its observers; its reports are ignored. */
    stop(): void;
    /** The metrics as reported so far. */
    collect(): WebVitalsMetrics;
    private onMetric;
    private send;
}
export {};
//# sourceMappingURL=index.d.ts.map