/**
 * Copyright 2019, OpenCensus Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
import * as webTypes from '@opencensus/web-types';
import { Span } from './span';
import { TracerBase } from './tracer-base';
/** Tracer manages the current root span and trace header propagation. */
export declare class Tracer extends TracerBase implements webTypes.Tracer {
    private static singletonInstance;
    /** Gets the tracer instance. */
    static readonly instance: Tracer;
    private currentRootSpanNoZone;
    /**
     * Gets the current root span associated to Zone.current.
     * If the current zone does not have a root span (e.g. root zone) or
     * `Zone` is not present return the value store in the unique root span.
     */
    /**
    * Sets the current root span to the current Zone.
    * If the current zone does not have a 'data' property (e.g. root zone)
    * or `Zone` is not present, just assign the root span to a variable.
    */
    currentRootSpan: Span;
    /**
     * Creates a new Zone (in case `Zone` global variable is present) and start
     * a new RootSpan to `currentRootSpan` associating the new RootSpan to the
     * new Zone. Thus, there might be several root spans at the same time.
     * If `Zone` is not present, just create the root span and store it in the current
     * root span.
     * Currently no sampling decisions are propagated or made here.
     * @param options Options for tracer instance
     * @param fn Callback function
     * @returns The callback return
     */
    startRootSpan<T>(options: webTypes.TraceOptions, fn: (root: Span) => T): T;
    /** Clears the current root span. */
    clearCurrentTrace(): void;
    /**
     * Start a new Span instance to the currentRootSpan
     * @param name Span name or SpanOptions object.
     * @param kind Span kind
     * @returns The new Span instance started
     */
    startChildSpan(options?: webTypes.SpanOptions): Span;
    /**
     * Binds the trace context to the given function - but because opencensus-web
     * currently only supports a single trace context at a time, this just returns
     * the function.
     */
    wrap<T>(fn: webTypes.Func<T>): webTypes.Func<T>;
    /** Binds trace context to NodeJS event emitter. No-op for opencensus-web. */
    wrapEmitter(emitter: webTypes.NodeJsEventEmitter): void;
    isZonePresent(): boolean;
}