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// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package event
import (
// Event holds the information about an event that occurred.
// It combines the event metadata with the user supplied labels.
type Event struct {
Parent uint64 // id of the parent event for this event
Namespace string // namespace of event; if empty, set by exporter to import path
At time.Time // time at which the event is delivered to the exporter.
Labels []Label
// Handler is a the type for something that handles events as they occur.
type Handler interface {
// Log indicates a logging event.
Log(context.Context, *Event)
// Metric indicates a metric record event.
Metric(context.Context, *Event)
// Annotate reports label values at a point in time.
Annotate(context.Context, *Event)
// Start indicates a trace start event.
Start(context.Context, *Event) context.Context
// End indicates a trace end event.
End(context.Context, *Event)
// Matcher is the interface to something that can check if an event matches
// a condition.
type Matcher interface {
Matches(ev *Event) bool
// WithExporter returns a context with the exporter attached.
// The exporter is called synchronously from the event call site, so it should
// return quickly so as not to hold up user code.
func WithExporter(ctx context.Context, e *Exporter) context.Context {
return newContext(ctx, e, 0, time.Time{})
// SetDefaultExporter sets an exporter that is used if no exporter can be
// found on the context.
func SetDefaultExporter(e *Exporter) {
// Is uses the matcher to check if the event is a match.
// This is a simple helper to convert code like
// event.End.Matches(ev)
// to the more readable
// ev.Is(event.End)
func (ev *Event) Is(m Matcher) bool {
return m.Matches(ev)
// NopHandler is a handler that does nothing. It can be used for tests, or
// embedded in a struct to avoid having to implement all the Handler methods.
type NopHandler struct{}
func (NopHandler) Log(context.Context, *Event) {}
func (NopHandler) Metric(context.Context, *Event) {}
func (NopHandler) Annotate(context.Context, *Event) {}
func (NopHandler) End(context.Context, *Event) {}
func (NopHandler) Start(ctx context.Context, _ *Event) context.Context {
return ctx