slog/example_level_handler_test.go

// Copyright 2022 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 slog_test

import (
	"context"
	"os"

	"golang.org/x/exp/slog"
	"golang.org/x/exp/slog/internal/testutil"
)

// A LevelHandler wraps a Handler with an Enabled method
// that returns false for levels below a minimum.
type LevelHandler struct {
	level   slog.Leveler
	handler slog.Handler
}

// NewLevelHandler returns a LevelHandler with the given level.
// All methods except Enabled delegate to h.
func NewLevelHandler(level slog.Leveler, h slog.Handler) *LevelHandler {
	// Optimization: avoid chains of LevelHandlers.
	if lh, ok := h.(*LevelHandler); ok {
		h = lh.Handler()
	}
	return &LevelHandler{level, h}
}

// Enabled implements Handler.Enabled by reporting whether
// level is at least as large as h's level.
func (h *LevelHandler) Enabled(_ context.Context, level slog.Level) bool {
	return level >= h.level.Level()
}

// Handle implements Handler.Handle.
func (h *LevelHandler) Handle(ctx context.Context, r slog.Record) error {
	return h.handler.Handle(ctx, r)
}

// WithAttrs implements Handler.WithAttrs.
func (h *LevelHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
	return NewLevelHandler(h.level, h.handler.WithAttrs(attrs))
}

// WithGroup implements Handler.WithGroup.
func (h *LevelHandler) WithGroup(name string) slog.Handler {
	return NewLevelHandler(h.level, h.handler.WithGroup(name))
}

// Handler returns the Handler wrapped by h.
func (h *LevelHandler) Handler() slog.Handler {
	return h.handler
}

// This example shows how to Use a LevelHandler to change the level of an
// existing Handler while preserving its other behavior.
//
// This example demonstrates increasing the log level to reduce a logger's
// output.
//
// Another typical use would be to decrease the log level (to LevelDebug, say)
// during a part of the program that was suspected of containing a bug.
func ExampleHandler_levelHandler() {
	th := slog.HandlerOptions{ReplaceAttr: testutil.RemoveTime}.NewTextHandler(os.Stdout)
	logger := slog.New(NewLevelHandler(slog.LevelWarn, th))
	logger.Info("not printed")
	logger.Warn("printed")

	// Output:
	// level=WARN msg=printed
}