example_level_handler_test.go

     1  // Copyright 2022 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package slog_test
     6  
     7  import (
     8  	"context"
     9  	"log/slog"
    10  	"os"
    11  )
    12  
    13  // A LevelHandler wraps a Handler with an Enabled method
    14  // that returns false for levels below a minimum.
    15  type LevelHandler struct {
    16  	level   slog.Leveler
    17  	handler slog.Handler
    18  }
    19  
    20  // NewLevelHandler returns a LevelHandler with the given level.
    21  // All methods except Enabled delegate to h.
    22  func NewLevelHandler(level slog.Leveler, h slog.Handler) *LevelHandler {
    23  	// Optimization: avoid chains of LevelHandlers.
    24  	if lh, ok := h.(*LevelHandler); ok {
    25  		h = lh.Handler()
    26  	}
    27  	return &LevelHandler{level, h}
    28  }
    29  
    30  // Enabled implements Handler.Enabled by reporting whether
    31  // level is at least as large as h's level.
    32  func (h *LevelHandler) Enabled(_ context.Context, level slog.Level) bool {
    33  	return level >= h.level.Level()
    34  }
    35  
    36  // Handle implements Handler.Handle.
    37  func (h *LevelHandler) Handle(ctx context.Context, r slog.Record) error {
    38  	return h.handler.Handle(ctx, r)
    39  }
    40  
    41  // WithAttrs implements Handler.WithAttrs.
    42  func (h *LevelHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
    43  	return NewLevelHandler(h.level, h.handler.WithAttrs(attrs))
    44  }
    45  
    46  // WithGroup implements Handler.WithGroup.
    47  func (h *LevelHandler) WithGroup(name string) slog.Handler {
    48  	return NewLevelHandler(h.level, h.handler.WithGroup(name))
    49  }
    50  
    51  // Handler returns the Handler wrapped by h.
    52  func (h *LevelHandler) Handler() slog.Handler {
    53  	return h.handler
    54  }
    55  
    56  // This example shows how to Use a LevelHandler to change the level of an
    57  // existing Handler while preserving its other behavior.
    58  //
    59  // This example demonstrates increasing the log level to reduce a logger's
    60  // output.
    61  //
    62  // Another typical use would be to decrease the log level (to LevelDebug, say)
    63  // during a part of the program that was suspected of containing a bug.
    64  func ExampleHandler_levelHandler() {
    65  	removeTime := func(groups []string, a slog.Attr) slog.Attr {
    66  		if a.Key == slog.TimeKey && len(groups) == 0 {
    67  			return slog.Attr{}
    68  		}
    69  		return a
    70  	}
    71  	th := slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{ReplaceAttr: removeTime})
    72  	logger := slog.New(NewLevelHandler(slog.LevelWarn, th))
    73  	logger.Info("not printed")
    74  	logger.Warn("printed")
    75  
    76  	// Output:
    77  	// level=WARN msg=printed
    78  }
    79
View as plain text