//構(gòu)造函數(shù)
func entry(logger *logger) *Entry {
  return &Entry{logger: logger, Buffer: new(bytes.Buffer), Map: make(map[string]interface{}, 5)}
}
/*entry主要方法是write方法，它首先通過e.logger.opt.level 與 level的比較，來判斷
是否要將日志輸出，其中DEBUG最低，F(xiàn)ATAL最高，這個(gè)從
https://github.com/marmotedu/gopractise-demo/blob/master/log/cuslog/options.go#L19
中可以看到。
*/
func (e *Entry) write(level Level, format string, args ...interface{}) {
  if e.logger.opt.level > level {
    return
  }
  e.Time = time.Now()
  e.Level = level
  e.Format = format
  e.Args = args
  if !e.logger.opt.disableCaller {
    if pc, file, line, ok := runtime.Caller(2); !ok {
      e.File = "???"
      e.Func = "???"
    } else {
      e.File, e.Line, e.Func = file, line, runtime.FuncForPC(pc).Name()
      e.Func = e.Func[strings.LastIndex(e.Func, "/")+1:]
    }
  }
  e.format()
  e.writer()
  e.release()
}

/*創(chuàng)建方法，通過sync.Pool緩存對(duì)象，提升性能，initOptions是用于初始化logger的options的
各種屬性*/
func New(opts ...Option) *logger {
  logger := &logger{opt: initOptions(opts...)}
  logger.entryPool = &sync.Pool{New: func() interface{} { return entry(logger) }}
  return logger
}

/*opts 參數(shù)是一系列的用于設(shè)置options屬性的函數(shù)，比如下面的WithOutput和WithLevel
都是這種函數(shù)，initOptions會(huì)接收這些函數(shù)作為輸入，對(duì)o = &options{}進(jìn)行設(shè)置*/
type Option func(*options)

func initOptions(opts ...Option) (o *options) {
  o = &options{}
  for _, opt := range opts {
    opt(o)
  }

  if o.output == nil {
    o.output = os.Stderr
  }

  if o.formatter == nil {
    o.formatter = &TextFormatter{}
  }

  return
}

func WithOutput(output io.Writer) Option {
  return func(o *options) {
    o.output = output
  }
}

func WithLevel(level Level) Option {
  return func(o *options) {
    o.level = level
  }
}

// 輸出到文件
  fd, err := os.OpenFile("test.log", os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
  if err != nil {
    log.Fatalln("create file test.log failed")
  }
  defer fd.Close()

  l := cuslog.New(cuslog.WithLevel(cuslog.InfoLevel),
    cuslog.WithOutput(fd),
    cuslog.WithFormatter(&cuslog.JsonFormatter{IgnoreBasicFields: false}),
  )
  l.Info("custom log with json formatter")