Add WithInit option

README.md

@@ -51,9 +51,18 @@ w := writer.MustNew("/path/to/example.log.%Y%m%d", writer.WithMutex())
 with Debug (stdout and stderr)
 ```go
 w := writer.MustNew("/path/to/example.log.%Y%m%d", writer.WithDebug())
+w.Write([]byte("test"))
 
 // output file, stdout and stderr
-// /path/to/2017/02/04/example.log
+// /path/to/example.log.20170204
+```
+
+with Init
+```go
+w := writer.MustNew("/path/to/example.log.%Y%m%d", writer.WithInit())
+
+// open the file when New() method is called
+// /path/to/example.log.20170204
 ```
 
 ## Format

writer.go

@@ -11,13 +11,14 @@ import (
 )
 
 type cronoWriter struct {
-	pattern *strftime.Strftime
-	path    string
-	fp      *os.File
+	pattern *strftime.Strftime // given pattern
+	path    string             // current file path
+	fp      *os.File           // current file pointer
 	loc     *time.Location
 	mux     sync.Locker
 	stdout  io.Writer
 	stderr  io.Writer
+	init    bool // if true, open the file when New() method is called
 }
 
 type option func(*cronoWriter)
@@ -48,11 +49,18 @@ func New(pattern string, options ...option) (*cronoWriter, error) {
 		mux:     new(NoMutex), // default mutex off
 		stdout:  &noopWriter{},
 		stderr:  &noopWriter{},
+		init:    false,
 	}
 
 	for _, option := range options {
 		option(c)
 	}
+
+	if c.init {
+		if _, err := c.Write([]byte("")); err != nil {
+			return nil, err
+		}
+	}
 	return c, nil
 }
 
@@ -85,6 +93,12 @@ func WithDebug() option {
 	}
 }
 
+func WithInit() option {
+	return func(c *cronoWriter) {
+		c.init = true
+	}
+}
+
 func (c *cronoWriter) Write(b []byte) (int, error) {
 	c.mux.Lock()
 	defer c.mux.Unlock()

writer_test.go

@@ -17,6 +17,11 @@ func stubNow(value string) {
 }
 
 func TestNew(t *testing.T) {
+	tmpDir, err := ioutil.TempDir("", "cronowriter")
+	if err != nil {
+		t.Fatal(err)
+	}
+
 	c, _ := New("/path/to/file")
 	if c.pattern.Pattern() != "/path/to/file" {
 		t.Errorf("Expected pattern file, got %s", c.pattern.Pattern())
@@ -37,10 +42,19 @@ func TestNew(t *testing.T) {
 		t.Error("Expected mutex object, got nil")
 	}
 
-	c, err := New("/path/to/%")
+	c, err = New("/path/to/%")
 	if err == nil {
 		t.Errorf("Expected failed compile error, got %v", err)
 	}
+
+	initPath := filepath.Join(tmpDir, "init_test.log")
+	_, err = New(initPath, WithInit())
+	if err != nil {
+		t.Error(err)
+	}
+	if _, err := os.Stat(initPath); err != nil {
+		t.Error(err)
+	}
 }
 
 func TestMustNew_Panic(t *testing.T) {