Refactor pager functionality into a package

Make it possible to output to the pager for other commands without
having to duplicate deciding whether we're calling an external pager or
writing directly STDOUT based on our configuration.

Author: msp301

cmd/outline.go

@@ -1,13 +1,8 @@
 package cmd
 
 import (
-	"fmt"
-	"io"
 	"log"
-	"os"
-	"os/exec"
 	"strings"
-	"sync"
 
 	"github.com/msp301/zb/graph"
 	"github.com/msp301/zb/pager"
@@ -19,52 +14,29 @@ var outlineCmd = &cobra.Command{
 	Use:   "outline",
 	Short: "Output notes as a tree",
 	Run: func(cmd *cobra.Command, args []string) {
-		pager := pager.FindPager()
-
-		var pipe io.WriteCloser
-
-		var command *exec.Cmd
-		if pager == "" {
-			pipe = os.Stdout
-		} else {
-			command = exec.Command(pager)
-			command.Stdout = os.Stdout
-			command.Stderr = os.Stderr
-
-			var err error
-			pipe, err = command.StdinPipe()
-			if err != nil {
-				log.Fatal(err)
-			}
+		pager, err := pager.Open()
+		if err != nil {
+			log.Fatal(err)
 		}
 
-		var wg sync.WaitGroup
-		wg.Add(1)
-
 		go func() {
-			defer pipe.Close()
+			defer pager.Close()
 
 			book().Notes.Walk(func(vertex graph.Vertex, depth int) bool {
 				indent := strings.Repeat("\t", depth)
 				switch val := vertex.Properties["Value"].(type) {
 				case parser.Note:
-					_, err := fmt.Fprintf(pipe, "%s%s - %s\n", indent, val.File, val.Title)
+					_, err := pager.Writef("%s%s - %s\n", indent, val.File, val.Title)
 					if err != nil {
 						log.Fatal(err)
 					}
 				}
 				return true
 			}, -1)
-
-			wg.Done()
 		}()
 
-		if pager == "" {
-			wg.Wait()
-		} else {
-			if err := command.Run(); err != nil {
-				log.Fatal(err)
-			}
+		if err := pager.Wait(); err != nil {
+			log.Fatal(err)
 		}
 	},
 }

pager/pager.go

@@ -1,7 +1,11 @@
 package pager
 
 import (
+	"fmt"
+	"io"
 	"os"
+	"os/exec"
+	"sync"
 
 	"github.com/spf13/viper"
 )
@@ -19,3 +23,50 @@ func FindPager() string {
 
 	return ""
 }
+
+type Pager struct {
+	pipe    io.WriteCloser
+	command *exec.Cmd
+	wg      sync.WaitGroup
+}
+
+func Open() (*Pager, error) {
+	pagerCmd := FindPager()
+
+	var pager = Pager{}
+	pager.wg.Add(1)
+
+	if pagerCmd == "" {
+		pager.pipe = os.Stdout
+	} else {
+		pager.command = exec.Command(pagerCmd)
+		pager.command.Stdout = os.Stdout
+		pager.command.Stderr = os.Stderr
+
+		var err error
+		pager.pipe, err = pager.command.StdinPipe()
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return &pager, nil
+}
+
+func (pager *Pager) Close() {
+	pager.pipe.Close()
+	pager.wg.Done()
+}
+
+func (pager *Pager) Wait() error {
+	if pager.command == nil {
+		pager.wg.Wait()
+		return nil
+	}
+
+	return pager.command.Run()
+}
+
+func (pager *Pager) Writef(format string, args ...interface{}) (int, error) {
+	return fmt.Fprintf(pager.pipe, format, args...)
+}