Support retaining screen contents after exit

Fixes #76.

m/embed-api.go

@@ -1,8 +1,6 @@
 package m
 
 import (
-	"fmt"
-
 	"github.com/walles/moar/twin"
 )
 
@@ -13,20 +11,12 @@ func (p *Pager) Page() error {
 		// Screen setup failed
 		return e
 	}
-	defer func() {
-		screen.Close()
-		if p.DeInit {
-			return
-		}
-
-		// FIXME: Consider moving this logic into the twin package, or into Pager.
-		_, height := p.screen.Size()
-		lines := p.reader.GetLines(p.firstLineOneBased, height-1).lines
-		for _, line := range lines {
-			fmt.Println(line.raw)
-		}
-	}()
 
 	p.StartPaging(screen)
-	return nil
+	screen.Close()
+	if p.DeInit {
+		return nil
+	}
+
+	return p.ReprintAfterExit()
 }

m/pager.go

@@ -710,3 +710,18 @@ func (p *Pager) StartPaging(screen twin.Screen) {
 		log.Warnf("Reader reported an error: %s", p.reader.err.Error())
 	}
 }
+
+// After the pager has exited and the normal screen has been restored, you can
+// call this method to print the pager contents to screen again, faking
+// "leaving" pager contents on screen after exit.
+func (p *Pager) ReprintAfterExit() error {
+	// Figure out how many screen lines are used by pager contents
+	_, height := p.screen.Size()
+	heightWithoutFooter := height - 1
+	lineCount := len(p.reader.GetLines(p.firstLineOneBased, heightWithoutFooter).lines)
+
+	if lineCount > 0 {
+		p.screen.ShowNLines(lineCount)
+	}
+	return nil
+}

moar.go

@@ -150,6 +150,7 @@ func main() {
 		"Highlighting style from https://xyproto.github.io/splash/docs/longer/all.html")
 	colorsOption := flagSet.String("colors", "16M", "Highlighting palette size: 8, 16, 256, 16M")
 	noLineNumbers := flagSet.Bool("no-linenumbers", false, "Hide line numbers on startup, press left arrow key to show")
+	noClearOnExit := flagSet.Bool("no-clear-on-exit", false, "Retain screen contents when exiting moar")
 
 	// Combine flags from environment and from command line
 	flags := os.Args[1:]
@@ -246,7 +247,7 @@ func main() {
 	if stdinIsRedirected {
 		// Display input pipe contents
 		reader := m.NewReaderFromStream("", os.Stdin)
-		startPaging(reader, *wrap, *noLineNumbers)
+		startPaging(reader, *wrap, *noLineNumbers, *noClearOnExit)
 		return
 	}
 
@@ -256,16 +257,21 @@ func main() {
 		fmt.Fprintf(os.Stderr, "ERROR: %v\n", err)
 		os.Exit(1)
 	}
-	startPaging(reader, *wrap, *noLineNumbers)
+	startPaging(reader, *wrap, *noLineNumbers, *noClearOnExit)
 }
 
-func startPaging(reader *m.Reader, wrapLongLines bool, noLineNumbers bool) {
+func startPaging(reader *m.Reader, wrapLongLines bool, noLineNumbers bool, noClearOnExit bool) {
 	screen, e := twin.NewScreen()
 	if e != nil {
 		panic(e)
 	}
 
 	var loglines strings.Builder
+	log.SetOutput(&loglines)
+	pager := m.NewPager(reader)
+	pager.WrapLongLines = wrapLongLines
+	pager.ShowLineNumbers = !noLineNumbers
+
 	defer func() {
 		// Restore screen...
 		screen.Close()
@@ -276,6 +282,13 @@ func startPaging(reader *m.Reader, wrapLongLines bool, noLineNumbers bool) {
 			panic(err)
 		}
 
+		if noClearOnExit {
+			err := pager.ReprintAfterExit()
+			if err != nil {
+				log.Error("Failed reprinting pager view after exit", err)
+			}
+		}
+
 		if len(loglines.String()) > 0 {
 			printProblemsHeader()
 
@@ -286,9 +299,5 @@ func startPaging(reader *m.Reader, wrapLongLines bool, noLineNumbers bool) {
 		}
 	}()
 
-	log.SetOutput(&loglines)
-	pager := m.NewPager(reader)
-	pager.WrapLongLines = wrapLongLines
-	pager.ShowLineNumbers = !noLineNumbers
 	pager.StartPaging(screen)
 }

twin/fake-screen.go

@@ -64,6 +64,10 @@ func (screen *FakeScreen) Show() {
 	// This method intentionally left blank
 }
 
+func (screen *FakeScreen) ShowNLines(int) {
+	// This method intentionally left blank
+}
+
 func (screen *FakeScreen) Size() (int, int) {
 	return screen.width, screen.height
 }

twin/screen.go

@@ -25,6 +25,10 @@ type Screen interface {
 	// Render our contents into the terminal window
 	Show()
 
+	// Can be called after Close()ing the screen to fake retaining its output.
+	// Plain Show() is what you'd call during normal operation.
+	ShowNLines(lineCountToShow int)
+
 	// Returns screen width and height.
 	//
 	// NOTE: Never cache this response! On window resizes you'll get an
@@ -444,19 +448,24 @@ func renderLine(row []Cell) (string, int) {
 	return builder.String(), headerLength
 }
 
-// Render our contents into the terminal window
-//
-// Note that we start by prepping everything we want to write, then write it all
-// in one go. This is to make the screen update experience as atomic and flicker
-// free as possible.
 func (screen *UnixScreen) Show() {
+	_, height := screen.Size()
+	screen.showNLines(height, true)
+}
+
+func (screen *UnixScreen) ShowNLines(height int) {
+	screen.showNLines(height, false)
+}
+
+func (screen *UnixScreen) showNLines(height int, clearFirst bool) {
 	var builder strings.Builder
 
-	// Start in the top left corner:
-	// https://en.wikipedia.org/wiki/ANSI_escape_code#CSI_(Control_Sequence_Introducer)_sequences
-	builder.WriteString("\x1b[1;1H")
+	if clearFirst {
+		// Start in the top left corner:
+		// https://en.wikipedia.org/wiki/ANSI_escape_code#CSI_(Control_Sequence_Introducer)_sequences
+		builder.WriteString("\x1b[1;1H")
+	}
 
-	_, height := screen.Size()
 	for row := 0; row < height; row++ {
 		rendered, lineLength := renderLine(screen.cells[row])
 		builder.WriteString(rendered)