feature: Add a App.Reader that defaults to os.Stdin

Closes: #1190

app.go

@@ -72,6 +72,8 @@ type App struct {
 	Authors []*Author
 	// Copyright of the binary if any
 	Copyright string
+	// Reader reader to write input to (useful for tests)
+	Reader io.Reader
 	// Writer writer to write output to
 	Writer io.Writer
 	// ErrWriter writes error output
@@ -117,6 +119,7 @@ func NewApp() *App {
 		BashComplete: DefaultAppComplete,
 		Action:       helpCommand.Action,
 		Compiled:     compileTime(),
+		Reader:       os.Stdin,
 		Writer:       os.Stdout,
 		ErrWriter:    os.Stderr,
 	}
@@ -160,6 +163,10 @@ func (a *App) Setup() {
 		a.Compiled = compileTime()
 	}
 
+	if a.Reader == nil {
+		a.Reader = os.Stdin
+	}
+
 	if a.Writer == nil {
 		a.Writer = os.Stdout
 	}

app_test.go

@@ -433,6 +433,12 @@ func TestApp_Command(t *testing.T) {
 	}
 }
 
+func TestApp_Setup_defaultsReader(t *testing.T) {
+	app := &App{}
+	app.Setup()
+	expect(t, app.Reader, os.Stdin)
+}
+
 func TestApp_Setup_defaultsWriter(t *testing.T) {
 	app := &App{}
 	app.Setup()
@@ -850,6 +856,15 @@ func TestApp_ParseSliceFlagsWithMissingValue(t *testing.T) {
 	}
 }
 
+func TestApp_DefaultStdin(t *testing.T) {
+	app := &App{}
+	app.Setup()
+
+	if app.Reader != os.Stdin {
+		t.Error("Default input reader not set.")
+	}
+}
+
 func TestApp_DefaultStdout(t *testing.T) {
 	app := &App{}
 	app.Setup()
@@ -859,6 +874,29 @@ func TestApp_DefaultStdout(t *testing.T) {
 	}
 }
 
+func TestApp_SetStdin(t *testing.T) {
+	buf := make([]byte, 12)
+
+	app := &App{
+		Name:   "test",
+		Reader: strings.NewReader("Hello World!"),
+		Action: func(c *Context) error {
+			_, err := c.App.Reader.Read(buf)
+			return err
+		},
+	}
+
+	err := app.Run([]string{"help"})
+
+	if err != nil {
+		t.Fatalf("Run error: %s", err)
+	}
+
+	if string(buf) != "Hello World!" {
+		t.Error("App did not read input from desired reader.")
+	}
+}
+
 func TestApp_SetStdout(t *testing.T) {
 	var w bytes.Buffer