flush underlying when implementing flusher

lib.go

@@ -140,6 +140,15 @@ func (w *Writer) flush() error {
 		return w.err
 	}
 
+	if f, ok := w.out.(interface{ Flush() error }); ok {
+		if err := f.Flush(); err != nil {
+			w.err = err
+			return w.err
+		}
+	} else if f, ok := w.out.(interface{ Flush() }); ok {
+		f.Flush()
+	}
+
 	return nil
 }
 
@@ -161,6 +170,9 @@ func (w *Writer) WriteRune(r rune) (int, error) {
 // the underlying io.Writer, but first passes it to the subsequent editors for editing. When the used editor
 // instances comply with the expectations of the Editor interface, the writer will have a fresh state and can be
 // reused for further editing.
+//
+// If the underlying io.Writer also implements Flusher (Flush() error or Flush()), then it will be implicitly
+// called.
 func (w *Writer) Flush() error {
 	return w.flush()
 }

lib_test.go

@@ -15,6 +15,17 @@ type failingWriter struct {
 	fail bool
 }
 
+type flusherWriter struct {
+	out     io.Writer
+	fail    bool
+	flushed bool
+}
+
+type flusherWriterNoErr struct {
+	out     io.Writer
+	flushed bool
+}
+
 func (w *failingWriter) Write(p []byte) (int, error) {
 	if w.fail {
 		return 0, errTest
@@ -23,6 +34,27 @@ func (w *failingWriter) Write(p []byte) (int, error) {
 	return w.out.Write(p)
 }
 
+func (w *flusherWriter) Write(p []byte) (int, error) {
+	return w.out.Write(p)
+}
+
+func (w *flusherWriter) Flush() error {
+	w.flushed = true
+	if w.fail {
+		return errTest
+	}
+
+	return nil
+}
+
+func (w *flusherWriterNoErr) Write(p []byte) (int, error) {
+	return w.out.Write(p)
+}
+
+func (w *flusherWriterNoErr) Flush() {
+	w.flushed = true
+}
+
 func TestNoop(t *testing.T) {
 	t.Run("editor", func(t *testing.T) {
 		var b bytes.Buffer
@@ -105,3 +137,61 @@ func TestFailingWriter(t *testing.T) {
 		}
 	})
 }
+
+func TestFlush(t *testing.T) {
+	t.Run("with err succeed", func(t *testing.T) {
+		var b bytes.Buffer
+		fw := flusherWriter{out: &b}
+		w := textedit.New(&fw, textedit.Replace("o", "e"))
+		if _, err := w.Write([]byte("foo bar")); err != nil {
+			t.Fatal(err)
+		}
+
+		if err := w.Flush(); err != nil {
+			t.Fatal(err)
+		}
+
+		if b.String() != "fee bar" {
+			t.Fatal(b.String())
+		}
+
+		if !fw.flushed {
+			t.Fatal("failed to flush underlying flusher")
+		}
+	})
+
+	t.Run("with err fail", func(t *testing.T) {
+		var b bytes.Buffer
+		fw := flusherWriter{out: &b}
+		w := textedit.New(&fw, textedit.Replace("o", "e"))
+		if _, err := w.Write([]byte("foo bar")); err != nil {
+			t.Fatal(err)
+		}
+
+		fw.fail = true
+		if err := w.Flush(); !errors.Is(err, errTest) {
+			t.Fatal("failed to fail with the right error", err)
+		}
+	})
+
+	t.Run("without err", func(t *testing.T) {
+		var b bytes.Buffer
+		fw := flusherWriterNoErr{out: &b}
+		w := textedit.New(&fw, textedit.Replace("o", "e"))
+		if _, err := w.Write([]byte("foo bar")); err != nil {
+			t.Fatal(err)
+		}
+
+		if err := w.Flush(); err != nil {
+			t.Fatal(err)
+		}
+
+		if b.String() != "fee bar" {
+			t.Fatal(b.String())
+		}
+
+		if !fw.flushed {
+			t.Fatal("failed to flush underlying flusher")
+		}
+	})
+}