mocking testing with io.Reader and io.Writer

io_reader_writer_testing.go

// GetTextAndDisplay reads and display data to io
func GetTextAndDisplay(r io.Reader, w io.Writer) error {
	buf := make([]byte, 10<<20) // 10MB
	readBytes, err := r.Read(buf)
	if err != nil {
		return err
	}

	_, err = w.Write(buf[0:readBytes])
	if err != nil {
		return err
	}

	return nil
}

// Testing
type mockReaderOk struct {
	bytes []byte
}

func (mro *mockReaderOk) Read(bytes []byte) (int, error) {
	src := []byte("reader ok")
	for i, b := range src {
		bytes[i] = b
	}
	return len(src), nil
}

type mockReaderErr struct{}

func (mre *mockReaderErr) Read(b []byte) (int, error) {
	return 0, fmt.Errorf("error on Reader")
}

type mockWriterOk struct {
	bytes []byte
}

func (mwo *mockWriterOk) Write(b []byte) (int, error) {
	mwo.bytes = b
	return len(b), nil
}

type mockWriterErr struct {
	bytes []byte
}

func (mwe *mockWriterErr) Write(b []byte) (int, error) {
	return 0, fmt.Errorf("error on Writer")
}

func TestGetTextAndDisplay(t *testing.T) {
	t.Run("reader ok, writer ok", func(t *testing.T) {
		r := &mockReaderOk{}
		w := &mockWriterOk{}
		var wantErr error
		wantStr := "reader ok"

		if err := GetTextAndDisplay(r, w); !reflect.DeepEqual(wantErr, err) {
			t.Errorf("GetTextAndDisplay() error = %v, wantErr %v", err, wantErr)
			return
		}
		if gotStr := string(w.bytes); gotStr != wantStr {
			t.Errorf("GetTextAndDisplay() = %v, want %v", gotStr, wantStr)
		}
	})

	t.Run("reader error, writer ok", func(t *testing.T) {
		r := &mockReaderErr{}
		w := &mockWriterOk{}
		wantErr := fmt.Errorf("error on Reader")
		wantStr := ""

		if err := GetTextAndDisplay(r, w); !reflect.DeepEqual(wantErr, err) {
			t.Errorf("GetTextAndDisplay() error = %v, wantErr %v", err, wantErr)
			return
		}
		if gotStr := string(w.bytes); gotStr != wantStr {
			t.Errorf("GetTextAndDisplay() = %v, want %v", gotStr, wantStr)
		}
	})

	t.Run("reader ok, writer error", func(t *testing.T) {
		r := &mockReaderOk{}
		w := &mockWriterErr{}
		wantErr := fmt.Errorf("error on Writer")
		wantStr := ""

		if err := GetTextAndDisplay(r, w); !reflect.DeepEqual(wantErr, err) {
			t.Errorf("GetTextAndDisplay() error = %v, wantErr %v", err, wantErr)
			return
		}
		if gotStr := string(w.bytes); gotStr != wantStr {
			t.Errorf("GetTextAndDisplay() = %v, want %v", gotStr, wantStr)
		}
	})

	t.Run("reader error, writer error", func(t *testing.T) {
		r := &mockReaderErr{}
		w := &mockWriterErr{}
		wantErr := fmt.Errorf("error on Reader")
		wantStr := ""

		if err := GetTextAndDisplay(r, w); !reflect.DeepEqual(wantErr, err) {
			t.Errorf("GetTextAndDisplay() error = %v, wantErr %v", err, wantErr)
			return
		}
		if gotStr := string(w.bytes); gotStr != wantStr {
			t.Errorf("GetTextAndDisplay() = %v, want %v", gotStr, wantStr)
		}
	})
}