From e66b3deed158c750f20e578495a6262393c85902 Mon Sep 17 00:00:00 2001
From: Daisy Guo <guoyingc@cn.ibm.com>
Date: Wed, 8 Apr 2020 00:31:08 +0800
Subject: [PATCH] change to flags/listprint.go

---
 pkg/kn/commands/flags/listprint.go      | 22 ++++++++++++++++++++++
 pkg/kn/commands/flags/listprint_test.go | 15 +++++++++++++++
 pkg/kn/commands/service/list.go         | 16 +---------------
 pkg/printers/tableprinter.go            |  4 ++++
 4 files changed, 42 insertions(+), 15 deletions(-)

diff --git a/pkg/kn/commands/flags/listprint.go b/pkg/kn/commands/flags/listprint.go
index a19cca498c..da73381c01 100644
--- a/pkg/kn/commands/flags/listprint.go
+++ b/pkg/kn/commands/flags/listprint.go
@@ -15,11 +15,15 @@
 package flags
 
 import (
+	"io"
+
 	"github.com/spf13/cobra"
+	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/cli-runtime/pkg/genericclioptions"
 
 	"knative.dev/client/pkg/kn/commands"
 	hprinters "knative.dev/client/pkg/printers"
+	"knative.dev/client/pkg/util"
 )
 
 // ListFlags composes common printer flag structs
@@ -56,6 +60,24 @@ func (f *ListPrintFlags) ToPrinter() (hprinters.ResourcePrinter, error) {
 	return p, nil
 }
 
+// Print is to print an Object to a Writer
+func (f *ListPrintFlags) Print(obj runtime.Object, w io.Writer) error {
+	printer, err := f.ToPrinter()
+	if err != nil {
+		return err
+	}
+
+	if f.GenericPrintFlags.OutputFlagSpecified() {
+		unstructuredList, err := util.ToUnstructuredList(obj)
+		if err != nil {
+			return err
+		}
+		return printer.PrintObj(unstructuredList, w)
+	}
+
+	return printer.PrintObj(obj, w)
+}
+
 // AddFlags receives a *cobra.Command reference and binds
 // flags related to humanreadable and template printing.
 func (f *ListPrintFlags) AddFlags(cmd *cobra.Command) {
diff --git a/pkg/kn/commands/flags/listprint_test.go b/pkg/kn/commands/flags/listprint_test.go
index 7a9afb0c2b..da17e88de8 100644
--- a/pkg/kn/commands/flags/listprint_test.go
+++ b/pkg/kn/commands/flags/listprint_test.go
@@ -48,3 +48,18 @@ func TestListPrintFlags(t *testing.T) {
 	_, ok := p.(hprinters.ResourcePrinter)
 	assert.Check(t, ok == true)
 }
+
+func TestListPrintFlagsPrint(t *testing.T) {
+	var cmd *cobra.Command
+	flags := NewListPrintFlags(func(h hprinters.PrintHandler) {})
+
+	cmd = &cobra.Command{}
+	flags.AddFlags(cmd)
+
+	pr, err := flags.ToPrinter()
+	assert.NilError(t, err)
+	assert.Assert(t, pr != nil)
+
+	err = flags.Print(nil, cmd.OutOrStdout())
+	assert.NilError(t, err)
+}
diff --git a/pkg/kn/commands/service/list.go b/pkg/kn/commands/service/list.go
index 974db35340..10a3308cc2 100644
--- a/pkg/kn/commands/service/list.go
+++ b/pkg/kn/commands/service/list.go
@@ -24,7 +24,6 @@ import (
 	"knative.dev/client/pkg/kn/commands"
 	"knative.dev/client/pkg/kn/commands/flags"
 	clientservingv1 "knative.dev/client/pkg/serving/v1"
-	"knative.dev/client/pkg/util"
 )
 
 // NewServiceListCommand represents 'kn service list' command
@@ -66,11 +65,6 @@ func NewServiceListCommand(p *commands.KnParams) *cobra.Command {
 				serviceListFlags.EnsureWithNamespace()
 			}
 
-			printer, err := serviceListFlags.ToPrinter()
-			if err != nil {
-				return err
-			}
-
 			// Sort serviceList by namespace and name (in this order)
 			sort.SliceStable(serviceList.Items, func(i, j int) bool {
 				a := serviceList.Items[i]
@@ -82,15 +76,7 @@ func NewServiceListCommand(p *commands.KnParams) *cobra.Command {
 				return a.ObjectMeta.Name < b.ObjectMeta.Name
 			})
 
-			if serviceListFlags.GenericPrintFlags.OutputFlagSpecified() {
-				unstructedList, err := util.ToUnstructuredList(serviceList)
-				if err != nil {
-					return err
-				}
-				return printer.PrintObj(unstructedList, cmd.OutOrStdout())
-			}
-
-			return printer.PrintObj(serviceList, cmd.OutOrStdout())
+			return serviceListFlags.Print(serviceList, cmd.OutOrStdout())
 		},
 	}
 	commands.AddNamespaceFlags(serviceListCommand.Flags(), true)
diff --git a/pkg/printers/tableprinter.go b/pkg/printers/tableprinter.go
index f51bc1390f..0f309b40d9 100644
--- a/pkg/printers/tableprinter.go
+++ b/pkg/printers/tableprinter.go
@@ -41,6 +41,10 @@ func NewTablePrinter(options PrintOptions) *HumanReadablePrinter {
 
 // PrintObj prints the obj in a human-friendly format according to the type of the obj.
 func (h *HumanReadablePrinter) PrintObj(obj runtime.Object, output io.Writer) error {
+	if obj == nil {
+		return nil
+	}
+
 	w, found := output.(*tabwriter.Writer)
 	if !found {
 		w = NewTabWriter(output)