Add WithField transformer

This should allow people to access fields within their objects fetched 
by the Matcher. This allows easier assertions by allowing specific 
fields to be compared during tests.

pkg/envtest/komega/transforms.go

@@ -0,0 +1,52 @@
+/*
+Copyright 2021 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package komega
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+
+	"github.com/onsi/gomega"
+	gtypes "github.com/onsi/gomega/types"
+)
+
+// WithField gets the value of the named field from the object.
+// This is intended to be used in assertions with the Matcher make it easy
+// to check the value of a particular field in a resource.
+// To access nested fields uses a `.` separator.
+// Eg.
+//    m.Eventually(deployment).Should(WithField("spec.replicas", BeZero()))
+// To access nested lists, use one of the Gomega list matchers in conjunction with this.
+// Eg.
+//    m.Eventually(deploymentList).Should(WithField("items", ConsistOf(...)))
+func WithField(field string, matcher gtypes.GomegaMatcher) gtypes.GomegaMatcher {
+	// Addressing Field by <struct>.<field> can be recursed
+	fields := strings.SplitN(field, ".", 2)
+	if len(fields) == 2 {
+		matcher = WithField(fields[1], matcher)
+	}
+
+	return gomega.WithTransform(func(obj interface{}) interface{} {
+		r := reflect.ValueOf(obj)
+		f := reflect.Indirect(r).FieldByName(fields[0])
+		if !f.IsValid() {
+			panic(fmt.Sprintf("Object '%s' does not have a field '%s'", reflect.TypeOf(obj), fields[0]))
+		}
+		return f.Interface()
+	}, matcher)
+}