refactor: improve code structure and add tests

with_value.go

@@ -0,0 +1,37 @@
+package gomockctx
+
+import (
+	"context"
+	"fmt"
+	"reflect"
+
+	"github.com/golang/mock/gomock"
+)
+
+// WithValue returns a gomock.Matcher which matches any context that has the
+// specified key and value.
+func WithValue(key interface{}, value interface{}) gomock.Matcher {
+	return &valueMatcher{
+		key:   key,
+		value: value,
+	}
+}
+
+type valueMatcher struct {
+	key   interface{}
+	value interface{}
+}
+
+var _ gomock.Matcher = &valueMatcher{}
+
+func (cm *valueMatcher) Matches(x interface{}) bool {
+	if ctx, ok := x.(context.Context); ok {
+		return reflect.DeepEqual(cm.value, ctx.Value(cm.key))
+	}
+
+	return false
+}
+
+func (cm *valueMatcher) String() string {
+	return fmt.Sprintf(`context with "%+v" = "%+v"`, cm.key, cm.value)
+}