[#19] transformer: Add a target which sends parts to a channel

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

object/transformer/channel.go

@@ -0,0 +1,44 @@
+package transformer
+
+import (
+	objectSDK "github.com/TrueCloudLab/frostfs-sdk-go/object"
+	"github.com/nspcc-dev/neo-go/pkg/util/slice"
+)
+
+type chanTarget struct {
+	header  *objectSDK.Object
+	payload []byte
+	ch      chan<- *objectSDK.Object
+}
+
+// NewChannelTarget returns ObjectTarget which writes
+// object parts to a provided channel.
+func NewChannelTarget(ch chan<- *objectSDK.Object) ObjectTarget {
+	return &chanTarget{
+		ch: ch,
+	}
+}
+
+// WriteHeader implements the ObjectTarget interface.
+func (c *chanTarget) WriteHeader(object *objectSDK.Object) error {
+	c.header = object
+	return nil
+}
+
+// Write implements the ObjectTarget interface.
+func (c *chanTarget) Write(p []byte) (n int, err error) {
+	c.payload = append(c.payload, p...)
+	return len(p), nil
+}
+
+// Close implements the ObjectTarget interface.
+func (c *chanTarget) Close() (*AccessIdentifiers, error) {
+	if len(c.payload) != 0 {
+		c.header.SetPayload(slice.Copy(c.payload))
+	}
+	c.ch <- c.header
+
+	c.header = nil
+	c.payload = nil
+	return new(AccessIdentifiers), nil
+}