test(client-go): strengthen WriteArrayElements assertions and add README example

clients/go/README.md

@@ -153,6 +153,20 @@ integer `0`, float `0.0`, string `""`, time = Unix epoch). This is a **RESET**
 of unmentioned indices, not a preserve of existing values. Use the full-array
 form (read-modify-write) when existing element values must be preserved.
 
+```go
+// Set element [3] of a 10-element float array; all other indices reset to 0.0.
+err = session.WriteArrayElements(
+    ctx,
+    serverHandle, itemHandle,
+    mxgateway.DataTypeFloat,
+    10, // totalLength
+    map[uint32]*mxgateway.MxValue{
+        3: mxgateway.FloatValue(1.5),
+    },
+    userID,
+)
+```
+
 `AddItem` (and `AddItem2`) now auto-normalize a bare attribute name to the `[]`
 array address form expected by MXAccess, so callers do not need to append `[]`
 themselves. Both forms are accepted; duplicates are deduplicated by the gateway.

clients/go/mxgateway/client_session_test.go

@@ -742,7 +742,7 @@ func TestWriteArrayElementsSendsWriteCommandWithSparseOneof(t *testing.T) {
 		DataTypeFloat,
 		10,
 		map[uint32]*MxValue{3: FloatValue(1.5)},
-		0,
+		42,
 	)
 	if err != nil {
 		t.Fatalf("WriteArrayElements() error = %v", err)
@@ -752,7 +752,20 @@ func TestWriteArrayElementsSendsWriteCommandWithSparseOneof(t *testing.T) {
 	if cmd.GetKind() != pb.MxCommandKind_MX_COMMAND_KIND_WRITE {
 		t.Fatalf("command kind = %s, want WRITE", cmd.GetKind())
 	}
-	val := cmd.GetWrite().GetValue()
+	w := cmd.GetWrite()
+	if w.GetServerHandle() != 1 {
+		t.Fatalf("server handle = %d, want 1", w.GetServerHandle())
+	}
+	if w.GetItemHandle() != 2 {
+		t.Fatalf("item handle = %d, want 2", w.GetItemHandle())
+	}
+	if w.GetUserId() == 0 {
+		t.Fatal("user id = 0, want non-zero")
+	}
+	if w.GetUserId() != 42 {
+		t.Fatalf("user id = %d, want 42", w.GetUserId())
+	}
+	val := w.GetValue()
 	sa, ok := val.Kind.(*pb.MxValue_SparseArrayValue)
 	if !ok {
 		t.Fatalf("value kind is %T, want *pb.MxValue_SparseArrayValue", val.Kind)
@@ -763,4 +776,14 @@ func TestWriteArrayElementsSendsWriteCommandWithSparseOneof(t *testing.T) {
 	if sa.SparseArrayValue.GetElementDataType() != DataTypeFloat {
 		t.Errorf("ElementDataType = %v, want DataTypeFloat", sa.SparseArrayValue.GetElementDataType())
 	}
+	if len(sa.SparseArrayValue.GetElements()) != 1 {
+		t.Fatalf("len(Elements) = %d, want 1", len(sa.SparseArrayValue.GetElements()))
+	}
+	elem := sa.SparseArrayValue.GetElements()[0]
+	if elem.GetIndex() != 3 {
+		t.Errorf("element index = %d, want 3", elem.GetIndex())
+	}
+	if elem.GetValue().GetFloatValue() != 1.5 {
+		t.Errorf("element float value = %v, want 1.5", elem.GetValue().GetFloatValue())
+	}
 }