Implemented selective marshaling of struct fields

- Skips private fields by default
- Allows overriding of that default by an UDA on the struct type
- Allows per-field overriding using UDA on field

source/ddbus/conv.d

@@ -5,6 +5,7 @@ import ddbus.exception : InvalidValueException, TypeMismatchException;
 import ddbus.util;
 import ddbus.thin;
 import std.exception : enforce;
+import std.meta: allSatisfy;
 import std.string;
 import std.typecons;
 import std.range;
@@ -116,7 +117,22 @@ void buildIter(TS...)(DBusMessageIter *iter, TS args) if(allCanDBus!TS) {
     } else static if(is(T == struct)) {
       DBusMessageIter sub;
       dbus_message_iter_open_container(iter, 'r', null, &sub);
-      buildIter(&sub, arg.tupleof);
+
+      // Following failed because of missing 'this' for members of arg.
+      // That sucks. It worked without Filter.
+      // Reported: https://issues.dlang.org/show_bug.cgi?id=17692
+//    buildIter(&sub, Filter!(isAllowedField, arg.tupleof));
+
+      // Using foreach to work around the issue
+      foreach(i, member; arg.tupleof) {
+        // Ugly, but we need to use tupleof again in the condition, because when
+        // we use `member`, isAllowedField will fail because it'll find this
+        // nice `buildIter` function instead of T when it looks up the parent
+        // scope of its argument.
+        static if (isAllowedField!(arg.tupleof[i]))
+          buildIter(&sub, member);
+      }
+
       dbus_message_iter_close_container(iter, &sub);
     } else static if(basicDBus!T) {
       dbus_message_iter_append_basic(iter,typeCode!T,&arg);
@@ -289,10 +305,70 @@ void readIterTuple(Tup)(DBusMessageIter *iter, ref Tup tuple) if(isTuple!Tup &&
 }
 
 void readIterStruct(S)(DBusMessageIter *iter, ref S s) if(is(S == struct) && allCanDBus!(Fields!S)) {
-  alias FieldNameTuple!S names;
   foreach(index, T; Fields!S) {
+    static if (isAllowedField!(s.tupleof[index])) {
       s.tupleof[index] = readIter!T(iter);
     }
+  }
+}
+
+/++
+  Flags for use with dbusMarshaling UDA
+
+  Default is to include public fields only
++/
+enum MarshalingFlag : ubyte {
+  includePrivateFields = 1 << 0,  /// Automatically include private fields
+  manualOnly           = 1 << 7   /// Only include fields with explicit
+                                  ///   `@Yes.DBusMarshal`. This overrides any
+                                  ///   `include` flags.
+}
+
+private template isMarshalingFlag(T) {
+  enum isMarshalingFlag = is(T == MarshalingFlag);
+}
+
+/++
+  UDA for specifying DBus marshaling options on structs
++/
+auto dbusMarshaling(Args)(Args args ...)
+  if (allSatisfy!(isMarshalingFlag, Args)) {
+  return BitFlags!MarshalingFlag(args);
+}
+
+private template marshalingFlags(S) if (is(S == struct)) {
+  private alias getUDAs!(S, BitFlags!MarshalingFlag) UDAs;
+
+  static if (UDAs.length == 0)
+    enum marshalingFlags = BitFlags!MarshalingFlag.init;
+  else {
+    static assert (UDAs.length == 1,
+      "Only one @dbusMarshaling UDA allowed on type.");
+    static assert (is(typeof(UDAs[0]) == BitFlags!MarshalingFlag),
+      "Huh? Did you intend to use @dbusMarshaling UDA?");
+    enum marshalingFlags = UDAs[0];
+  }
+}
+
+private template isAllowedField(alias field) {
+  private enum flags = marshalingFlags!(__traits(parent, field));
+  private alias getUDAs!(field, Flag!"DBusMarshal") UDAs;
+
+  static if (UDAs.length != 0) {
+    static assert (UDAs.length == 1,
+      "Only one UDA of type Flag!\"DBusMarshal\" allowed on struct field.");
+    static assert (is(typeof(UDAs[0]) == Flag!"DBusMarshal"),
+      "Did you intend to add UDA Yes.DBusMarshal or No.DBusMarshal?");
+    enum isAllowedField = cast(bool) UDAs[0];
+  } else static if (!(flags & MarshalingFlag.manualOnly)) {
+    static if (__traits(getProtection, field) == "public")
+      enum isAllowedField = true;
+    else static if (cast(bool) (flags & MarshalingFlag.includePrivateFields))
+      enum isAllowedField = true;
+    else
+      enum isAllowedField = false;
+  } else
+    enum isAllowedField = false;
 }
 
 unittest {

source/ddbus/thin.d

@@ -573,14 +573,49 @@ struct Connection {
 unittest {
   import dunit.toolkit;
 
-  struct S1 { private int a; double b; string s; }
+  struct S1 {
-  struct S2 { Variant!int c; string d; S1 e; uint f; }
+    private int a;
+    private @(Yes.DBusMarshal) double b;
+    string s;
+  }
+
+  @dbusMarshaling(MarshalingFlag.manualOnly)
+  struct S2 {
+    int h, i;
+    @(Yes.DBusMarshal) int j, k;
+  }
+
+  @dbusMarshaling(MarshalingFlag.includePrivateFields)
+  struct S3 {
+    private Variant!int c;
+    string d;
+    S1 e;
+    S2 f;
+    @(No.DBusMarshal) uint g;
+  }
 
   Message msg = Message("org.example.wow", "/wut", "org.test.iface", "meth3");
 
-  enum testStruct = S2(variant(5), "blah", S1(-7, 63.5, "test"), 16);
+  enum testStruct = S3(
+    variant(5), "blah",
+    S1(-7, 63.5, "test"),
+    S2(84, -123, 78, 432),
+    16
+  );
+
   msg.build(testStruct);
-  msg.read!S2().assertEqual(testStruct);
+
+  // Non-marshaled fields should appear as freshly initialized
+  enum expectedResult = S3(
+    variant(5), "blah",
+    S1(int.init, 63.5, "test"),
+    S2(int.init, int.init, 78, 432),
+    uint.init
+  );
+
+  msg.read!S3().assertEqual(expectedResult);
+
+
 }
 
 Connection connectToBus(DBusBusType bus = DBusBusType.DBUS_BUS_SESSION) {