Unable to initialise a null map field by field number

[dave]

If a map field isn't set (e.g. it's null), then it doesn't seem to be possible to modify it by field number. It looks like getField returns the default value, and updates to that value are not reflected in the message... I've prepared a simplified example:

message Holder {
  map<string, int32> numbers = 1;
}

setMapByField(GeneratedMessage message, int field, dynamic key, dynamic value) {
  var map = message.getField(field) as PbMap;
  map[key] = value;
}

void main() {
  // this test passes:
  test("empty map", () {
    var holder = Holder()..numbers.clear(); // holder.numbers map is initialised but empty
    setMapByField(holder, 1, "a", 1);
    expect(holder.numbers["a"], 1);
  });

  // this test fails:
  test("null map", () {
    var holder = Holder(); // holder.numbers map is not initialised
    setMapByField(holder, 1, "a", 1);
    expect(holder.numbers["a"], 1);
  });
}

Is there any way around this?

[dave]

I believe this can be fixed with a change to the _getDefault method... This method already initialises repeated fields, and can be simply extended to also initialise maps.

I've sent a quick PR showing a potential fix. I'm not totally familiar with the internals so it's possible I've missed something, but this fixes my bug and the protobuf tests all pass. Let me know what you think.

[nwparker]

@sigurdm - Do you mind taking a look at this?

This is going back awhile, but it's related to this: #340 (comment)

In other words, your suggestion there won't work for the reason @dave has pointed out

[tp]

Would be awesome to get dave@f0605ba in, as I also ran into this issue, and there doesn't seem to the a "client side" workaround (apart from switching to Dave's branch).

[osa1]

Unfortunately dave@f0605ba is not a good fix for this issue, and it breaks a few things. I'm currently working on fixing this. In the meantime here's a hacky workaround:

Proto file:

syntax = "proto3";

message A {
  map<int32, int32> a = 1;
}

Dart:

var a = A.create();

// Get index of the map field in generated message. 1 here is the tag of
// the field as specified in the proto definition.
var mapFieldInfo = a.info_.fieldInfo[1]!;
var mapFieldIndex = mapFieldInfo.index!;

PbMap<int, int> map = a.$_getMap(mapFieldIndex) as PbMap<int, int>;

map[1] = 2;
print(a); // prints "a : {1 : 2}"