[dccd] Changing ref to Iterables cause an exception

[vicb]

@chirayuk could you please take a look at this:

      iit('should support switching refs', async(() {
        var list = [0];

        var record = detector.watch(list, null, null);
        microLeap();
        var iterator = detector.collectChanges()..moveNext();

        record.object = [1, 0];
        microLeap();
        iterator = detector.collectChanges()..moveNext();
        expect(iterator.current.currentValue, toEqualCollectionRecord(
            collection: ['1[null -> 0]', '0[0->1]'],
            additions: ['1[null -> 0]'],
            moves: ['0[0->1]'],
            removals: []));
      }));

The error is
`Test failed: Caught The null object does not have a setter '_next@0x1d3970e2='.

Is the usage wrong or is this a bug ?

(You can ignore the async and microLeap - they are not related to the issue)

[vicb]

@chirayuk seems like detector.collectChangesshould be called twice for this to work, is that expected ?

      iit('should support switching from List to ObservableList', sync(() {
        var list = [0];

        var record = detector.watch(list, null, null);
        detector.collectChanges();
        detector.collectChanges();

        var iterator;
        record.object = [1, 0];
        iterator = detector.collectChanges()..moveNext();
        expect(iterator.current.currentValue, toEqualCollectionRecord(
            collection: ['1[null -> 0]', '0[0 -> 1]'],
            previous: ['0[0 -> 1]'],
            additions: ['1[null -> 0]'],
            moves: ['0[0 -> 1]'],
            removals: []));

        record.object = [2, 1, 0];
        iterator = detector.collectChanges()..moveNext();
        expect(iterator.current.currentValue, toEqualCollectionRecord(
            collection: ['2[null -> 0]', '1[null -> 1]', '0[0 -> 2]'],
            previous: ['0[0 -> 2]'],
            additions: ['2[null -> 0]', '1[null -> 1]'],
            moves: ['0[0 -> 2]'],
            removals: []));
      }));

Notice how the third list is compared to the first one (the test passes)

[chirayuk]

In your test case, you're only calling collectChanges once.  In real dirty checking, we call again if there were changes.  This is required for the proper resetting for collection watching (specifically to stay efficient while detecting simultaneous mutation + ref change).

You could rewrite it like this (ref: chirayuk@9f69f73):

iit('ckck: issue 1158: should support switching refs', async(() {
  var list = [0];

  var record = detector.watch(list, null, null);
  if (detector.collectChanges().moveNext()) {
    detector.collectChanges();
  }

  record.object = [1, 0];
  var iterator = detector.collectChanges()..moveNext();
  expect(iterator.current.currentValue, toEqualCollectionRecord(
      collection: ['1[null -> 0]', '0[0 -> 1]'],
      previous: ['0[0 -> 1]'],
      additions: ['1[null -> 0]'],
      moves: ['0[0 -> 1]'],
      removals: []));
}));

[vicb]

@chirayuk thanks for your help, I've submitted a PR (#1253)