ImmutableIterationHolder is a concrete subclass of the Iteration<E> mixin that exposes a read‑only concatenation of multiple Iteration<E> sequences. It differs from IterationHolder in that it does not allow mutation of the underlying segments; it simply presents the items one after another as a single iterable view.
ImmutableIterationHolder({
required Iterable<Iteration<E>> items,
int startIndex = 0,
int indexTake = 0,
})
items – the list of child iterations to concatenate. Each entry may itself be a view into some larger collection, and all their lengths are summed to form the total source length.startIndex, indexTake – offsets that hide a number of elements at the start and/or end of the concatenated sequence (see Iteration). These offsets apply across the entire concatenation, not per child.On construction the class pre‑computes sourceLength by folding the lengths of all items. If you modify the lengths of those items after construction, the ImmutableIterationHolder will not update its cached length; it is intended to be used with immutable children.
| Field | Description |
|---|---|
items |
The ordered collection of child Iteration<E> objects being concatenated. |
startIndex |
Inherited from Iteration; number of elements at the beginning of the concatenation that are not visible. |
indexTake |
Inherited from Iteration; number of elements at the end of the concatenation that are hidden. |
sourceLength |
Total number of elements across all child iterations (computed once at construction). |
ImmutableIterationHolder extends the Iteration<E> mixin. As such it inherits default implementations for length, isEmpty, isNotEmpty, element look‑up (first, last, single, elementAt), searching (indexOf, indexWhere, etc.), reduction, mapping and slicing. These operations respect startIndex and indexTake across the concatenated view.sourceIterator – overrides the getter to return a HolderIterator<E> constructed with the items iterable. The iterator yields all elements of the first child, then the second, and so on.sourceGet(int sourceIndex) – when an element at a raw source index is requested, the method walks through items subtracting each child’s length until it finds the segment that contains sourceIndex. It then forwards to that child’s elementAt() to retrieve the element. If the index is out of range, an IndexError is thrown.Use ImmutableIterationHolder when you need to present several Iteration sequences as a single read‑only iterable. Because it caches sourceLength and does not expose any mutating methods, it is lightweight and suitable for building composite views.