Composition

Composition fills an aggregation

A composition fills an aggregation with concrete child Block instances on a Block definition — nested inside the aggregation member it fills, never standalone.

One container concept, two forms

Composition is the initial fill of an aggregation on a Block definition only; bml:container is the general virtual container used everywhere else, on Block instances — both support the same flow-control directives.

Composition nests recursively

Every Block instance inside a composition may itself contain aggregation members, opened directly by name just like any other instance — complex nested structures build up this way without ever needing a nested composition wrapper.

Filling an aggregation with one child

An aggregation on a Block definition can declare an initial fill, nesting its composition directly inside the aggregation member.

<!-- composition sits nested inside "items", never standalone -->
<acme:Basket xmlns="http://blockml.org/bml"
  xmlns:acme="com.acme.example"
  xmlns:core="org.blockml.bml.core">

  <baseType>
    core:Block
  </baseType>
  <aggregations>
    <items type="acme:Widget">
      <composition>
        <acme:Widget bml:id="firstWidget" />
      </composition>
    </items>
  </aggregations>
</acme:Basket>

The composition holding firstWidget only makes sense as the fill of the items aggregation it sits inside — the same element written outside any aggregation member would be a standalone composition and invalid.

What to carry into the next pages

After this page, readers should be able to fill an aggregation with the correct composition syntax, before Sub-Blocks introduces nested Block definitions that composition children may also be typed by.

Continue with Sub-Blocks