MultiChildLayoutDelegate class abstract

A delegate that controls the layout of multiple children.

Used with CustomMultiChildLayout (in the widgets library) and RenderCustomMultiChildLayoutBox (in the rendering library).

Delegates must be idempotent. Specifically, if two delegates are equal, then they must produce the same layout. To change the layout, replace the delegate with a different instance whose shouldRelayout returns true when given the previous instance.

Override getSize to control the overall size of the layout. The size of the layout cannot depend on layout properties of the children. This was a design decision to simplify the delegate implementations: This way, the delegate implementations do not have to also handle various intrinsic sizing functions if the parent's size depended on the children. If you want to build a custom layout where you define the size of that widget based on its children, then you will have to create a custom render object. See MultiChildRenderObjectWidget with ContainerRenderObjectMixin and RenderBoxContainerDefaultsMixin to get started or RenderStack for an example implementation.

Override performLayout to size and position the children. An implementation of performLayout must call layoutChild exactly once for each child, but it may call layoutChild on children in an arbitrary order. Typically a delegate will use the size returned from layoutChild on one child to determine the constraints for performLayout on another child or to determine the offset for positionChild for that child or another child.

Override shouldRelayout to determine when the layout of the children needs to be recomputed when the delegate changes.

The most efficient way to trigger a relayout is to supply a relayout argument to the constructor of the MultiChildLayoutDelegate. The custom layout will listen to this value and relayout whenever the Listenable notifies its listeners, such as when an Animation ticks. This allows the custom layout to avoid the build phase of the pipeline.

Each child must be wrapped in a LayoutId widget to assign the id that identifies it to the delegate. The LayoutId.id needs to be unique among the children that the CustomMultiChildLayout manages.

// Define your own slot numbers, depending upon the id assigned by LayoutId.
// Typical usage is to define an enum like the one below, and use those
// values as the ids.
enum _Slot {
  leader,
  follower,
}

class FollowTheLeader extends MultiChildLayoutDelegate {
  @override
  void performLayout(Size size) {
    Size leaderSize = Size.zero;

    if (hasChild(_Slot.leader)) {
      leaderSize = layoutChild(_Slot.leader, BoxConstraints.loose(size));
      positionChild(_Slot.leader, Offset.zero);
    }

    if (hasChild(_Slot.follower)) {
      layoutChild(_Slot.follower, BoxConstraints.tight(leaderSize));
      positionChild(_Slot.follower, Offset(size.width - leaderSize.width,
          size.height - leaderSize.height));
    }
  }

  @override
  bool shouldRelayout(MultiChildLayoutDelegate oldDelegate) => false;
}

The delegate gives the leader widget loose constraints, which means the child determines what size to be (subject to fitting within the given size). The delegate then remembers the size of that child and places it in the upper left corner.

The delegate then gives the follower widget tight constraints, forcing it to match the size of the leader widget. The delegate then places the follower widget in the bottom right corner.

The leader and follower widget will paint in the order they appear in the child list, regardless of the order in which layoutChild is called on them.

See also:

• CustomMultiChildLayout, the widget that uses this delegate.
• RenderCustomMultiChildLayoutBox, render object that uses this delegate.