Creating Custom Deltas

If you are not yet familiar with deltas, see the customizing widget styles guide first.

When the built-in delta factory constructors do not cover your use case, implement the delta class directly and override call.

Suppose you want to mirror a widget's EdgeInsets padding. To do so, implement EdgeInsetsDelta with a call method that swaps the edges:

1class MirrorDelta implements EdgeInsetsDelta {
2  const MirrorDelta();
3
4  @override
5  EdgeInsets call(EdgeInsets? insets) {
6    final resolved = insets ?? .zero;
7    return .only(
8      left: resolved.right,
9      top: resolved.top,
10      right: resolved.left,
11      bottom: resolved.bottom,
12    );
13  }
14}
15

Pass it anywhere an EdgeInsetsDelta is accepted:

1FTooltip(
2  style: const .delta(padding: MirrorDelta()),
3  tipBuilder: (_, _) => const Text('Tooltip'),
4  child: const Placeholder(),
5);
6

The same pattern works for any delta class. Implement it and override call.