AnnotatedRegionLayers return local position relative to clipping region

Summary

The local position returned by AnnotatedRegionLayers in an annotation search has been changed to be relative to the clipping region instead of the layer. This makes the local position more meaningful and reliable, but breaks code that directly performs annotation searches and uses the local position.

Context

Annotations are metadata that are assigned during the rendering phase to regions on the screen. Searching the annotations with a location gives the contextual information that contains that location. They are used to detect mouse events and the theme of app bars.

When localPosition was first added to the search result, it was defined as relative to the layer that owned the annotation, which turned out to be a design mistake. The offset from the layer is meaningless and unreliable. For example, a Transform widget draws on the same layer with an offset if the transform matrix is a simple translation, or push a dedicated TransformLayer if the matrix is non-trivial. The former case keeps the previous coordinate origin (for example, the top left corner of the app), while the latter case moves the position origin since it’s on a new layer. The two cases might not produce noticeable visual differences, since the extra layer might just be a scale of 99%, despite that the annotation search returns different results. In order to make this local position reliable, we have to choose one of the results to stick to.

Description of change

The localPosition returned by an AnnotatedRegionLayer is now the local position it received subtracted by offset, where offset is the location of the clipping area relative to the layer.

class AnnotatedRegionLayer<T> extends ContainerLayer {
  @override
  bool findAnnotations<S>(AnnotationResult<S> result, Offset localPosition, { @required bool onlyFirst }) {
    ...
    if (/* shouldAddAnnotation */) {
      result.add(AnnotationEntry<S>(
        annotation: typedValue,
        // Used to be:
        // localPosition: localPosition,
        localPosition: localPosition - offset,
      ));
    }
    ...
  }
}

Conceptually, this has changed how AnnotatedRegionLayer.offset and size are defined. They used to mean “the clipping rectangle that restricts the annotation search”, while they now jointly represent “the region of the annotation object”.

Migration guide

Code that is actively using this local position is probably directly interacting with layers, since using render objects or widgets will already have made this result unreliable. In order to preserve the previous behavior, you can reimplement AnnotatedRegionLayer that returns a local position without subtracting the offset.

Timeline

This change was introduced in 1.15.2.

References

API documentation:

Relevant issues:

Relevant PRs: