findAnnotations<S extends Object> method
Null safety
- AnnotationResult<
S> result, - Offset localPosition,
- {required bool onlyFirst}
Search this layer and its subtree for annotations of type S at the
location described by localPosition.
This method is called by the default implementation of find and findAllAnnotations. Override this method to customize how the layer should search for annotations, or if the layer has its own annotations to add.
The default implementation simply returns false, which means neither
the layer nor its children has annotations, and the annotation search
is not absorbed either (see below for explanation).
About layer annotations
An annotation is an optional object of any type that can be carried with a layer. An annotation can be found at a location as long as the owner layer contains the location and is walked to.
The annotations are searched by first visiting each child recursively, then this layer, resulting in an order from visually front to back. Annotations must meet the given restrictions, such as type and position.
The common way for a value to be found here is by pushing an AnnotatedRegionLayer into the layer tree, or by adding the desired annotation by overriding findAnnotations.
Parameters and return value
The result parameter is where the method outputs the resulting
annotations. New annotations found during the walk are added to the tail.
The onlyFirst parameter indicates that, if true, the search will stop
when it finds the first qualified annotation; otherwise, it will walk the
entire subtree.
The return value indicates the opacity of this layer and its subtree at this position. If it returns true, then this layer's parent should skip the children behind this layer. In other words, it is opaque to this type of annotation and has absorbed the search so that its siblings behind it are not aware of the search. If the return value is false, then the parent might continue with other siblings.
The return value does not affect whether the parent adds its own annotations; in other words, if a layer is supposed to add an annotation, it will always add it even if its children are opaque to this type of annotation. However, the opacity that the parents return might be affected by their children, hence making all of its ancestors opaque to this type of annotation.
Implementation
@override
bool findAnnotations<S extends Object>(AnnotationResult<S> result, Offset localPosition, { required bool onlyFirst }) {
for (Layer? child = lastChild; child != null; child = child.previousSibling) {
final bool isAbsorbed = child.findAnnotations<S>(result, localPosition, onlyFirst: onlyFirst);
if (isAbsorbed) {
return true;
}
if (onlyFirst && result.entries.isNotEmpty) {
return isAbsorbed;
}
}
return false;
}