build method
Describes the part of the user interface represented by this component.
The framework calls this method when this component is inserted into the tree
in a given BuildContext
and when the dependencies of this component change
(e.g., an InheritedComponent
referenced by this component changes). This
method can potentially be called in every frame and should not have any side
effects beyond building a component.
The framework replaces the subtree below this component with the component
returned by this method, either by updating the existing subtree or by
removing the subtree and inflating a new subtree, depending on whether the
component returned by this method can update the root of the existing
subtree, as determined by calling Component.canUpdate
.
Typically implementations return a newly created constellation of components
that are configured with information from this component's constructor and
from the given BuildContext
.
The given BuildContext
contains information about the location in the
tree at which this component is being built. For example, the context
provides the set of inherited components for this location in the tree. A
given component might be built with multiple different BuildContext
arguments over time if the component is moved around the tree or if the
component is inserted into the tree in multiple places at once.
The implementation of this method must only depend on:
- the fields of the component, which themselves must not change over time, and
- any ambient state obtained from the
context
usingBuildContext.dependOnInheritedComponentOfExactType
.
If a component's build method is to depend on anything else, use a
StatefulComponent
instead.
See also:
StatelessComponent
, which contains the discussion on performance considerations.
Implementation
@override
Iterable<Component> build(BuildContext context) sync* {
yield a(
href: to,
target: target,
referrerPolicy: referrer,
classes: classes,
styles: styles,
attributes: attributes,
events: {
if (preload)
'mouseover': (event) {
var router = Router.maybeOf(context);
if (router != null) {
router.preload(to);
}
},
'click': (event) {
var router = Router.maybeOf(context);
if (router != null) {
event.preventDefault();
if (!replace) {
router.push(to, extra: extra);
} else {
router.replace(to, extra: extra);
}
}
}
},
[if (child != null) child!, ...?children],
);
}