build method

@override Widget build(BuildContext context)

Describes the part of the user interface represented by this widget.

The framework calls this method when this widget is inserted into the tree in a given BuildContext and when the dependencies of this widget change (e.g., an InheritedWidget referenced by this widget changes). This method can potentially be called in every frame and should not have any side effects beyond building a widget.

The framework replaces the subtree below this widget with the widget returned by this method, either by updating the existing subtree or by removing the subtree and inflating a new subtree, depending on whether the widget returned by this method can update the root of the existing subtree, as determined by calling Widget.canUpdate.

Typically implementations return a newly created constellation of widgets that are configured with information from this widget's constructor and from the given BuildContext.

The given BuildContext contains information about the location in the tree at which this widget is being built. For example, the context provides the set of inherited widgets for this location in the tree. A given widget might be built with multiple different BuildContext arguments over time if the widget is moved around the tree or if the widget is inserted into the tree in multiple places at once.

The implementation of this method must only depend on:

• the fields of the widget, which themselves must not change over time, and
• any ambient state obtained from the context using BuildContext.dependOnInheritedWidgetOfExactType.

If a widget's build method is to depend on anything else, use a StatefulWidget instead.

See also:

• StatelessWidget, which contains the discussion on performance considerations.

Implementation

@override
Widget build(BuildContext context) {
  final scope = TreeRowDragScope.maybeOf(context);
  final armed = enabled && (scope?.canDrag ?? false);
  return MouseRegion(
    cursor: armed ? SystemMouseCursors.grab : MouseCursor.defer,
    // `RenderMouseRegion` defaults its hit-test behaviour to
    // `HitTestBehavior.opaque` (rendering/proxy_box.dart), which would
    // make this wrapper absorb hits on its own account. Defer instead,
    // and let the `Listener` below make the single decision.
    hitTestBehavior: HitTestBehavior.deferToChild,
    child: Listener(
      // OPAQUE while armed, and this is not cosmetic. `Listener`
      // defaults to `deferToChild`, so a handle wrapping a child that
      // does not hit-test itself (a bare `SizedBox`, a `Padding` around
      // nothing, a `CustomPaint` with no hit-test override) would
      // render, show no cursor, and silently never drag. That is
      // exactly the footgun a caller-placed handle invites, so the
      // behaviour is decided here rather than left to whatever the
      // caller happens to pass as [child].
      //
      // TRANSPARENT while disarmed, which is the other half. An opaque
      // disarmed handle would swallow taps the app wants for its own
      // menu, and would shadow whatever it overlaps during a FLIP
      // slide, where `RenderSliverTree.hitTestChildren` returns on the
      // first row that reports a hit.
      //
      // Flipping a FIELD, never the widget type, so `Widget.canUpdate`
      // still matches and a `canReorder` flip does not re-inflate the
      // row subtree beneath this handle.
      behavior: armed ? HitTestBehavior.opaque : HitTestBehavior.deferToChild,
      // Disarmed by NULLING, never by omitting, for the shape-stability
      // reason spelled out on [enabled].
      onPointerDown: armed
          ? (event) {
              _start(context, scope!, event);
            }
          : null,
      child: child,
    ),
  );
}