build method

  1. @override
Widget build(
  1. BuildContext context
)
override

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:

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) {
  // Local property to store the composed Widget return
  Widget _child = child;

  // If the flex argument is not null, build the Expanded; otherwise if the width or height arguments are not null, build the SizedBox
  if (flex != null) {
    _child = _buildExpanded(_child);
  } else if (width != null || height != null) {
    _child = _buildSizedBox(_child);
  }

  // If the canScroll argument is true, build the SingleChildScrollView
  if (canScroll != null && canScroll == true) {
    _child = _buildScrollView(_child);
  }

  // If the onTap argument is not null, build the GestureDetector
  if (onTap != null) {
    _child = _buildGestureDetector(_child);
  }

  // If the padding argument is not null, build the inside Padding
  if (padding != null) {
    _child = _buildPadding(_child, padding!);
  }

  // If the decoration argument is not null, built the DecoratedBox
  if (decoration != null) {
    _child = _buildDecoratedBox(_child);
  }

  // If the margin argument is not null, build the outside Padding
  if (margin != null) {
    _child = _buildPadding(_child, margin!);
  }

  // Return the composed Widget
  return _child;
}