build method Null safety

  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) {
  assert(
    !isSelected.any((bool val) => val == null),
    'All elements of isSelected must be non-null.\n'
    'The current list of isSelected values is as follows:\n'
    '$isSelected',
  );
  assert(
    focusNodes == null || !focusNodes!.any((FocusNode val) => val == null),
    'All elements of focusNodes must be non-null.\n'
    'The current list of focus node values is as follows:\n'
    '$focusNodes',
  );
  assert(
    () {
      if (focusNodes != null) {
        return focusNodes!.length == children.length;
      }
      return true;
    }(),
    'focusNodes.length must match children.length.\n'
    'There are ${focusNodes!.length} focus nodes, while '
    'there are ${children.length} children.',
  );
  final ThemeData theme = Theme.of(context);
  final ToggleButtonsThemeData toggleButtonsTheme = ToggleButtonsTheme.of(context);
  final TextDirection textDirection = Directionality.of(context);

  final List<Widget> buttons = List<Widget>.generate(children.length, (int index) {
    final BorderRadius edgeBorderRadius = _getEdgeBorderRadius(index, children.length, textDirection, toggleButtonsTheme);
    final BorderRadius clipBorderRadius = _getClipBorderRadius(index, children.length, textDirection, toggleButtonsTheme);

    final BorderSide leadingBorderSide = _getLeadingBorderSide(index, theme, toggleButtonsTheme);
    final BorderSide borderSide = _getBorderSide(index, theme, toggleButtonsTheme);
    final BorderSide trailingBorderSide = _getTrailingBorderSide(index, theme, toggleButtonsTheme);

    final Set<MaterialState> states = <MaterialState>{
        if (isSelected[index] && onPressed != null) MaterialState.selected,
        if (onPressed == null) MaterialState.disabled,
    };
    final Color effectiveFillColor = _ResolveFillColor(fillColor
      ?? toggleButtonsTheme.fillColor).resolve(states)
      ?? _DefaultFillColor(theme.colorScheme).resolve(states);
    final Color currentColor;
    if (onPressed != null && isSelected[index]) {
      currentColor = selectedColor
        ?? toggleButtonsTheme.selectedColor
        ?? theme.colorScheme.primary;
    } else if (onPressed != null && !isSelected[index]) {
      currentColor = color
        ?? toggleButtonsTheme.color
        ?? theme.colorScheme.onSurface.withOpacity(0.87);
    } else {
      currentColor = disabledColor
        ?? toggleButtonsTheme.disabledColor
        ?? theme.colorScheme.onSurface.withOpacity(0.38);
    }
    final TextStyle currentTextStyle = textStyle
      ?? toggleButtonsTheme.textStyle
      ?? theme.textTheme.bodyText2!;
    final BoxConstraints? currentConstraints = constraints
      ?? toggleButtonsTheme.constraints;
    final Size minimumSize = currentConstraints == null
      ? const Size.square(kMinInteractiveDimension)
      : Size(currentConstraints.minWidth, currentConstraints.minHeight);
    final Size? maximumSize = currentConstraints == null
      ? null
      : Size(currentConstraints.maxWidth, currentConstraints.maxHeight);
    final Size minPaddingSize;
    switch (tapTargetSize ?? theme.materialTapTargetSize) {
      case MaterialTapTargetSize.padded:
        if (direction == Axis.horizontal) {
          minPaddingSize = const Size(
            0.0,
            kMinInteractiveDimension,
          );
        } else {
          minPaddingSize = const Size(
            kMinInteractiveDimension,
            0.0,
          );
        }
        assert(minPaddingSize.width >= 0.0);
        assert(minPaddingSize.height >= 0.0);
        break;
      case MaterialTapTargetSize.shrinkWrap:
        minPaddingSize = Size.zero;
        break;
    }

    Widget button = _SelectToggleButton(
      leadingBorderSide: leadingBorderSide,
      borderSide: borderSide,
      trailingBorderSide: trailingBorderSide,
      borderRadius: edgeBorderRadius,
      isFirstButton: index == 0,
      isLastButton: index == children.length - 1,
      direction: direction,
      verticalDirection: verticalDirection,
      child: ClipRRect(
        borderRadius: clipBorderRadius,
        child: TextButton(
          focusNode: focusNodes != null ? focusNodes![index] : null,
          style: ButtonStyle(
            backgroundColor: MaterialStatePropertyAll<Color?>(effectiveFillColor),
            foregroundColor: MaterialStatePropertyAll<Color?>(currentColor),
            overlayColor: _ToggleButtonDefaultOverlay(
              selected:  onPressed != null && isSelected[index],
              unselected: onPressed != null && !isSelected[index],
              colorScheme: theme.colorScheme,
              disabledColor: disabledColor ?? toggleButtonsTheme.disabledColor,
              focusColor: focusColor ?? toggleButtonsTheme.focusColor,
              highlightColor: highlightColor ?? toggleButtonsTheme.highlightColor,
              hoverColor: hoverColor ?? toggleButtonsTheme.hoverColor,
              splashColor: splashColor ?? toggleButtonsTheme.splashColor,
            ),
            elevation: const MaterialStatePropertyAll<double>(0),
            textStyle: MaterialStatePropertyAll<TextStyle?>(currentTextStyle.copyWith(
              color: currentColor,
            )),
            padding: const MaterialStatePropertyAll<EdgeInsetsGeometry>(EdgeInsets.zero),
            minimumSize: MaterialStatePropertyAll<Size?>(minimumSize),
            maximumSize: MaterialStatePropertyAll<Size?>(maximumSize),
            shape: const MaterialStatePropertyAll<OutlinedBorder>(RoundedRectangleBorder()),
            mouseCursor: MaterialStatePropertyAll<MouseCursor?>(mouseCursor),
            visualDensity: VisualDensity.standard,
            tapTargetSize: MaterialTapTargetSize.shrinkWrap,
            animationDuration: kThemeChangeDuration,
            enableFeedback: true,
            alignment: Alignment.center,
            splashFactory: InkRipple.splashFactory,
          ),
          onPressed: onPressed != null
            ? () {onPressed!(index);}
            : null,
          child: children[index],
        ),
      ),
    );

    if (currentConstraints != null) {
      button = Center(child: button);
    }

    return MergeSemantics(
      child: Semantics(
        container: true,
        toggled: isSelected[index],
        enabled: onPressed != null,
        child: _InputPadding(
          minSize: minPaddingSize,
          direction: direction,
          child: button,
        ),
      ),
    );
  });

  if (direction == Axis.vertical) {
    return IntrinsicWidth(
      child: Column(
        mainAxisSize: MainAxisSize.min,
        crossAxisAlignment: CrossAxisAlignment.stretch,
        verticalDirection: verticalDirection,
        children: buttons,
      ),
    );
  }

  return IntrinsicHeight(
    child: Row(
      mainAxisSize: MainAxisSize.min,
      crossAxisAlignment: CrossAxisAlignment.stretch,
      children: buttons,
    ),
  );
}