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// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
import 'dart:math' as math;
import 'basic.dart';
import 'debug.dart';
import 'framework.dart';
/// [NavigationToolbar] is a layout helper to position 3 widgets or groups of
/// widgets along a horizontal axis that's sensible for an application's
/// navigation bar such as in Material Design and in iOS.
/// The [leading] and [trailing] widgets occupy the edges of the widget with
/// reasonable size constraints while the [middle] widget occupies the remaining
/// space in either a center aligned or start aligned fashion.
/// Either directly use the themed app bars such as the Material [AppBar] or
/// the iOS [CupertinoNavigationBar] or wrap this widget with more theming
/// specifications for your own custom app bar.
class NavigationToolbar extends StatelessWidget {
/// Creates a widget that lays out its children in a manner suitable for a
/// toolbar.
const NavigationToolbar({
this.centerMiddle = true,
this.middleSpacing = kMiddleSpacing,
/// The default spacing around the [middle] widget in dp.
static const double kMiddleSpacing = 16.0;
/// Widget to place at the start of the horizontal toolbar.
final Widget? leading;
/// Widget to place in the middle of the horizontal toolbar, occupying
/// as much remaining space as possible.
final Widget? middle;
/// Widget to place at the end of the horizontal toolbar.
final Widget? trailing;
/// Whether to align the [middle] widget to the center of this widget or
/// next to the [leading] widget when false.
final bool centerMiddle;
/// The spacing around the [middle] widget on horizontal axis.
/// Defaults to [kMiddleSpacing].
final double middleSpacing;
Widget build(BuildContext context) {
final TextDirection textDirection = Directionality.of(context);
return CustomMultiChildLayout(
delegate: _ToolbarLayout(
centerMiddle: centerMiddle,
middleSpacing: middleSpacing,
textDirection: textDirection,
children: <Widget>[
if (leading != null) LayoutId(id: _ToolbarSlot.leading, child: leading!),
if (middle != null) LayoutId(id: _ToolbarSlot.middle, child: middle!),
if (trailing != null) LayoutId(id: _ToolbarSlot.trailing, child: trailing!),
enum _ToolbarSlot {
class _ToolbarLayout extends MultiChildLayoutDelegate {
required this.centerMiddle,
required this.middleSpacing,
required this.textDirection,
// If false the middle widget should be start-justified within the space
// between the leading and trailing widgets.
// If true the middle widget is centered within the toolbar (not within the horizontal
// space between the leading and trailing widgets).
final bool centerMiddle;
/// The spacing around middle widget on horizontal axis.
final double middleSpacing;
final TextDirection textDirection;
void performLayout(Size size) {
double leadingWidth = 0.0;
double trailingWidth = 0.0;
if (hasChild(_ToolbarSlot.leading)) {
final BoxConstraints constraints = BoxConstraints(
maxWidth: size.width,
minHeight: size.height, // The height should be exactly the height of the bar.
maxHeight: size.height,
leadingWidth = layoutChild(_ToolbarSlot.leading, constraints).width;
final double leadingX;
switch (textDirection) {
case TextDirection.rtl:
leadingX = size.width - leadingWidth;
case TextDirection.ltr:
leadingX = 0.0;
positionChild(_ToolbarSlot.leading, Offset(leadingX, 0.0));
if (hasChild(_ToolbarSlot.trailing)) {
final BoxConstraints constraints = BoxConstraints.loose(size);
final Size trailingSize = layoutChild(_ToolbarSlot.trailing, constraints);
final double trailingX;
switch (textDirection) {
case TextDirection.rtl:
trailingX = 0.0;
case TextDirection.ltr:
trailingX = size.width - trailingSize.width;
final double trailingY = (size.height - trailingSize.height) / 2.0;
trailingWidth = trailingSize.width;
positionChild(_ToolbarSlot.trailing, Offset(trailingX, trailingY));
if (hasChild(_ToolbarSlot.middle)) {
final double maxWidth = math.max(size.width - leadingWidth - trailingWidth - middleSpacing * 2.0, 0.0);
final BoxConstraints constraints = BoxConstraints.loose(size).copyWith(maxWidth: maxWidth);
final Size middleSize = layoutChild(_ToolbarSlot.middle, constraints);
final double middleStartMargin = leadingWidth + middleSpacing;
double middleStart = middleStartMargin;
final double middleY = (size.height - middleSize.height) / 2.0;
// If the centered middle will not fit between the leading and trailing
// widgets, then align its left or right edge with the adjacent boundary.
if (centerMiddle) {
middleStart = (size.width - middleSize.width) / 2.0;
if (middleStart + middleSize.width > size.width - trailingWidth) {
middleStart = size.width - trailingWidth - middleSize.width - middleSpacing;
} else if (middleStart < middleStartMargin) {
middleStart = middleStartMargin;
final double middleX;
switch (textDirection) {
case TextDirection.rtl:
middleX = size.width - middleSize.width - middleStart;
case TextDirection.ltr:
middleX = middleStart;
positionChild(_ToolbarSlot.middle, Offset(middleX, middleY));
bool shouldRelayout(_ToolbarLayout oldDelegate) {
return oldDelegate.centerMiddle != centerMiddle
|| oldDelegate.middleSpacing != middleSpacing
|| oldDelegate.textDirection != textDirection;