BaselineBox complete, widget and example

baseline_box_with_example.dart

import 'package:flutter/widgets.dart';
import 'package:flutter/foundation.dart';
import 'package:flutter/rendering.dart';

import 'package:flutter/material.dart';

void main() => runApp(new BaselineExampleApp());

class BaselineExampleApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new Scaffold(
        body: new Center(
          child: Container(
            height: 200.0,
            width: 220.0,
            color: Colors.green,
            child: Baseline(
              baseline: 0.0,
              baselineType: TextBaseline.alphabetic,
              child: BaselineBox(
                baseline: 30.0,
                //fromTop: true,
                child: Container(
                  color: Colors.red,
                  height: 100.0,
                  width: 100.0,
                ),
              ),
            ),
          ),
        ),
      ),
    );
  }
}


/// A widget that sets a baseline based on the distance in logical pixels to
/// its bottom or to its top.
///
/// By default, when no values are provided, the baseline coincides with the
/// bottom of the widget, as this is de default behavior for widgets without a
/// baseline. When the param `baseline` has a value, it represents the offset
/// from the bottom.
///
/// If `fromTop` is true, the baseline is the offset from the top of the widget.
///
/// ```dart
/// Baselined(
///   baseline: 15.0,
///   child: Container(
///     padding: 20.0,
///     child: Text('The baseline is now 5.0 below this text widget'),
/// )
/// ```
///
/// The baseline is set regardless of the [TextBaseline] type, meaning that
/// when the baseline for this widget is queried from the parent, the same
/// baseline will be returned regardless of the [TextBaseline] type queried.
///
/// See also:
///
///  * [Baseline], a widget that positions its child according to the child's
///    baseline.
///  * [Align], a widget that aligns its child within itself and optionally
///    sizes itself based on the child's size.
///  * [Center], a widget that centers its child within itself.
///  * The [catalog of layout widgets](https://flutter.io/widgets/layout/).
class BaselineBox extends SingleChildRenderObjectWidget {
  /// Creates a widget that sets a baseline to its child.
  ///
  /// By default, [baseline] is 0.0 and [fromTop] is false.
  /// The [baseline] argument must not be null.
  const BaselineBox(
      {Key key, this.baseline = 0.0, this.fromTop = false, Widget child})
      : assert(baseline != null),
        super(key: key, child: child);

  /// The number of logical pixels from the bottom or from the top of this
  /// widget at which the baseline is positioned.
  final double baseline;

  /// Whether the baseline distance is measured from the bottom of the widget
  /// or from the top
  final bool fromTop;

  @override
  RenderBaselineBox createRenderObject(BuildContext context) =>
      new RenderBaselineBox(baseline: baseline, fromTop: fromTop);

  @override
  void updateRenderObject(BuildContext context, RenderBaselineBox renderObject) {
    renderObject
      ..baseline = baseline
      ..fromTop = fromTop;
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(new DoubleProperty('baseline', baseline));
    properties.add(new DiagnosticsProperty<bool>('fromTop', fromTop));
  }
}

/// Sets a baseline based on the distance in logical pixels to its
/// bottom or to its top.
///
/// By default, when no values are provided, the baseline coincides with the
/// bottom of the widget, as this is de default behavior for widgets without a
/// baseline. When the param `baseline` has a value, it represents the offset
/// from the bottom.
///
/// If `fromTop` is true, the baseline is the offset from the top of the widget.
///
/// The baseline is set regardless of the [TextBaseline] type, meaning that
/// when the baseline for this widget is queried from the parent, the same
/// baseline will be returned regardless of the [TextBaseline] type queried.
///
class RenderBaselineBox extends RenderProxyBox {
  /// Creates a render object that adds a baseline to its child.
  ///
  /// The [baseline] argument must be a finite value.
  RenderBaselineBox({
    RenderBox child,
    @required double baseline,
    bool fromTop = false,
  })  : assert(baseline != null),
        assert(baseline.isFinite),
        assert(fromTop != null),
        _baseline = baseline,
        _fromTop = fromTop,
        super(child);

  /// Value used as the offset for the baseline.
  double get baseline => _baseline;
  double _baseline;

  set baseline(double value) {
    assert(value != null);
    assert(value.isFinite);
    if (_baseline == value) return;
    _baseline = value;
    markNeedsLayout();
  }

  /// Whether the baseline distance is measured from the bottom of the widget
  /// or from the top
  bool get fromTop => _fromTop;
  bool _fromTop;

  set fromTop(bool value) {
    assert(value != null);
    if (_fromTop == value) return;
    _fromTop = value;
    markNeedsLayout();
  }

  @override
  double computeDistanceToActualBaseline(TextBaseline baselineType) {
    return _fromTop ? _baseline : size.height - _baseline;
  }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(new DoubleProperty('baseline', baseline));
    properties.add(new DiagnosticsProperty<bool>('fromTop', fromTop));
  }
}