SliverAppBar 有一个属性 bottom，它必须有 preferredSize。

现在我让它返回常量值:

  ...
  new SliverAppBar(
    expandedHeight: _kFlexibleSpaceMaxHeight,
    flexibleSpace: new FlexibleSpaceBar(.....)
    ...                   
    bottom: new BottomBar(...), // has to have preferredSize
  ),
  ...

class BottomBar extends StatelessWidget implements PreferredSizeWidget {
    ...
    @override
      Size get preferredSize {
        return new Size.fromHeight(my_constant_height_value);
      }

    ...
    }

我想在这个底部的 Widget 中放置一个文本，但我不知道其中的文本将有多长。

如何实现底部小部件的动态高度？

有没有办法在布局之前测量小部件的高度？

编辑 25/04/2018

最终，我遵循了 Thibault 的指示并得到了这个:

// 'as rendering' to avoid conflict with 'package:intl/intl.dart'
import 'package:flutter/rendering.dart' as rendering; 

...

// this is the function that returns the height of a Text widget
// given the text
double getHeight(String text, BuildContext context, bool isTitle) {
  var rp = rendering.RenderParagraph(
    new TextSpan(
        style: isTitle
            ? Theme.of(context).primaryTextTheme.title
            : Theme.of(context).primaryTextTheme.subhead,
        text: text,
        children: null,
        recognizer: null),

    // important as the user can have increased text on his device
    textScaleFactor: MediaQuery.of(context).textScaleFactor, 

    textDirection: rendering.TextDirection.ltr,
  );
  var horizontalPaddingSum = 20; // optional 
  var width = MediaQuery.of(context).size.width - horizontalPaddingSum;
  // if your Text widget has horizontal padding then you have to 
  // subtract it from available width to get the needed results
  var ret = rp.computeMinIntrinsicHeight(width);
  return ret;
}

...


  _kPreferredBBTextHeight =
      getHeight(mTitle ?? "", context, true);

  var verticalPaddingSum = 10;
  _kPreferredBBSubTextHeight = getHeight(mSubtitle ?? "", context,false) + verticalPaddingSum;

  _kPreferredBottomBarSize =
      _kPreferredBBTextHeight + _kPreferredBBSubTextHeight + 48;

  _kFlexibleSpaceMaxHeight =
      _kPreferredBottomBarSize + _kPreferredBottomBarSize + kToolbarHeight;

  _backgroudBottomPadding = _kPreferredBottomBarSize;

...
new CustomSliverAppBar(
                pinned: true,
                automaticallyImplyLeading: false,
                primary: true,
                expandedHeight: _kFlexibleSpaceMaxHeight,
                flexibleSpace: new FlexibleSpaceBar(
                  background: new Padding(
                      padding:
                          new EdgeInsets.only(bottom: _backgroudBottomPadding),
                      child: new Image(
                        image: new NetworkImage(mImageUrl),
                        fit: BoxFit.cover,
                      )),
                ),
                bottom: new BottomBar(
                  fixedHeight: _kPreferredBottomBarSize,
                ),
              ),

...

class BottomBar extends StatelessWidget implements PreferredSizeWidget {
  final double fixedHeight;

  BottomBar({this.fixedHeight});

  @override
  Size get preferredSize {
    return new Size.fromHeight(this.fixedHeight);
  }

  @override
  Widget build(BuildContext context) {
    // https://github.com/flutter/flutter/issues/3782
    return new Container(
        height: this.fixedHeight,
        child: new Material(
            color: Theme.of(context).primaryColor,
            child: new Column(
              children: <Widget>[
                new Row(
                  children: <Widget>[
                    new IconButton(
                      icon: new Icon(Icons.arrow_back, color: Colors.white),
                      onPressed: () {
                        Navigator.of(context).pop();
                      },
                    ),
                    new Expanded(
                      child: new Container(),
                    ),
                    new IconButton(
                      icon: new Icon(Icons.share, color: Colors.white),
                      onPressed: () {
                        print("share pressed");
                      },
                    )
                  ],
                ),
                new Column(
                  mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                  children: <Widget>[
                    new Padding(
                        padding: new EdgeInsets.only(left: 10.0, right: 10.0),
                        child: new Container(
                          child: new Container(
                            alignment: Alignment.centerLeft,
                            child: new Text(
                              mTitle ?? "",
                              style: Theme.of(context).primaryTextTheme.title,
                            ),
                          ),
                        )),
                    new Container(
                      padding: new EdgeInsets.only(
                          left: 10.0, right: 10.0, top: 5.0, bottom: 5.0),
                      alignment: Alignment.centerLeft,
                      child: new Text(
                        mSubtitle ?? "",
                        style: Theme.of(context).primaryTextTheme.subhead,
                      ),
                    ),
                  ],
                ),
              ],
            )));
  }

最佳答案

PreferredSizeWidget 的全部意义在于不，您不能动态调整此小部件的大小。

其背后的原因是 Scaffold 使用首选大小进行一些计算。如果 appbar 的大小在渲染之前是未知的，那将是不可能的。

您必须相应地重新考虑您的 UI。

关于layout - 有没有办法在 SliverAppBar 的底部小部件中实现动态高度，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/49268040/