如何修复ListTile触发RenderBox was not laid out布局异常问题

• 布局错误的根源是Row的子元素没有明确横向宽度约束：ListTile默认会填充父组件提供的最大横向宽度，你直接将返回ListTile的StreamBuilder放在Row的children列表中时，Row默认不会给子元素限制横向尺寸，导致ListTile布局时计算出无限宽度，最终触发hasSize断言失败。

• 你只需要用Expanded直接包裹整个StreamBuilder即可。Expanded会强制其子组件在Row的横向主轴上填充剩余可用空间，给ListTile明确的宽度约束，即可解决布局异常。如果你的Row后续要添加其他横向排列的组件，Expanded也会自动计算剩余宽度分配给ListTile，不需要额外调整。

修改后的代码示例：

Row get upperSection {
    return Row(
      children: <Widget>[
        // 用Expanded包裹整个StreamBuilder
        Expanded(
          child: StreamBuilder<BluetoothDeviceState>(
            stream: widget.device.state,
            initialData: BluetoothDeviceState.connecting,
            builder: (c, snapshot) => ListTile(
              leading: (snapshot.data == BluetoothDeviceState.connected)
                  ? const Icon(Icons.bluetooth_connected)
                  : const Icon(Icons.bluetooth_disabled),
              title: Text('Device is ${snapshot.data.toString().split('.')[0]}.'),
              subtitle: Text('${widget.device.id}'),
              trailing: StreamBuilder<bool>(
                //The below stream is to show either a refresh button or
                //CircularProgress based on the service discovering status,
                //and a IndexedStack widget is used to present the needed widget
                stream: widget.device.isDiscoveringServices,
                initialData: false,
                builder: (c, snapshot) => IndexedStack(
                  index: snapshot.data ? 0 : 0,
                  children: <Widget>[
                    IconButton(
                      icon: const Icon(Icons.refresh),
                      onPressed: () => widget.device.discoverServices(),
                    ),
                    const IconButton(
                      icon: SizedBox(
                        child: CircularProgressIndicator(
                          valueColor: AlwaysStoppedAnimation(Colors.grey),
                        ),
                        width: 17.0,
                        height: 17.0,
                      ),
                      onPressed: null,
                    ),
                  ],
                ),
              ),
            ),
          ),
        ),
      ],
    );
  }

内容的提问来源于stack exchange，提问作者Ahmad Zaklouta