如何获得小部件的高度？

获取大小/小部件在屏幕上的位置，您可以使用 GlobalKey 获取其 BuildContext 来找到 RenderBox 特定的小部件，其中将包含其全局位置和呈现的大小。

To get the size/position of a widget on screen, you can use GlobalKey to get its BuildContext to then find the RenderBox of that specific widget, which will contain its global position and rendered size.

请注意以下几点：如果未呈现窗口小部件，则可能不存在。这可能会导致 ListView 出现问题，因为只有在可能可见的情况下才渲染窗口小部件。

Just one thing to be careful of: That context may not exist if the widget is not rendered. Which can cause a problem with ListView as widgets are rendered only if they are potentially visible.

另一个问题是您无法在 build 调用期间获得窗口小部件的 RenderBox ，因为窗口小部件尚未呈现。

Another problem is that you can't get a widget's RenderBox during build call as the widget hasn't been rendered yet.

但是我需要在构建过程中调整大小！我该怎么办？

有一个很酷的小工具可以帮助您：覆盖及其 OverlayEntry 。
它们用于在所有其他事物（类似于堆栈）的顶部显示小部件。

There's one cool widget that can help: Overlay and its OverlayEntry. They are used to display widgets on top of everything else (similar to stack).

但是最酷的是它们在不同的构建流程；它们是在 常规小部件之后构建的。

But the coolest thing is that they are on a different build flow; they are built after regular widgets.

具有一种超酷的含义： OverlayEntry

That have one super cool implication: OverlayEntry can have a size that depends on widgets of the actual widget tree.

好。但是，OverlayEntry不需要手动重建吗？

是的，它们确实需要。但是还有另一件事要注意：传递给 Scrollable 的 ScrollController 与 AnimationController 。

Yes, they do. But there's another thing to be aware of: ScrollController, passed to a Scrollable, is a listenable similar to AnimationController.

这意味着您可以将 AnimatedBuilder 与 ScrollController 组合在一起>，它会产生很好的效果，可以自动滚动重建窗口小部件。

Which means you could combine an AnimatedBuilder with a ScrollController, it would have the lovely effect to rebuild your widget automatically on a scroll. Perfect for this situation, right?

将所有内容组合为示例：

Combining everything into an example:

在下面的示例中，您将看到一个叠加层，该叠加层位于 ListView 内的小部件之后，并具有相同的高度。

In the following example, you'll see an overlay that follows a widget inside ListView and shares the same height.

import 'package:flutter/material.dart';
import 'package:flutter/scheduler.dart';

class MyHomePage extends StatefulWidget {
  const MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final controller = ScrollController();
  OverlayEntry sticky;
  GlobalKey stickyKey = GlobalKey();

  @override
  void initState() {
    if (sticky != null) {
      sticky.remove();
    }
    sticky = OverlayEntry(
      builder: (context) => stickyBuilder(context),
    );

    SchedulerBinding.instance.addPostFrameCallback((_) {
      Overlay.of(context).insert(sticky);
    });

    super.initState();
  }

  @override
  void dispose() {
    sticky.remove();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: ListView.builder(
        controller: controller,
        itemBuilder: (context, index) {
          if (index == 6) {
            return Container(
              key: stickyKey,
              height: 100.0,
              color: Colors.green,
              child: const Text("I'm fat"),
            );
          }
          return ListTile(
            title: Text(
              'Hello $index',
              style: const TextStyle(color: Colors.white),
            ),
          );
        },
      ),
    );
  }

  Widget stickyBuilder(BuildContext context) {
    return AnimatedBuilder(
      animation: controller,
      builder: (_,Widget child) {
        final keyContext = stickyKey.currentContext;
        if (keyContext != null) {
          // widget is visible
          final box = keyContext.findRenderObject() as RenderBox;
          final pos = box.localToGlobal(Offset.zero);
          return Positioned(
            top: pos.dy + box.size.height,
            left: 50.0,
            right: 50.0,
            height: box.size.height,
            child: Material(
              child: Container(
                alignment: Alignment.center,
                color: Colors.purple,
                child: const Text("^ Nah I think you're okay"),
              ),
            ),
          );
        }
        return Container();
      },
    );
  }
}

注意：

导航到其他屏幕时，呼叫以下内容将保持可见。

When navigating to a different screen, call following otherwise sticky would stay visible.

sticky.remove();