Flutter onStretchTrigger 不工作。在帧期间安排构建

Question

如果我使用刷新指示器或点击按钮，此代码将完全正常工作。

但如果在 SliverAppBar 中用于 onStretchTrigger 函数.

，将无法工作

onStretchTrigger: () async {
 setState(() {
 myVariable = myFutureData();
 });
},

我得到的错误：

I/flutter (31102): Build scheduled during frame.

I/flutter (31102): While the widget tree was being built, laid out, and painted, a new frame was scheduled to rebuild the widget tree.

I/flutter (31102): This might be because setState() was called from a layout or paint callback. If a change is needed to the widget tree, it should be applied as the tree is being built. Scheduling a change for the subsequent frame instead results in an interface that lags behind by one frame. If this was done to make your build dependent on a size measured at layout time, consider using a LayoutBuilder, CustomSingleChildLayout, or CustomMultiChildLayout.

If, on the other hand, the one frame delay is the desired effect, for example because this is an animation, consider scheduling the frame in a post-frame callback using SchedulerBinding.addPostFrameCallback or using an AnimationController to trigger the animation. I/flutter (31102): I/flutter (31102): #0
WidgetsBinding._handleBuildScheduled. package:flutter/…/widgets/binding.dart:747

I/flutter (31102): #1 WidgetsBinding._handleBuildScheduled package:flutter/…/widgets/binding.dart:770

I/flutter (31102): #2 BuildOwner.scheduleBuildFor package:flutter/…/widgets/framework.dart:2476

I/flutter (31102): #3 Element.markNeedsBuild package:flutter/…/widgets/framework.dart:4324

I/flutter (31102): #4 State.setState package:flutter/…/widgets/framework.dart:1108

I/flutter (31102): #5 _HomeScreenState.build. package:fesale/screens/screen_home.dart:197

I/flutter (31102): #6 _HomeScreenState.build. package:fesale/screens/screen_home.dart:195

I/flutter (31102): #7 RenderSliverPersistentHeader.layoutChild package:flutter/…/rendering/sliver_persistent_header.dart:257

I/flutter (31102): #8
RenderSliverFloatingPersistentHeader.performLayout package:flutter/…/rendering/sliver_persistent_header.dart:708

I/flutter (31102): #9 RenderObject.layout package:flutter/…/rendering/object.dart:1858

I/flutter (31102): #10 RenderViewportBase.layoutChildSeq

请帮助找不到任何解决方案:(

Answer 1

INITIAL：首先：您不需要在此处放置 async，因为您没有使用 await 关键字，这只是一个函数调用，将 return 值分配给您的变量。

UPDATE：onStretchTrigger 实际上需要一个 AsyncCallback 所以对于这个用例必须使用 async 关键字 - 抱歉！

---

其次：您的错误消息说明了导致此问题的原因并提供了几种解决方案：

[...] This might be because setState() was called from a layout or paint callback. [...] consider scheduling the frame in a post-frame callback using SchedulerBinding.addPostFrameCallback [...]

所以现在你可以做：

SchedulerBinding.instance?.addPostFrameCallback((_) {
  setState(() => myVariable = myFutureData());
});