fix: borrow list ANR from infinite-height grid measurement

[A1nair]

问题

起因

在一加Ace2上点击图书馆信息之后卡死闪退

使用 adb logcat 获取原因

ApplicationExitInfo ... reason=6 (ANR) ...
description=user request after error:
Input dispatching timed out (
  io.github.benderblog.traintime_pda/...MainActivity is not responding.
  Waited 5000ms for MotionEvent(..., action=DOWN, ...)

Input dispatching timed out ... Waited 5000ms 说明点击后主线程被阻塞超过 5 秒，不是普通 Dart 异常崩溃，而是主 isolate 陷入同步死循环 / 无限布局。DartWorker 后台线程当时占用 ~6.2% CPU，但主 UI isolate 卡死。

布局循环

#304  _LayoutBuilderElement._rebuildWithConstraints.updateChildCallback
       (package:flutter/src/widgets/layout_builder.dart:247)
#306  _LayoutBuilderElement._rebuildWithConstraints
       (package:flutter/src/widgets/layout_builder.dart:270)
#307  RenderAbstractLayoutBuilderMixin.layoutCallback
       (package:flutter/src/widgets/layout_builder.dart:333)
...
#313  _RenderLayoutBuilder.performLayout
       (package:flutter/src/widgets/layout_builder.dart:447)
#315  RenderProxyBoxMixin.performLayout
       (package:flutter/src/rendering/proxy_box.dart:118)
#317  RenderProxyBoxMixin.performLayout   ← 上述两段交替出现 300+ 层
...
#350  _RenderTheater.performLayout        (Overlay / 路由树根)

LayoutBuilder 无限布局循环：外层 _RenderLayoutBuilder（来自 BorrowListView 的 LayoutBuilder）与内层 _LayoutBuilderElement（来自 AlignedGridView 内部的 SliverLayoutBuilder）互相触发布局，Flutter 的布局深度保护触发 Layout cycle。

去掉外层 LayoutBuilder 后变成 OOM

把 BorrowListView 的 LayoutBuilder 临时去掉后，profile 日志由"布局循环"变为：

E/DartVM  (23541): Exhausted heap space, trying to allocate 228... bytes.
E/DartVM  (23541): Exhausted heap space, trying to allocate 228... bytes.
...（持续）

这证明循环发生在测量/分配阶段（不是 build 阶段），且是无限分配——与"网格对每张卡片反复测量"的假设一致。

为什么卡片会被测成无限高

flutter_staggered_grid_view-0.7.0 源码：

SliverAlignedGrid.build（sliver_aligned_grid.dart:132）

return SliverLayoutBuilder(
  builder: (context, constraints) {
    final crossAxisCount = gridDelegate.getCrossAxisCount(constraints, crossAxisSpacing);
    ...
    return SliverList(
      delegate: SliverChildBuilderDelegate(
        (context, index) {
          ...
          return UniformTrack(                 // 一行卡片
            direction: constraints.crossAxisDirection,
            division: crossAxisCount,
            spacing: crossAxisSpacing,
            children: [...children],
          );
        },
        childCount: listItemCount,
      ),
    );
  },
);

每行用 UniformTrack 把一行里所有卡片拉伸到最宽那张的高度。

RenderUniformTrack._computeSize（uniform_track.dart:155-180）

Size _computeSize(BoxConstraints constraints, ChildLayouter layoutChild) {
  final mainAxisExtent = _isHorizontal ? constraints.maxWidth : constraints.maxHeight;
  final childMainAxisExtent = ((mainAxisExtent + spacing) / division) - spacing;
  final childConstraints = _isHorizontal
      ? BoxConstraints.tightFor(width: childMainAxisExtent)
      : BoxConstraints.tightFor(height: childMainAxisExtent);   // ← 纵向滚动走这里
  RenderBox? child = firstChild;
  ...
  while (child != null) {
    final size = layoutChild(child, childConstraints);          // ← 用 childConstraints 测量每张卡
    maxChildCrossAxisExtent = math.max(maxChildCrossAxisExtent, getChildCrossAxisExtent(size));
    child = childAfter(child);
  }
  ...
}

关键点：UniformTrack 是 SliverList 的子项。在滚动 sliver 中，子 box 收到的主轴（纵向）约束 maxHeight = double.infinity（sliver 主轴高度无界，子项高度由其自身内容决定）。于是：

• mainAxisExtent = constraints.maxHeight = double.infinity
• childMainAxisExtent = ((infinity + spacing) / division) - spacing = double.infinity
• 每张 BorrowInfoCard 被以 BoxConstraints.tightFor(height: double.infinity) 布局，即 minHeight = maxHeight = 无限。

修复

lib/page/library/borrow_info_card.dart

把卡片内容的最外层 Row 用 SizedBox(height: 140) 包裹：

return SizedBox(
  height: 140,
  child: [
    CachedNetworkImage(...).clipRRect(all: 14),
    const VerticalDivider(),
    [ ... 文字列 ... ].toColumn(...).expanded(),
  ].toRow(),
).padding(horizontal: 12, vertical: 4).card(elevation: 0);

AlignedGridView 内部 UniformTrack 会以 tightFor(height: infinity) 测量每张卡；BorrowInfoCard 顶层的 .expanded() 会把卡片撑成无限高，导致整张网格测量不收敛。用 SizedBox(height: 140) 给卡片一个有限且确定的高度，覆盖传入的无限高约束，使网格测量一次收敛。

140 为 250 * 0.5 留有余量。

lib/page/library/borrow_list_view.dart

去掉外层的 LayoutBuilder，改用一次性计算的 MediaQuery 求列数，并增加最小值约束：

@override
Widget build(BuildContext context) {
  final crossAxisCount = max(1, MediaQuery.of(context).size.width ~/ 360);
  return Scaffold(
    body: Stack(
      children: <Widget>[
        if (borrowList.isEmpty) EmptyListView(...),
        AlignedGridView.count(
          ...
          crossAxisCount: crossAxisCount,
          ...
        ),
      ],
    ),
    bottomNavigationBar: ...,
  );
}