listview - 图像的无限 ListView 卡住应用程序或在布局期间抛出异常 [包括小应用程序]

Question

我试图用可重复的图像制作一个水平滚动的 ListView，让它看起来像是一个永远滚动的长图像(就像在 2d 平台游戏中一样)。

但是，当我将图像添加到 ListView.build(...) 方法时，我无法让它们正确缩放。我希望图像为 .fitHeight，在尝试了所有我能想到的小部件之后，我得到了两种结果之一。

1. 应用程序启动但卡住
2. 应用运行但抛出异常

完整代码如下:

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: Scaffold(
        body: MyHomePage(),
      ),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

  @override
  Widget build(BuildContext context) {

    /// Freezes app and prevents it from hot reload/rebuild until app is manually terminated.
    Widget image = Image.asset('assets/repeatable-forest.jpg', fit: BoxFit.fitHeight);

    /// Wrapping Image in a FittedBox works, but with a lot of exceptions/warnings
    Widget fittedWrappedImage =
        FittedBox(child: Image.asset('assets/repeatable-forest.jpg'), fit: BoxFit.fitHeight);

    return ListView.builder(
      scrollDirection: Axis.horizontal,
      controller: ScrollController(),
      itemBuilder: (BuildContext context, int index) {
        return image; //fittedWrappedImage;  <-- choose which one to test.
      },
    );
  }
}

如果您想尝试使用示例图片(实际上看起来不错)，您可以在此处下载“完整”存储库: https://github.com/joelbrostrom/infinite_listview_image_freeze_example

正如您在代码注释中看到的那样，Image.asset() 卡住应用程序并将其包装在 FittedBox 中，使其可运行但有异常(exception)。

在我的主要项目中，第二种方法在调试配置中出现异常，但在发布时卡住。也许这是一个线索？

这是我在 FittedBox 中包装图像时遇到的异常:

I/flutter (27099): ══╡ EXCEPTION CAUGHT BY RENDERING LIBRARY ╞═════════════════════════════════════════════════════════
I/flutter (27099): The following assertion was thrown during performLayout():
I/flutter (27099): 'package:flutter/src/rendering/box.dart': Failed assertion: line 307 pos 12: 'width > 0.0': is not
I/flutter (27099): true.
I/flutter (27099): 
I/flutter (27099): Either the assertion indicates an error in the framework itself, or we should provide substantially
I/flutter (27099): more information in this error message to help you determine and fix the underlying cause.
I/flutter (27099): In either case, please report this assertion by filing a bug on GitHub:
I/flutter (27099):   https://github.com/flutter/flutter/issues/new?template=BUG.md
I/flutter (27099): 
I/flutter (27099): When the exception was thrown, this was the stack:
I/flutter (27099): #2      BoxConstraints.constrainSizeAndAttemptToPreserveAspectRatio (package:flutter/src/rendering/box.dart:307:12)
I/flutter (27099): #3      RenderFittedBox.performLayout (package:flutter/src/rendering/proxy_box.dart:2288:26)
I/flutter (27099): #4      RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #5      _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #6      RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #7      _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #8      RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #9      RenderSliverList.performLayout (package:flutter/src/rendering/sliver_list.dart:164:27)
I/flutter (27099): #10     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #11     RenderSliverPadding.performLayout (package:flutter/src/rendering/sliver_padding.dart:182:11)
I/flutter (27099): #12     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #13     RenderViewportBase.layoutChildSequence (package:flutter/src/rendering/viewport.dart:407:13)
I/flutter (27099): #14     RenderViewport._attemptLayout (package:flutter/src/rendering/viewport.dart:1322:12)
I/flutter (27099): #15     RenderViewport.performLayout (package:flutter/src/rendering/viewport.dart:1240:20)
I/flutter (27099): #16     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #17     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #18     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #19     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #20     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #21     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #22     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #23     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #24     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #25     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #26     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #27     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #28     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #29     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #30     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #31     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #32     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #33     MultiChildLayoutDelegate.layoutChild (package:flutter/src/rendering/custom_layout.dart:142:11)
I/flutter (27099): #34     _ScaffoldLayout.performLayout (package:flutter/src/material/scaffold.dart:350:7)
I/flutter (27099): #35     MultiChildLayoutDelegate._callPerformLayout (package:flutter/src/rendering/custom_layout.dart:212:7)
I/flutter (27099): #36     RenderCustomMultiChildLayoutBox.performLayout (package:flutter/src/rendering/custom_layout.dart:356:14)
I/flutter (27099): #37     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #38     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #39     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #40     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #41     _RenderCustomClip.performLayout (package:flutter/src/rendering/proxy_box.dart:1206:11)
I/flutter (27099): #42     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #43     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #44     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #45     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #46     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #47     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #48     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #49     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #50     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #51     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #52     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #53     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #54     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #55     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #56     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #57     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #58     RenderOffstage.performLayout (package:flutter/src/rendering/proxy_box.dart:3032:13)
I/flutter (27099): #59     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #60     RenderStack.performLayout (package:flutter/src/rendering/stack.dart:510:15)
I/flutter (27099): #61     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #62     __RenderTheatre&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #63     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #64     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #65     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #66     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #67     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #68     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #69     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #70     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #71     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #72     _RenderProxyBox&RenderBox&RenderObjectWithChildMixin&RenderProxyBoxMixin.performLayout (package:flutter/src/rendering/proxy_box.dart:105:13)
I/flutter (27099): #73     RenderObject.layout (package:flutter/src/rendering/object.dart:1632:7)
I/flutter (27099): #74     RenderView.performLayout (package:flutter/src/rendering/view.dart:151:13)
I/flutter (27099): #75     RenderObject._layoutWithoutResize (package:flutter/src/rendering/object.dart:1507:7)
I/flutter (27099): #76     PipelineOwner.flushLayout (package:flutter/src/rendering/object.dart:766:18)
I/flutter (27099): #77     _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding.drawFrame (package:flutter/src/rendering/binding.dart:329:19)
I/flutter (27099): #78     _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding&WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:701:13)
I/flutter (27099): #79     _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:268:5)
I/flutter (27099): #80     _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:988:15)
I/flutter (27099): #81     _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:928:9)
I/flutter (27099): #82     _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding.scheduleWarmUpFrame.<anonymous closure> (package:flutter/src/scheduler/binding.dart:749:7)
I/flutter (27099): #91     _Timer._runTimers (dart:isolate/runtime/libtimer_impl.dart:382:19)
I/flutter (27099): #92     _Timer._handleMessage (dart:isolate/runtime/libtimer_impl.dart:416:5)
I/flutter (27099): #93     _RawReceivePortImpl._handleMessage (dart:isolate/runtime/libisolate_patch.dart:171:12)
I/flutter (27099): (elided 10 frames from class _AssertionError and package dart:async)
I/flutter (27099): 
I/flutter (27099): The following RenderObject was being processed when the exception was fired:
I/flutter (27099):   RenderFittedBox#fc66f relayoutBoundary=up5 NEEDS-LAYOUT NEEDS-PAINT
I/flutter (27099):   creator: FittedBox ← RepaintBoundary-[<0>] ← IndexedSemantics ←
I/flutter (27099):   NotificationListener<KeepAliveNotification> ← KeepAlive ← AutomaticKeepAlive ← SliverList ←
I/flutter (27099):   MediaQuery ← SliverPadding ← Viewport ← IgnorePointer-[GlobalKey#aa491] ← Semantics ← ⋯
I/flutter (27099):   parentData: <none> (can use size)
I/flutter (27099):   constraints: BoxConstraints(0.0<=w<=Infinity, h=1089.8)
I/flutter (27099):   size: MISSING
I/flutter (27099):   fit: fitHeight
I/flutter (27099):   alignment: center
I/flutter (27099):   textDirection: ltr
I/flutter (27099): This RenderObject had the following descendants (showing up to depth 5):
I/flutter (27099):   RenderSemanticsAnnotations#bfe63 relayoutBoundary=up6 NEEDS-PAINT
I/flutter (27099):     RenderImage#90dc8 relayoutBoundary=up7 NEEDS-PAINT
I/flutter (27099): ════════════════════════════════════════════════════════════════════════════════════════════════════
I/flutter (27099): Another exception was thrown: RenderBox was not laid out: RenderFittedBox#fc66f relayoutBoundary=up5 NEEDS-PAINT
I/flutter (27099): Another exception was thrown: RenderBox was not laid out: RenderRepaintBoundary#4ba38 relayoutBoundary=up4 NEEDS-PAINT
I/flutter (27099): Another exception was thrown: 'package:flutter/src/rendering/sliver_multi_box_adaptor.dart': Failed assertion: line 467 pos 12: 'child.hasSize': is not true.
I/flutter (27099): Another exception was thrown: NoSuchMethodError: The getter 'scrollOffsetCorrection' was called on null.
I/flutter (27099): Another exception was thrown: NoSuchMethodError: The method 'debugAssertIsValid' was called on null.
I/flutter (27099): Another exception was thrown: NoSuchMethodError: The getter 'visible' was called on null.
I/flutter (27099): Another exception was thrown: NoSuchMethodError: The getter 'visible' was called on null.
D/libGLESv2(27099): STS_GLApi : DTS, ODTC are not allowed for Package : com.example.listview_image_scaling_issue

如有任何见解，我们将不胜感激!

Answer 1

嗯...文档说:

///  2. The [ListView.builder] constructor takes an [IndexedWidgetBuilder], which 
///     builds the children on demand. This constructor is appropriate for list views 
///     with a large (or infinite) number of children because the builder is called 
///     only for those children that are actually visible.

从理论上讲，这应该使您想做的事情成为可能。无论出于何种原因，您似乎都陷入了循环。 itemBuilder 不断被调用。我的猜测是，在 Assets 加载并实际呈现之前，无法确定它的尺寸。

您可以通过设置 itemCount: 1 来验证这一点，然后看到卡住消失了。此外，对 Image 的宽度设置初始约束有助于:

return ListView.builder(
  scrollDirection: Axis.horizontal,
  itemBuilder: (ctx, index) {
    return ConstrainedBox(
      constraints: BoxConstraints(
        minWidth: MediaQuery.of(context).size.width,
        maxWidth: double.infinity,
      ),
      child: Image.asset(
        'assets/repeatable-forest.jpg',
        fit: BoxFit.fitHeight,
      ),
    );
  },
);

如果有人能深入了解为什么 itemBuilder 一直被调用，那就太棒了。