出错场景
如果我们在使用约束时没有正确的传入宽高，比如以下代码
ConstrainedBox( /// 设置最小高度为150, 最大高度为100. constraints: BoxConstraints(minHeight: 150,maxHeight: 100), child: Container( color: Colors.red, child: Center( child: Text('呵呵'), ), ), ),
运行会报以下错误
======== Exception caught by widgets library ======================================================= The following assertion was thrown building AsyncTaskPage(dirty, dependencies: [MediaQuery, _ViewScope], state: _AsyncTaskPageState#e16b3): BoxConstraints has non-normalized height constraints. The offending constraints were: BoxConstraints(0.0<=w<=Infinity, 150.0<=h<=100.0; NOT NORMALIZED) The relevant error-causing widget was: AsyncTaskPage AsyncTaskPage:file:///Users/ado/work/flutter/flutter_app/lib/main.dart:115:30 When the exception was thrown, this was the stack: #0 BoxConstraints.debugAssertIsValid.<anonymous closure>.throwError (package:flutter/src/rendering/box.dart:520:9) #1 BoxConstraints.debugAssertIsValid.<anonymous closure> (package:flutter/src/rendering/box.dart:563:9) #2 BoxConstraints.debugAssertIsValid (package:flutter/src/rendering/box.dart:578:6) #3 new ConstrainedBox (package:flutter/src/widgets/basic.dart:2511:27) #4 _AsyncTaskPageState._centerScrollEffect (package:flutter_app/async_task_page.dart:74:11) #5 _AsyncTaskPageState.build (package:flutter_app/async_task_page.dart:48:13) #6 StatefulElement.build (package:flutter/src/widgets/framework.dart:5198:27) #7 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5086:15) #8 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5251:11) #9 Element.rebuild (package:flutter/src/widgets/framework.dart:4805:7) #10 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2780:19) #11 WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:903:21) #12 RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:358:5) #13 SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1284:15) #14 SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1214:9) #15 SchedulerBinding.scheduleWarmUpFrame.<anonymous closure> (package:flutter/src/scheduler/binding.dart:939:7) #19 _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:189:12) (elided 3 frames from class _Timer and dart:async-patch) ====================================================================================================
源码分析
进入BoxConstraints的debugAssertIsValid方法查看(package:flutter/src/rendering/box.dart:520:9)
if (minWidth < 0.0 && minHeight < 0.0) { throwError(ErrorSummary('BoxConstraints has both a negative minimum width and a negative minimum height.')); } if (minWidth < 0.0) { throwError(ErrorSummary('BoxConstraints has a negative minimum width.')); } if (minHeight < 0.0) { throwError(ErrorSummary('BoxConstraints has a negative minimum height.')); } if (maxWidth < minWidth && maxHeight < minHeight) { throwError(ErrorSummary('BoxConstraints has both width and height constraints non-normalized.')); } if (maxWidth < minWidth) { throwError(ErrorSummary('BoxConstraints has non-normalized width constraints.')); } if (maxHeight < minHeight) { throwError(ErrorSummary('BoxConstraints has non-normalized height constraints.')); } if (isAppliedConstraint) { if (minWidth.isInfinite && minHeight.isInfinite) { throwError(ErrorSummary('BoxConstraints forces an infinite width and infinite height.')); } if (minWidth.isInfinite) { throwError(ErrorSummary('BoxConstraints forces an infinite width.')); } if (minHeight.isInfinite) { throwError(ErrorSummary('BoxConstraints forces an infinite height.')); } }
通过代码得知，只要出现以上最小宽高为负数、最小宽高大于最大宽高、最小宽高是无穷大就会报错，所以我们只需要在使用约束的时候注意设置正确的minWidth、maxWidth、minHeight、maxHeight即可。
正确代码
ConstrainedBox( /// 设置最小高度为100, 最大高度为150. constraints: BoxConstraints(minHeight: 100,maxHeight: 150), child: Container( color: Colors.red, child: Center( child: Text('呵呵'), ), ), ),
