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'),
),
),
),