这是代码：
Widget getSlider(BuildContext context) {
return SliderTheme(
data: SliderTheme.of(context).copyWith(
overlayShape: RoundSliderThumbShape(enabledThumbRadius: 25.0),
trackHeight: 0,
thumbShape: RoundSliderThumbShape(enabledThumbRadius: 25.0),
thumbColor: thumbColor,
),
child: Slider(
value: value,
min: minValue,
max: maxValue,
onChanged: onChanged,
onChangeEnd: onChangeEnd,
),
);
}
一切似乎都很好。但是在我将 Flutter 升级到版本 1.19.0-0.0.pre 后，它中断并给我这个错误：
════════ Exception caught by rendering library
The following NoSuchMethodError was thrown during paint():
The getter 'isEmpty' was called on null.
Receiver: null
Tried calling: isEmpty
The relevant error-causing widget was:
Slider file:///s_app/lib/components/ui_components/slider_components.dart:254:14
When the exception was thrown, this was the stack:
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
#1 RoundSliderThumbShape.paint (package:flutter/src/material/slider_theme.dart:2373:30)
#2 _RenderSlider.paint (package:flutter/src/material/slider.dart:1323:33)
#3 RenderObject._paintWithContext (package:flutter/src/rendering/object.dart:2264:7)
#4 PaintingContext.paintChild (package:flutter/src/rendering/object.dart:184:13)
...
The following RenderObject was being processed when the exception was fired: _RenderSlider#4d553
... parentData: <none> (can use size)
... constraints: BoxConstraints(0.0<=w<=270.0, 0.0<=h<=30.0)
... semantic boundary
... size: Size(270.0, 30.0)
RenderObject: _RenderSlider#4d553
parentData: <none> (can use size)
constraints: BoxConstraints(0.0<=w<=270.0, 0.0<=h<=30.0)
semantic boundary
size: Size(270.0, 30.0)
══════════════════════════════════════════════════════
这是我的颤动医生：
[✓] Flutter (Channel dev, 1.19.0-0.0.pre, on Mac OS X 10.15.3 19D76, locale en-US)
• Flutter version 1.19.0-0.0.pre at /Applications/flutter
• Framework revision a849daf283 (4 days ago), 2020-05-07 18:59:02 -0700
• Engine revision 3953c3ccd1
• Dart version 2.9.0 (build 2.9.0-5.0.dev 4da5b40fb6)
[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
• Android SDK at /Applications/Android/sdk
• Platform android-29, build-tools 29.0.3
• ANDROID_HOME = /Applications/Android/sdk
• Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
• Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b4-5784211)
• All Android licenses accepted.
[✓] Xcode - develop for iOS and macOS (Xcode 11.4.1)
• Xcode at /Applications/Xcode.app/Contents/Developer
• Xcode 11.4.1, Build version 11E503a
• CocoaPods version 1.9.1
[✓] Chrome - develop for the web
• Chrome at /Applications/Google
Chrome.app/Contents/MacOS/Google Chrome
[✓] Android Studio (version 3.6)
• Android Studio at /Applications/Android Studio.app/Contents
• Flutter plugin version 46.0.1-dev.5
• Dart plugin version 192.7761
• Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b4-5784211)
[✓] VS Code (version 1.42.0)
• VS Code at /Applications/Visual Studio Code.app/Contents
• Flutter extension version 3.8.0
[✓] Connected device (3 available)
• iPho • acd5efecf25da9ac75d28 • ios • iOS 13.4.1
• Web Server • web-server • web-javascript • Flutter Tools
• Chrome • chrome • web-javascript • Google Chrome 81.0.4044.138
• No issues found!
在我跟踪问题后，我发现 RoundSliderThumbShape 的绘制方法中的 [sizeWithOverflow] 为空。但是我不知道如何使其不为空，因为这是 flutter 自己的代码。
此外，仅作为旁注，如果我删除滑块主题，一切正常。
终于我找到了解决方案。这可能不漂亮，但这是我目前能找到的。我对更好的解决方案持开放态度。
- 我创建了一个继承自 SliderComponentShape 的新类。
- 在它的绘画方法中，我注释掉了
assert(!sizeWithOverflow.isEmpty); - 我添加了
sizeWithOverflow = parentBox.size;
这行得通。
所以代码看起来像这样：
class SpecialSliderThumbShape extends
SliderComponentShape {
// .... All the same codes as the original RoundSliderThumbShape
@override
void paint(
PaintingContext context,
Offset center, {
Animation<double> activationAnimation,
@required Animation<double> enableAnimation,
bool isDiscrete,
TextPainter labelPainter,
RenderBox parentBox,
@required SliderThemeData sliderTheme,
TextDirection textDirection,
double value,
double textScaleFactor,
Size sizeWithOverflow,
}) {
assert(context != null);
assert(center != null);
assert(enableAnimation != null);
assert(sliderTheme != null);
assert(sliderTheme.disabledThumbColor != null);
assert(sliderTheme.thumbColor != null);
// This is the difference here:
// assert(!sizeWithOverflow.isEmpty);
sizeWithOverflow = parentBox.size;
//... Continue same as the original
你是对的。 1.19 附带了一个更改，开始显示"空参数"的错误。 我也有这个，我们不是唯一的人。一个好人向Flutter的团队开了一张票。 欢迎您关注更多更新。
颤振：将空值传递到不可为空的参数值 启用运行时空安全时，这将成为失败