动画控制器 vsync 未将“this”识别为有效表达式

Posted 2023-02-23

【中文标题】动画控制器 vsync 未将“this”识别为有效表达式

【英文标题】：Animation controller vsync not recognizing 'this' as valid expression

【发布时间】：2019-10-01 22:28:57

【问题描述】：

尝试在 dart 中填写 TextFormField 时实现加载动画。声明 AnimationController 时，我的声明无法识别“this”。

我发现的每个文档都使用“this”作为 vsync，由于某种原因我无法识别它。

class _SettingsScreenState extends State<SettingsScreen> with SingleTickerProviderStateMixin 
final _animationController = AnimationController(vsync: this, duration: Duration(seconds: 1));

...

ListTile(
                  title: new TextFormField(
                    controller: _serverController,
                    keyboardType: TextInputType.emailAddress,
                    style: TextStyle(
                        color: darkGray,
                        fontSize: 18,
                        fontWeight: FontWeight.w500,
                        fontFamily: mainFontFamily
                    ),
                    onFieldSubmitted: _handleSubmitted,
                    autocorrect: false,
                    decoration: new InputDecoration(
                      icon: AnimatedIcon(icon: AnimatedIcons.search_ellipsis, progress: _animationController, color: darkGray,), /*TODO get this to actually show up and animate while checking server*/
                      contentPadding: EdgeInsets.symmetric(
                          vertical: 12, horizontal: 12),
                      border: OutlineInputBorder(
                        borderRadius: BorderRadius.circular(10.0),
                      ),
                    ),
                  ),
                ),

颤振医生 -v 输出

[√] Flutter (Channel stable, v1.2.1, on Microsoft Windows [Version 10.0.17763.475], locale en-US)
    • Flutter version 1.2.1 at C:\flutter
    • Framework revision 8661d8aecd (3 months ago), 2019-02-14 19:19:53 -0800
    • Engine revision 3757390fa4
    • Dart version 2.1.2 (build 2.1.2-dev.0.0 0a7dcf17eb)

[!] android toolchain - develop for Android devices (Android SDK version 28.0.3)
    • Android SDK at C:\Users\amehta\AppData\Local\Android\sdk
    • Android NDK location not configured (optional; useful for native profiling support)
    • Platform android-28, build-tools 28.0.3
    • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1343-b01)
    ! Some Android licenses not accepted.  To resolve this, run: flutter doctor --android-licenses

[√] Android Studio (version 3.4)
    • Android Studio at C:\Program Files\Android\Android Studio
    • Flutter plugin version 35.3.1
    • Dart plugin version 183.6270
    • Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1343-b01)

[!] VS Code, 64-bit edition (version 1.21.0)
    • VS Code at C:\Program Files\Microsoft VS Code
    X Flutter extension not installed; install from
      https://marketplace.visualstudio.com/items?itemName=Dart-Code.flutter

[√] Connected device (1 available)
    • GT N7100 • 4d00db7fb27a90e9 • android-arm • Android 4.4.2 (API 19)

! Doctor found issues in 2 categories.

我希望图标显示出来，一旦我在 TextFormField 中提交，它将运行动画，直到服务器检查完成

【参考方案1】：

您必须将该行放在 initState 方法中，如下所示：

@override
  void initState() 
    super.initState();
    _controller = AnimationController(duration: expandDuration, vsync: this);

希望对您有所帮助。

【讨论】：

哇。是的，这行得通。我不知道我是怎么错过这个细节的。我应该删除这个问题还是将其标记为已回答？

留着吧，说不定能帮到别人。当你对 Dart 不太熟悉时，这可能是一个常见的错误。

你能解释一下为什么 initState() 中的双关语有效吗？而不是在 build()