如何在 Flutter 中发现 Widget 何时出现/消失？

Question

我想在有状态的小部件中找出整个小部件何时出现在屏幕上或消失，类似于 iOS onViewWillAppear / 消失。这有可能吗？我在 Flutter 文档中没有找到任何相关内容。

谢谢！

Answer 1

您要找的东西就在flutter_widgets包裹里

将以下内容添加到您的pubspec.yaml

flutter_widgets: ^0.1.7+1

这个包里面有一个小部件，称为VisibilityDetector它需要 a key、 achild和一个函数onVisibilityChanged

return VisibilityDetector(
key: Key("1"),
onVisibilityChanged: (visibility) {
//This will give you a range of values between 0 and 1,
//0 being not visible and 1 being fully visible.
print(visibility.visibleFraction) 
}
child: Container(
width:double.infinity,
height: 300,
   ),
),

Answer 2

如果您想在小部件构建后执行某些操作，您应该使用以下代码：

void initState() {
super.initState();
if (SchedulerBinding.instance.schedulerPhase ==
    SchedulerPhase.persistentCallbacks) {
  SchedulerBinding.instance.addPostFrameCallback((_) => onWidgetBuild());
}

/// appear
void onWidgetBuild() {
/// This block will be called onWidgetBuild
/// do your code here
}

/// disappear
@override
void dispose() {
  super.dispose();
  /// release whatever you have consume
}

希望这会对您有所帮助。