1.使用AutomaticKeepAliveClientMixin，根据文档说明
/// A mixin with convenience methods for clients of [AutomaticKeepAlive]. Used
/// with [State] subclasses.
///
/// Subclasses must implement [wantKeepAlive], and their [build] methods must
/// call super.build (the return value will always return null, and should be
/// ignored).
使用条件
1.必须是 Statefull 的widget
2.重写 wantKeepAlive
@override
bool get wantKeepAlive => true;
3.在build方法中，在返回你的内容之前必须调用super.build(context);
Widget build(BuildContext context) {
super.build(context);
return Scaffold(...);
}
国内的文章中都忽略了第三点，会导致意想不到的问题，所以建议大家还是多看官方文档
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