Flutter BLoC `buildWhen` 属性

Question

我_CastError 在这段代码的最后一行遇到错误

BlocBuilder buildUsernameField() {
  return BlocBuilder<ProfileBloc, ProfileState>(
    buildWhen: (previous, current) => previous != current && current is EditingUserInfo,
    builder: (context, state) => TextField(
      keyboardType: TextInputType.name,
      controller: TextEditingController(
          text: (state as EditingUserInfo).username.value),

这么说

I/flutter (26787): The following _CastError was thrown building BlocBuilder<ProfileBloc, ProfileState>(dirty, state:
I/flutter (26787): _BlocBuilderBaseState<ProfileBloc, ProfileState>#25b87):
I/flutter (26787): type 'Success' is not a subtype of type 'EditingUserInfo' in type cast

所以发生的事情是，当我处于另一种状态（成功）时，它会尝试构建该小部件。但在buildWhen参数中，我指定小部件仅应在状态为 时构建EditingUserInfo。

据我了解，这个错误不应该发生。

这是我的ProfileState：

I/flutter (26787): The following _CastError was thrown building BlocBuilder<ProfileBloc, ProfileState>(dirty, state:
I/flutter (26787): _BlocBuilderBaseState<ProfileBloc, ProfileState>#25b87):
I/flutter (26787): type 'Success' is not a subtype of type 'EditingUserInfo' in type cast

Answer 1

您仍然需要检查状态变量是否处于正确的状态。每次更改时都会检查状态，因此状态变量仍然可以是不同的状态，除非 buildWhen 条件为真，否则它不会重建。

BlocBuilder buildUsernameField() {
return BlocBuilder<ProfileBloc, ProfileState>(
buildWhen: (previous, current) => previous != current && current is EditingUserInfo,
builder: (context, state) {
 if(state is EditingUserInfo) {
  return TextField(
  keyboardType: TextInputType.name,
  controller: TextEditingController(
      text: state.username.info)
}
}