为什么我不能在 build() 中使用 context.read，但我可以使用 Provider.of with listen: false？

Question

文档中说它们是一样的，context.read只是Provider.of<x>(context, listen: false)的一个快捷方式。如果我尝试在构建方法中使用 context.read，控制台也会出现错误，但它没有解释原因。

我也找到了这个话题：但它没有回答“为什么”。

Answer 1

context.read不允许在build里面使用，因为在那里使用非常危险，而且有更好的解决方案。
Provider.of 在 build 中被允许用于向后兼容。

总体而言，build 中不允许使用 context.read 的原因在 its documentation 中进行了解释：

DON'T call [read] inside build if the value is used only for events:
Widget build(BuildContext context) {
  // counter is used only for the onPressed of RaisedButton
  final counter = context.read<Counter>();

  return RaisedButton(
    onPressed: () => counter.increment(),
  );
}
While this code is not bugged in itself, this is an anti-pattern. It could easily lead to bugs in the future after refactoring the widget to use counter for other things, but forget to change [read] into [watch].

CONSIDER calling [read] inside event handlers:
Widget build(BuildContext context) {
  return RaisedButton(
    onPressed: () {
      // as performant as the previous previous solution, but resilient to refactoring
      context.read<Counter>().increment(),
    },
  );
}
This has the same efficiency as the previous anti-pattern, but does not suffer from the drawback of being brittle.

DON'T use [read] for creating widgets with a value that never changes
Widget build(BuildContext context) {
  // using read because we only use a value that never changes.
  final model = context.read<Model>();

  return Text('${model.valueThatNeverChanges}');
}
While the idea of not rebuilding the widget if something else changes is good, this should not be done with [read]. Relying on [read] for optimisations is very brittle and dependent on an implementation detail.

CONSIDER using [select] for filtering unwanted rebuilds
Widget build(BuildContext context) {
  // Using select to listen only to the value that used
  final valueThatNeverChanges = context.select((Model model) => model.valueThatNeverChanges);

  return Text('$valueThatNeverChanges');
}
While more verbose than [read], using [select] is a lot safer. It does not rely on implementation details on Model, and it makes impossible to have a bug where our UI does not refresh.

Answer 2

问题是您尝试在小部件构建完成之前调用上下文，以便在小部件构建完成后运行您的代码将您的代码提供给 post 帧回调函数。

例如：

WidgetsBinding.instance.addPostFrameCallback((_) {
    // your code in here
});

为什么我不能在 build() 中使用 context.read，但我可以使用 Provider.of with listen: false？

Why can't I use context.read in build(), but I can use Provider.of with listen: false?

dart

flutter

flutter-provider