为什么toList()创建List< dynamic>在Dart? [英] Why toList() creates a List<dynamic> in Dart?

问题描述

我有这种方法，可以在Dart 2中正确编译.但是在运行时，出现以下错误

I have this method, which compiles with no problems in Dart 2. However at run-time I get the following error

类型' List< dynamic> '不是类型' List< ExchangeRate> '

正如您在代码中看到的那样，我在 .map()中创建并返回新的ExchangeRate对象，然后返回我期望的 rateEntries.toList()的类型为 List< ExchangeRate> ，但是似乎可以推断为类型为 List< dynamic> !!!

As you see in the code I create and return new ExchangeRate objects within .map() and then after that I return a rateEntries.toList() which I expect to be of type List<ExchangeRate>, however it seems to be inferred as type List<dynamic>!!!

@override
Future<List<ExchangeRate>> getExchangeRatesAt(DateTime time, Currency baseCurrency) async {
  final http.Client client = http.Client();
  final String uri = "some uri ...";
  return await client
    .get(uri)
    .then((response) {
      var jsonEntries = json.decode(response.body) as Map<String, dynamic>;
      var rateJsonEntries = jsonEntries["rates"].entries.toList();
      var rateEntries = rateJsonEntries.map((x) {
        return new ExchangeRate(x.value.toDouble());
      });
      return rateEntries.toList(); // WHY IS IT RETURNING A List<dynamic> here?
    })
    .catchError((e) => print(e))
    .whenComplete(() => client.close());
}

但是，如果我将其专门转换为ExchangeRate，那就没问题了.

However if I cast it specifically to ExchangeRate it would be fine.

    return rateEntries.toList().cast<ExchangeRate>();

这种转换对我来说似乎是多余的，为什么我需要它?

This casting at the end seems redundant to me, why should I need it?

推荐答案

好吧，似乎必须使用强制类型转换来完全定义类型.

Well, it seems that the cast is necessary to fully define the type.

但是，如果添加以下任何代码段，则可以避免强制转换:

But, you can avoid the cast if you add any of the following snippets:

1. 为 rateJsonEntries 变量提供正确的类型

List< dynamic>rateJsonEntries = jsonEntries ["rates"].entries.toList();

无论出于何种原因，这对我来说都是有效的.

For whatever reason this works in my case.

将参数类型添加到 map()方法

var rateEntries = rateJsonEntries.map< ExchangeRate>(((x){返回新的ExchangeRate(x.value.toDouble());});

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