如何将JSON资产加载到Flutter App中 [英] How to load JSON assets into Flutter App

问题描述

如何将JSON资产加载到Flutter应用程序中？

How do I load a JSON asset into my Flutter app?

我的 pubspec.yaml 文件具有以下：

  assets:
    - assets/data.json

我一直在尝试加载数据时陷入困境。我尝试了

I keep getting stuck trying to load the data. I tried

final json = JSON.decode(
    DefaultAssetBundle.of(context).loadString("assets/data.json")
);

但是我得到了错误

参数类型'Future<不能将String>分配给参数类型'String'。

The argument type 'Future< String>' can't be assigned to the parameter type 'String'.

谢谢！

推荐答案

@Alexandre Beaudet的答案是正确的，但没有给出很多有关正在发生的事情的背景。

@Alexandre Beaudet's answer is correct but doesn't give a lot of context about what is going on.

当您调用 loadString 时，它实际上是一个异步方法调用。您可以知道，因为它返回的是 Future< value> 而不是仅返回的 value 。这意味着它不会立即得到String的结果，但是会在将来的某个时刻出现。

When you're calling loadString, it's actually an asynchronous method call. You can tell because it returns a Future<value> rather than just a value. This means that it doesn't immediately have a result of String, but will at some point in the future.

在Dart中处理异步的主要方法有两种；第一种是使用 async 和 await ，第二种是直接使用期货。请参见异步编程飞镖指南。

There are two main ways of dealing with asynchronicity in Dart; the first being to use async and await, the second being to use the futures directly. See the dart guide on Asynchronous Programming.

如果直接使用 future.then ，则可以从常规函数（即initState等）中进行操作。您只需指定回调并在回调中处理结果。

If you use future.then directly, you can do it from a normal function (i.e. from initState etc). You simply specify the callback and in the callback what to do with the result.

void printDailyNewsDigest() {
  final future = gatherNewsReports();
  future.then((news) => print(news));
}

如果要使用 await 如@Alexandre所示，您需要将正在使用的功能标记为 async ，即：

If you want to use await as @Alexandre has illustrated, you need to mark the function you are using it from as async, i.e.:

Future<Void> printDailyNewsDigest() async {
  String news = await gatherNewsReports();
  print(news);
}

如果您覆盖某个函数（即initState），则还需要确保不要更改返回值。多数情况下，应该会被dart 2的输入捕获，但是void-> Future似乎不是。

If you override a function (i.e. initState) you also need to make sure you don't change the return value. That should get caught by dart 2's typing most of the time, but void -> Future doesn't seem to be.

最后一件事-如果您使用数据结果构建窗口小部件，您可能需要使用 FutureBuilder 。

One last thing to note - if you're using the result of the data to build widgets, you'll probably want to look at using a FutureBuilder.

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