Dart扩展了Map以方便延迟加载 [英] Dart extends Map to facilitate lazy loading

问题描述

我正尝试将服务器中的数据延迟加载到Map中。

出于这个原因，我想向Map添加功能，以便在不存在密钥时，

I am trying to lazy-load data from the server into a Map.
For that reason I would like to add functionality to Map, so that when a key doesn't exist, a call is done to get the value.

我尝试过的是这样：

class LazyMap extends Map {
  // use .length for now. When this works, go use xhr
  operator [](key) => LazyMap.putIfAbsent(key, () => key.length);
}

LazyMap test = new LazyMap();

main() {
  print(test.containsKey('hallo')); // false

  // I prefer to use this terse syntax and not have to use putIfAbsent
  // every time I need something from my map
  print(test['hello']); // 5

  print(test.containsKey('hallo')); // true
}

这会引发错误，指出无法解析构造函数映射以获取隐式超级

This raises an error stating "Cannot resolve constructor Map for implicit super call" which is cryptic for me.

这是我第一次尝试扩展任何内容，因此我可能会做一些愚蠢的事情。
对此做得更好的任何建议，或者可能告诉我我使用的是不良做法，都将受到赞赏。

This is the first time I try to extend anything, so I might be doing stupid things. Any advise on doing this better, or probably telling me that I am using a bad practice will be appreciated.

我已经研究了这个答案：< a href = https://stackoverflow.com/questions/16247045>如何在Dart中扩展列表，但这只是扩展列表而不是地图。我一直在寻找MapBase，但找不到它。

我已经调查了这个答案：我想将我自己的方法添加到一些Dart类中，但这似乎是一个很老的答案，没有实际解决方案。

I've looked into this answer: How do I extend a List in Dart, but this is about extending a List, not a Map. I've looked for a MapBase but could not find one.
And I've looked into this answer: I want to add my own methods to a few Dart Classes, but this seems to be a very old answer with no real solution.

亲切的问候，
Hendrik Jan

Kind regards, Hendrik Jan

推荐答案

查看此发布，您将无法扩展Map及其子类。我认为获得所需内容的最佳方法是实施它。

Looking at this post, you cannot extends Map and its subclass. I think the best way to get what you want is to implement it.

class LazyMap implements Map {
  Map _inner = {};

  operator [](key) => _inner.putIfAbsent(key, () => key.length);

  // forward all method to _inner
}

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