Dart extends Map to facilitate lazy loading
class, dart, dictionary, extends
Solution
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
}
Problem
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. What I tried is this: ``` 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 } ``` This raises an error stating "Cannot resolve constructor Map for implicit super call" which is cryptic for me. 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. 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. Kind regards, Hendrik Jan