在 Dart 中使用 ifAbsent 的地图更新方法

Question

I'd like to modify an existing item in a map, as in replace the value of an existing key with a new one, with an added clause if the key does not exist in the Map already, to simply create a new key and value pair. The Dart documentation suggests the update method for such a purpose, but I'm not quite sure about how to implement it with the optional ifAbsent() parameter, which I assume is a line of code called if the key to be updated does not exist.

V update(K key, V update(V value), {V ifAbsent()});

According to the documentation, there is an optional parameter to be taken, but it shows an error saying too many parameters, 2 expected but 3 found.

This shows no error (not yet tested, but theoretically should work):

userData.update(key, value);

This (with the added create if not exist clause) does:

userData.update(key, value,
  userData[key] = value;
  );

Any help to get the latter or equivalent to work is much appreciated! I assume I'm missing something rather obvious here...

Answer 1

That is a named parameter, you can use it like this:

userData.update(
  key, 
  // You can ignore the incoming parameter if you want to always update the value even if it is already in the map
  (existingValue) => value, 
  ifAbsent: () => value,
);

Answer 2

Updates the value for the given key.

user["age"] = 25;
user.update("age", (dynamic val) => ++val); // => 26

This also returns the new value. To prevent an error being thrown should the key not exist, there's a third parameter:

user.update("name", (dynamic val) => "Jim", ifAbsent: () => "Jane");
print(user); // => {"age": 26, "name": "Jane"};

In most cases you could update using the array bracket notation:

user["name"] = "Mary";