syntax - dart mixin 'with' 不能在没有 'extends' 的情况下使用？

Question

我在 Webstorm 6.0.2 中工作，在尝试使用 mixin 语法时出现错误:

class A{}

class B with A{} //error can't use with syntax without an extends?

为什么我不能使用 with没有 extends ?当然每个类都隐含 extends Object .

Answer 1

这是a really clear explanation from Ladislav Thon :

[...] there's a simple advice, which is actually semantically correct: in the declaration class C extends SC with M1, M2, M3 implements I1, I2 { ... }, imagine parenthesis around the content of the extends clause. They will look like this: class C extends (SC with M1, M2, M3) implements I1, I2 { ... }. Which means that class C doesn't extend SC, it extends SC_with_M1_with_M2_with_M3.

Or, put another way: the class declaration has an extends clause and an implements clause, but it doesn't have a with clause. Instead, the with clause belongs inside the extends clause.

和 another point from Florian Loitsch :

When you extend "Object" with a mixin the first mixin can always take the place of "Object".

所以你的 class B with A应该是 class B extends Object with A这也相当于 class B extends A .