类型转换和循环的位置

Question

我有以下场景:

protocol A {}
protocol B: A {}
protocol C: A {}

let objects: [A] = ...

我如何遍历数组并仅执行类型对象的逻辑B？

现在,我正在做这样的事情:

for object in objects {
    if let b = object as? B {
        ...
    }
}

但我想知道我是否可以where用来使这更具表现力和优雅.

for b in objects where b is B // <- compiles, but b is typed as A, not B
for b: B in objects where b is B // <- doesn't compile
for b in objects as! [B] where b is B // <- I get a warning that "is" will always be true

Answer 1

也有for case(几乎是相同的case,如switch报表),所以它看起来是这样的:

for case let b as B in objects {
  // use b which is now of type B
}

另一个好的表达是:

for case let b as protocol<B, C> in objects {
  // use b which is now of type protocol<B, C>
}

所以你可以同时使用两种协议的方法,属性等