使闭包符合typedef

Question

我正在将一些ObjC代码转换为Swift 3，并且在一个使用过的ObjC框架中的一个类需要一个符合typedef的完成块：

目标-C：

workflow = [[Workflow alloc] initWithSettings:settings completion:^(AuthenticationAccount *account, NSError *error)
{
    print("Do something ...")
}];

// The header for initWithSettings looks like this:

- (instancetype)initWithSettings:(Settings *)settings completion:(nullable authentication_account_completion_block_t)completion AUTH_DESIGNATED_INITIALIZER;

// authentication_account_completion_block_t typedef:

typedef void (^authentication_account_completion_block_t)(AuthenticationAccount * __nullable account, NSError * __nullable error);
#define AUTH_DESIGNATED_INITIALIZER NS_DESIGNATED_INITIALIZER AUTH_NO_DISCARD

如何将此正确转换为Swift，以便完成块仍符合authentication_account_completion_block_t typedef？ 如果我只是可选地将闭包强制转换为authentication_account_completion_block_t，则块永远不会被执行：

夫特：

let workflow = Workflow(settings: settings!, completion:
{
    (account:AuthenticationAccount, error:Error?) in
        // Never executed!
        print("Do something ...")
} as? authentication_account_completion_block_t)

workflow.start()