好的，我已经弄明白了。我已经将 UIButton 中的segue定义为下一个 UIViewController 。按照这种方式定义，没有办法有条件地执行segue。

Ok, I have figured it out. I had defined a segue from a UIButton to the next UIViewController. As defined in this manner, there is no way to conditionally execute the segue.

我没有把segue放在按钮上，而是从第一个 UIViewController 到另一个 UIViewController 。这定义了segue，但不与它相关联。从那里我可以简单地调用 performSegueWithIdentifier：sender：当我想要执行segue时（在我的情况下是异步任务完成时）。

Instead of putting the segue on the button, I made a segue from the first UIViewController to the other UIViewController. This defines the segue, but associates no action with it. From there I can simply call performSegueWithIdentifier:sender: when I want that segue to execute (in my case being when the async task is complete).

这对我来说的主要优势在于IB维持其结构。

The main advantage of this for me is that IB maintains its structure.

非常感谢@cli_hlt指出我正确的方向（ +1）。

Big thanks to @cli_hlt for pointing me in the right direction (+1).