使用 DEFER 离开 dispatchGroup 会导致崩溃

Question

我有一个休息Api异步方法，所以使用调度组：

let dispatchGroup = DispatchGroup()
dispatchGroup.enter()
Task.do { result in
   defer { dispatchGroup.leave() }
    //...
}

这会导致崩溃 Thread 1: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)

但是

let dispatchGroup = DispatchGroup()
dispatchGroup.enter()
Task.do { result in
    //...
    dispatchGroup.leave()
}

不要崩溃。为什么？

Answer 1

要点 1 是：当函数范围结束时调用 defer ...

要点 2 是：dispatchGroup enter 应该等于 leaves 否则你的应用程序会崩溃

Leaving dispatchGroup with DEFER cause a crash